WO2000007194A1 - Preparation of tungsten bronzes for nuclear waste storage forms and electronic materials - Google Patents

Abstract

A process for preparing a lanthanide or actinide-containing tungsten bronze including: (a) heating a solid composition containing a lanthanide or actinide-containing polyoxotungstate in the presence of a reducing agent until the temperature of said solid composition reaches a first predetermined temperature to form a reduced composition; (b) heating said reduced composition in the presence of an inert gas until the temperature of said reduced composition reaches a second predetermined temperature; and (c) maintaining said reduced composition at said second temperature for a predetermined period of time.

Description

TITLE OF THE INVENTION

PREPARATION OF TUNGSTEN BRONZES FOR NUCLEAR WASTE STORAGE FORMS AND ELECTRONIC MATERIALS This application claims benefit of priority to Provisional Application No. 60/094,233, filed on July 27, 1998, and Provisional Application No. , filed on July 21, 1999. The contents of the two Provisional

Applications are incorporated herein by reference in their entirety.

FIELD OF THE INVENTION

The present invention relates to tungsten bronzes useful for long term storage of radionuclides produced as nuclear waste. Particularly, the invention relates to new polyoxotungstates which form molecular units for preparing lanthanide and actinide containing tungsten bronzes. As well, the invention relates to a thermally efficient process for converting the polytungstates into tungsten bronzes.

This work was partially funded by the United States government through the Department of Energy Grant No. DE-Fg07-96ER14695.

BACKGROUND OF THE INVENTION

Tungsten bronzes are inert inorganic solids of general formula M_xW0₃, where M is an electropositive metal ion of appropriate size, and x has values between 0 and 1. Bronzes with alkali, alkaline earth, main group 3, NH/ or rare earth ions have been well studied due to their electronic and magnetic properties. The increasing interest in these materials is particularly due to the versatility of their composition, structure, physical and chemical properties. For example, tungsten bronzes may have drude type optical behavior, and electrochemical and electronic type properties which make them attractive as active electrodes in a number of devices including electrochromic windows. These materials can be regarded as solid solutions of the metal M in a

W0₃ matrix. The matrix is based on structural units formed by tungsten atoms surrounded by oxygen atoms in an octahedral arrangement, wherein the octahedrals are connected at their corners.

In light of the recent advances in solid state chemistry, particularly solid state synthesis and characterization, new avenues have become available for tailoring new bronzes with controlled structural, physical and/or chemical properties. The application of new solid state chemistry tools to the design of novel tungsten bronzes is particularly facilitated by the generally open crystallized structure of these bronzes. One avenue for the development of technological applications utilizing the unique properties of tungsten bronzes is based on the chemical inertness or resistance of these materials. In particular, tungsten bronzes incorporating radioactive elements have been proposed as matrices for long term storage of radioactive nuclear waste. The chemical resistance and thermal stability of tungsten bronzes suggests that they may be possible waste forms for radioactive materials.

Tungsten bronzes have been synthesized by conventional processes based on heating a mixture of powdered metal M and the oxide W0₃ at a temperature of at least 1000°C. The properties of bronzes produced by these conventional processes cannot be easily controlled, due in part to the lack of uniform dispersion of the ingredients in the powder mixture. Applicants have investigated alternative processes for making tungsten bronzes having controlled physical and chemical properties. One alternative process for preparing tungsten bronzes is based on the synthesis of polyoxoanions which encapsulate the metal ion to be sequestered in the Bronze. The polytungstates are employed as precursors which are treated to form the bronze.

For example, U.S. Patent No. 5,618,472 ('472) to Pope et al, the contents of which are hereby incorporated by reference in their entirety, describes tungsten glasses and bronzes which incorporate radioactive metals for long term storage. The long term storage of radioactive materials disclosed in '472 is based on the synthesis of metal ion containing polyoxotungstates capable of ion exchange to substitute the metals contained in a polyoxotungstate (generally alkaline or alkaline earth metals) by radionuclides from nuclear waste. The ion exchanged polyoxotungstates are then treated by known methods to form a tungsten glass or tungsten bronze for long term storage of radionuclides of nuclear waste origin.

The polytungstates disclosed in '472 are based on ZP₅W₃₀O₁₁₀ as a building block. While the tungstates disclosed in '472 have shown good incorporation properties for lanthanide ions having ionic radii smaller or equal to that of Sm₃ ⁺, ie r= 1.10 A, incorporation of larger lanthanide or actinide cations in those tungstate polyanions could not be successfully obtained. As well, the tungsten bronzes disclosed in the '472 were formed by a conventional heat treatment process, which requires substantive amounts of energy, thereby increasing the cost for long term storage of the nucleotides by sequestration in the bronze.

Thus, there remains a need for tungsten bronzes based on polyoxotungstates capable of selectively sequestering radionuclide cations of various sizes. As well, there remains a need for a more thermally efficient process for converting the polyoxotungstates into tungsten bronzes.

OBJECTS AND SUMMARY OF THE INVENTION

Therefore, it is an object of the invention to provide a process for preparing a lanthanide or actinide-containing tungsten bronze. The process comprises: (a) heating a solid composition comprising a lanthanide or actinide-containing polyoxotungstate and a reducing agent until the temperature reaches a first predetermined temperature to form a reduced composition; (b) heating the reduced composition in the presence of an inert gas until the temperature reaches a second predetermined temperature; and (c) maintaining the reduced composition at the second temperature for a predetermined period of time, preferably between about 3 and 6 hours, to form the tungsten bronze. Preferable first and second temperatures are about 500°C or less and about 1000°C or less, respectively. During the heat treatments in steps (a) and (b), the temperature of the composition being treated is increased from room temperature to the first or second predetermined temperature in temperature increments, preferably of about 5°C. The products obtained after step (c) are slowly cooled, preferably overnight. The process of the invention can be effectively practiced with a variety of reducing agents. Preferred reducing agents include hydrogen, ammonia and mixtures thereof. As well, a variety of inert gases are contemplated for conducting step (b). Preferred inert gases include argon and nitrogen.

In one embodiment, the invention provides a method of preparing a plyoxoanion containing a lanthanide. The method comprises: (a) mixing, in an aqueous solution, a metal oxide, a tungstate, and a compound containing said lanthanide to form a reaction mixture containing an alkaline metal with an alkaline metal concentration of at least 1M. The pH of the solution is then lowered by adding a first acid solution to said reaction mixture; (c) heating said reaction mixture for a predetermined period of time to form a lanthnide containing polyoxotungstate. The method further comprises precipitating said polytungstate by forming a polytungstate salt. The bronze is then formed by (i) heating said polytungstate salt in the presence of a reducing agent until the temperature of said salt reaches a first predetermined temperature to form a reduced composition; (ii) heating said reduced composition in the presence of a inert gas until the temperature of said reduced composition reaches a second predetermined temperature; and (iii) maintaining said reduced composition at said second temperature for a predetermined period of time.

In a specific embodiment, the polyoxotungstates of the invention are in the form of a salt, preferably a sodium free salt, wherein a lanthanide or actinide ion is incorporated into the polyanion during the synthesis of the polyoxoanion. Preferably, the salt comprises ammonium ions which are reduced to ammonia. The ammonia is then utilized as a reducing agent in step (a) of the bronze forming process of the invention.

The process of the invention is particularly suitable for the preparation of tungsten bronzes based on polyoxometalates selected from the group consisting of {M^III ₁₆(H₂0)₃₆As^11I ₁₂W₁₄₈0₅₂₄}-⁷⁶,

{M^III(H₂O)_π(M^III ₂OH)As^III ₄W₄₀O₁₄₀} {M^m ₄(H₂O)₄As^m ₅W₃₉O₁₄₃}-²⁵ and {(UO₂)₃(H₂O)₆A W₃₀O₁₀₅}-¹⁵.

In another aspect, the invention provides a tungsten bronze of formula M_xW0₃, wherein M is a lanthanide or actinide metal ion. The tungsten bronze of the invention is prepared by (a) heating a solid composition comprising a lanthanide or actinide-containing polyoxotungstate in the presence of a reducing agent until the temperature of the composition reaches a first predetermined temperature; (b) heating the composition in the presence of an inert gas until the temperature reaches a second predetermined temperature; and (c) maintaining the composition at said second temperature for a predetermined period of time, preferably between about 3 and 6 hours. The tungsten bronzes of the invention preferably have a cubic or hexagonal crystal structure. The value of x in the compounds of formula M_xW0₃ is controlled by adjusting the molar ration M/W in the polyoxotungstate. Preferred tungsten bronzes according to the invention include bronzes wherein x is between about 0.06 and 0.17.

In a preferred embodiment of the invention, the polyoxotungstate is preferably a lanthanide substituted polyoxometalate selected from the group consisting of {M^πι ₁₆(H₂0)₃₆As^m ₁₂W₁₄₈0₅₂₄}-⁷⁶, {M^m(H₂O)_π(M^III ₂OH)As^III ₄W₄₀O₁₄₀}-⁴⁰, {M^III ₄(H₂O)₄As^m ₅W₃₉O₁₄₃}-²⁵ and {(U0₂)₃(H₂0)₆As^III ₃W₃₀0₁o₅}^"15, wherein M^πι represents a trivalent lanthanide. In a further aspect, the invention provides a method for long term storage of radionuclides comprising :(a) heating a solid composition comprising a tungsten oxopolyanion containing a radionuclide in the presence of a reducing agent until the temperature of the composition reaches a first predetermined temperature; (b) heating the composition in the presence of an inert gas until the temperature of the composition reaches a second predetermined temperature; and (c) maintaining the composition at said second temperature for a predetermined period to form said radionuclide- containing tungsten bronze. In a preferred embodiment of the invention, the radionuclide is selected from the group consisting of Sr, Cs, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Ac, Th, Pa, U, Np, Pu, Am and mixtures thereof, and is preferably incorporated into the polyoxotungstate through ion exchange with a monovalent metal ion, such as Na⁺ or K⁺.

In yet another aspect, the invention provides a lanthanide substituted polyoxometalate useful in the preparation of tungsten bronzes selected from the group consisting of {M^πι ₁₆(H₂0)₃₆As^πι ₁₂W₁₄₈0₅₂₄}-⁷⁶,

{M^m(H₂O)₁₁(M"^I ₂OH)As^II1 ₄W₄₀O₁₄₀} {M^m ₄(H₂O)₄As^m ₅W₃₉O₁₄₃}-²⁵ and {(UO₂)₃(H₂O)₆As^πι ₃W₃₀O₁₀₅}^"15, wherein M^ιπ represents a trivalent lanthanide.

BRIEF DESCRIPTION OF THE FIGURES

Figures 1(a), 1(b) and 1(c) show the polyhedral 3-D structure of the Keggin anion [α-PW₁₂O₄₀]³, its lacunary fragment [B-α-As^ιπW₉0₃₃]⁹, and the polyoxotungstate [(As^πιW₉0₃₃)₄(W0₂)₄]^28" formed by combining four fragments [B-α-As^mW₉0₃₃]⁹\ respectively;

Figures 2(a) and 2(b) show polyhedral and space- filling views of the polytungstate [As^ιπ ₁₂Ce₁₆(H₂0)₃₆W₁₄₈0₅₂₄]⁷⁶\ respectively; Figures 3(a) and 3(b) show ^I83W NMR spectra in D₂0 for the polytungstate [M^ιπ ₁₆(H₂0)As^m ₁₂W₁₄₈0₅₂₄]⁷⁶-, wherein M is La and Ce, respectively;

Figures 4(a) and 4(b) show top and side views of the polytungstate [As^ιπ ₅Ce^,π ₁₄(H₂0)₄W₃₉0₁₄₃]²⁵-, respectively; Figures 5(a), 5(b) and 5(c) show polyhedral 3-D structure and partial top and side views of the polytungstate {[M"¹(H₂O)₁₁(M"^I ₂OH)As^II1W₄₀O₁₄₀]₂}⁴⁰-;

Figures 6(a), 6(b) and 6(c) show ¹⁸³W NMR spectra in D₂0 for {[M^III(H₂O)₁₁(M^II1 ₂OH)As^,1IW₄₀O₁₄₀]₂}⁴⁰ wherein M is Ce, Nd and Sm, respectively; Figures 7 shows a polyhedral 3-D structure of the polytungstate [As^ι ₃(UO₂)₃(H₂O)₆W₃₀O₁₀₅]¹⁵-;

Figure 8 shows a polyhedral 3-D structure of cubic bronze M_xW0₃;

Figure 9 shows powder deiffractometers of thermally treated NH₄ ⁺ salts of Ce₁₆As₁₂W₁₄₈ and U₃As₃W₃₀ polyoxoanions;

Figures 10(a) to 10(e) show polyhedral 3-D structures for five polyoxoanions of the structural types I, II, III, and IV or V: [As'"₁₂Ce₁₆(H₂0)₃₆W₁₄₈0₅₂₄]⁷⁶- (I), [As ^ι ₅Ce ^ι ₁₄(H₂0)₄W₃₉0₁₄₃]²⁵- (II), [As^m ₃(UO₂)₃(H₂O)₆W₃₀O₁₀₅]¹⁵- (III), and[(W₅0₁₈)₂Ce]⁹- (IV) and[(W₅0₁₈)₂Thf (V), respectively;

Figures 11 and 12 show powder diffractograms of the degradation products of la, lb and Ilia;

Figure 13 shows weight variation as a function of temperature during the formation of the cubic uranium bronze U₀ ,W0₃, Figure 14 shows the variation of cubic cell parameter a,, (in A) as a function of the variable x in cubic bronzes Ce_xW0₃.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides tungsten bronzes which incorporate radionuclides such as lanthanide and actinide cations. The tungsten bronzes of the invention provide chemically stable matrices for the long term storage of radionuclides present in nuclear waste. The present invention is based on the advantageous utilization of the structural properties of novel polyoxotungstates and a novel process for thermally efficient conversion of the polytungstates into tungsten bronzes having desired properties. The present invention provides novel polyoxotungstates which are useful as precursors for preparing tungsten bronzes incorporating radionuclides.

The invention provides a process for synthesizing radionuclide- containing polyoxotungstates by reacting the components of the polytungstate to be synthesized, including the radionuclide. The process involves mixing, in an aqueous solution, a metal oxide, such as As₂0₃, a tungstate, such as W0₄ ^2" and a compound containing the lanthanide to be encapsulated, for example, a lanthanum nitrate, such as Ce(N0₃)₃. The reaction conditions are adjusted such that a polyoxotungstate is formed around the lanthanide (cation), thereby providing a polyoxotungstate with a lanthanide sequestered therein.

The tungstate and metal oxide components are mixed in an aqueous solution to form an alkaline solution, i.e. a solution having a pH greater than 7. The pH of the solution is then lowered by adding a first acid solution to the mixture. The lanthanide component (in the form of a nitrate, for example) is then added to the solution. A second acid solution is then added to the mixture to offset the pH increase due to adding the lanthanide component. The second acid solution is added such that the pH of the solution mixture is between about 2 and 6. The mixture is then heated for a predetermined period to form the polytungstate with the lanthanide sequestered therein. The polytungstate is then precipitated from the solution in the form of a salt which is optionally processed for purification, such as by recrystalhzation or by other conventional purification techniques.

The process of the invention includes controlling key parameters to produce polytungstate anions of controlled size. The polytungstates of the invention are built based on building blocks. The size of the produced polyoxotungstate is determined by the number of polytungstate building blocks that are linked together to form a stable structure. Processes for synthesizing polytungstate anions have been previously reported. See for example the following references, the contents of which are hereby incorporated by reference in their entirety: K. Wassermann, M. H. Dickman, and M. T. Pope, Angew. Chem. Int. Ed. Engl, 36,1445 (1997); M. T. Pope, X. Wei, K. Wassermann, and M. H. Dickmann, C R. AcadSci, Ser. He, 1, 297 (1998); K.-C. Kim, Ph.D. Dissertation, Georgetown University, 1998; R. D. Peacook and T. J. R. Weakley, J Chem. Soc, (A), 1836 (1971); and A. V. Botar and T. J. R. Weakley, Rev. Roumaine de Chimie, 18,1155 (1973). The inventors have unexpectedly discovered a method for controlling the stability of the polytungstates being formed in the aqueous solution. For example, the inventors have discovered that in order for polytungstates having large and/or complex structures to be stable within the reaction mixture, the aqueous mixture must contain a large amount of an alkaline metal, such as sodium. The alkaline metal can be provided as part of the initial components mixed in the solution. Alternatively, the alkaline metal can be added to the aqueous solution, for example in the form of a hydroxide prior to adding the reagents for making the polytungstate. In particular, the inventors have discovered that the solution mixture containing the ingredients for forming the polytungstate must contain an alkaline metal, such as sodium, at a relatively high concentration. The alkaline metal is present at a concentration of at least 1M. Preferably, the concentration of the metal oxide is 2M or higher.

It is believed that the presence of the alkaline metal at relatively high concentrations stabilizes the polytungstate by interaction with the oxygen ions at the surface of the polytungstate. In the absence of the alkaline metal stabilization, the polyoxotungstate building blocks are believed to dissolve in the solution, which is detrimental to the formation of large polytungstates. The driving force for tungstate fragment dissolution is believed to be related to the relatively low pH required in the synthesis process.

The radionuclide containing polyoxotungstates synthesized according to the invention are employed as precursors of controlled composition and structure for the formation of tungsten bronzes. The bronzes are formed by a solid state reaction including reducing a solid composition containing the precursors by heat treatment to a first predetermined temperature in the presence of a reducing agent. The reduced composition is then heated to a second predetermined temperature in the presence of an inert gas. The products of the heat treatment are then maintained at the second temperature by isothermal heating for a predetermined period of time until a desired bronze phase is obtained. The bronze is then cooled down to room temperature at a slow rate.

The present invention provides numerous advantages. For example, the invention provides a process for synthesizing relatively large lanthanide- containing polytungstates, which in turn allows for a significant increase of the amount of lanthanides sequestered within the polytungstates. Additionally, the synthesis of relatively large polytungstates allows for the sequestration of large atomic and molecular ions. Another advantage provided by the process of synthesizing lanthanide- containing polyocotungstates according to the invention is the flexibility in controlling the stoichiometry of the precursor tungstates, thereby controlling the composition of the bronzes formed from those precursors. The stoichiometry of the final bronze is controlled by adjusting the relative amounts of the components added to the initial aqueous solution.

Still another advantage of the process of the invention relates to forming the polytungstate around the lanthanide, thereby eliminating the need for an ion exchange step to encapsulate a lanthanide by the polytungstate. Building a polytungstate around a lanthanide atom or compound allows more flexibility in producing polyoxotungstates which are tailored for sequestering a variety of lanthanides. The customization of making the encapsulating polyoxotungstates allows great flexibility in encapsulating lanthanides of various sizes.

The precursors of the invention can be advantageously designed to allow selective encapsulation of metal ions having a radius within a predetermined range. As well, the composition and structure of the polytungstate is adjusted as a function of the nature of the lanthanide to be encapsulated. Factors that influence the choice of the parameters of the synthesis process include, for example, the charge on the lanthanide. Based on the chrge of a lanthanide, the synthesis conditions are adjusted to provide a polytungstate structure which best complements the lanthanide cation, thereby increasing the stability of the lanthanide containing polytungstate.

The polyoxotungstate precursors synthesized according to the invention are also advantageous in that the amount of radionuclide content in the tungsten bronzes prepared from each precursor is controlled by adjusting the structure of the polytungstate as a function of the molar ratio between the radionuclide and the tungsten in the precursor. Precursors of different structure will encapsulate different amounts of radionuclide, thereby providing tungsten bronzes of formula M_xW0₃, wherein the variable x is determined by the structure of the polyoxotungstate precursor employed in forming the bronze. The bronzes of the invention are therefore designed to provide desirable qualitative and quantitative radionuclide encapsulation properties. That is, the structure of the polytungstate precursors is designed based on the nature of the radionuclide to be selectively encapsulated as well as the desired amount of radionuclide in the bronze formed from the precursor.

The enormous variety of polyoxometalate compositions available offers the prospect of synthesizing new bronze-based materials, e.g. with mixtures of guest atoms, that exhibit different ranges of optical, magnetic, and electronic properties.

The synthesis process of the invention is particularly advantageous in applications for the separation of nucleotides present in nuclear waste. In this regard, the invention provides a method for separating such radionuclide by conducting a polytungstate synthesis in an aqueous solution containing nuclear waste. The synthesis process parameters are adjusted to selectively form polyoxotungstates around the radionuclides in the waste, thereby encapsulating the radionuclide and forming polytungstate precursors. The precursors are then separated from the aquous solution by forming a salt thereof. The salt is precipitated from the solution in the form of a solid which is then optionally purified to minimize interference with the bronze forming process due to impurities that may be attached to the polytungstate salts. The salts are then treated as discussed above to form bronzes which in turn provide stable matrices for long term storage of the radionuclides. Another advantage is the increased homogeneity of the bronzes produced according to the invention. Preparing tungsten bronzes from precursor polytungstates containing the same amount of radionuclide provides highly homogeneous bronzes. Increasing the homogeneity of radionuclide distribution throughout a bronze crystal provides increased crystal stability and increased nuclide storage capacity. The quality of the bronzes of the invention is also enhanced due to the dispersion of the precursors at the molecular level, which can not be easily obtained when the bronze is prepared from powdered ingredients.

Yet another advantage of the invention is the thermal efficiency associated with the conversion of the polytungstate precursors to form the tungsten bronzes. Due to the molecular nature of the polytungstates of the invention, the temperature required for heating the precursors to form the bronzes is significantly reduced thereby converting the polytungstates into stable tungsten bronzes at temperatures which are significantly lower than those required by conventional processes. In one embodiment, the invention provides tungsten bronzes having properties which are controlled by adjusting the stereochemistry of the polyoxotungstates employed in forming the bronze. For example, the complex of An⁴⁺ with α₁-[P₂W₁₇0₆₁]^10" yields chiral syn and anti isomers ofTAn(P₂W,₇0₆₁)₂]^16", based on a nominal square antiprism coordination polyhedron for An⁴⁺. Similarly, P-NMR spectra of the U⁴⁺ and Th⁴⁺complexes display multiple sets of resonances consistent with the presence of several diastereomers. In this regard, anti and syn "conformations have been shown for the U and Th complexes.

EXAMPLES

EXAMPLE 1

This example illustrates the structural construction of polyoxometalate anions according to the invention which can be represented in terms of subunits based on lacunary fragments of a Keggin anion or its isomers. Figure 1(b) shows the polyhedral 3-D structure of the tungstoarsenate(III) anion [B- α-As^mW₉0₃₃]^9", which is a lacunary fragment of the Keggin anion [α- PW₁₂O₄₀]^3" shown in Figure 1(a). the anion [B-α-As^mW₉0₃₃]^9" shows a coordinative behavior which is believed to be dictated by the lone pair of electrons on As¹¹¹. The open structure of the anion [B-α-As^mW₉0₃₃]^9~ is advantageously employed as a subunit in the construction of multiple lanthanide and actinide substituted POMs, which are particularly useful as nuclide separators from nuclear waste.

As shown in Figures 1(c), 2(a), 4(a-b), 5, 7 and 8(b), the use of the anion [B-α-As^πιW₉0₃₃]^9~ provides cyclic structures in which the {AsW₉} units are augmented by additional tungsten, Ln or An atoms to form the polyoxotungstates [(As^πιW₉0₃₃)₄(W0₂)₄]²⁸ , [M^III ₁₆(H₂0)As^III ₁₂W₁₄₈0₅₂₄]⁷⁶-, [M^m(H₂O)₄._4xAs^III ₅O_M3._6x]²⁵-, {[M^III(H₂O)₁₁(M^III ₂OH)As^IIIW₄₀O₁₄₀]₂}⁴⁰- and [(UO₂)₃(H₂O)₆As^m ₃W₃₀O₁₀₅]¹⁵-, wherein M is La, Ce, Nd, Sm or Gd. The formed [B-α-As^mW₉0₃₃]^9" based polyoxotungstates have a molecular weight of up to 45,000 g/mol and were obtained as Na⁺ or

NH₄ ⁺salts. Figures 3(a) and 3(b) show the ¹⁸³W NMR spectra in D₂0 for [M^!II ₁₆(H₂0)As^m ₁₂W₁₄₈0₅₂₄]⁷⁶-, wherein M is La and Ce, respectively. Similarly, Figures 6(b), 6(c) and 6(d) show the ¹⁸³W NMR spectra in D₂0 for {[M^II1(H₂O)₁₁(M^,II ₂OH)As^IIIW₄₀O₁₄₀]₂}⁴⁰ wherein M is Ce, Nd and Sm, respectively. The NMR spectra show that the polyoxotungstates of the invention are readily soluble and stable in water. It should be noted, however, that a high Na⁺ concentration was found to be required for the synthesis of the [B-α-As^mW₉0₃₃]^9" based polyoxotungstates.

The [B-u As^mW₉0₃₃]^9" based polyoxotungstates of the invention were converted according to the thermally efficient process of the invention and provided, in quantitative yield, cubic Ln¹¹¹ and An^IV tungsten bronzes M_xW0₃ with x varying between about 0.006 and 0.17. Powder deiffractograms of thermally treated NH₄ ⁺ salts of Ce₁₆As₁₂W₁₄₈ and U₃As₃W₃₀ polyoxoanions are shown in Figure 9.

EXAMPLE 2

This example illustrates the preparation of tungsten bronzes from five polytungstate precursors corresponding to five different structures. Figures 10(a) to 10(e) are schematic diagrams of the polyoxotungstates [As^m ₁₂Ce^m ₁₆(H₂0)₃₆W₁₄₈0₅₂₄]⁷⁶- 1, [As^m ₅Ce^ι ₄(H₂0)₄W₃₉0₁₄₃]²⁵- II, [As^UO^H^W^O^]¹⁵- III, [(W₅0₁₈)₂Cef IV and [(W₅0₁₈)₂Thf V, respctively. The following analytical techniques and conditions were employed:

Thermal Analysis

The differential scanning calorimetric curves (DSC) were obtained in N₂ flow in the temperature range RT→725°C on a TA Instruments DSC 2910 analyzer, at a heating rate of 5°C/min and an initial sample weight of 15- 20mg using a platinum crucible. The thermogravimetric analyses (TGA) were performed under the same experimental conditions, RT→950°C on a TA Instruments TGA 2050 analyzer, using ceramic pans.

X-Ray Diffraction

Powder X-ray diffraction patterns were collected with a Rigaku D-Max Vertical Diffractometer using CuKa radiation with silicon as an internal standard) for all thermally treated products (TG, DSC and furnace). Elemental analysis

The elemental analyses for the starting materials and the thermally treated samples were carried out by E & R Microanalytical Laboratory, NY and Kanti Technologies Inc., NY. In order to determine the amount of reduced tungsten present in the bronzes M_xW0₃ (and therefore x), samples ranging from 25-150mg were oxidized in air and the increase in weight was determined, (TGA 2050, RT→850°C, 5K/min).

The compounds (NH₄)₇₀Na₆[As^!II ₁₂Ce^m ₁₆(H₂O)₃₆W_I48O₅₂₄]-175H₂O (la), (NH₄)₂₅._xNa_x[As^m ₅Ce^III ₄(H₂0)₄W₃₉0₁₄₃]-nH₂0 (IIa), (NH₄)15[As^πι ₃(UO₂)₃(H₂O)₆W₃₀O₁₀₅]-25H₂O (Ilia), Na₉[(W₅0₁₈)₂Ce]-nH₂0 (IVa) and Na₈[(W₅0₁₈)₂Th]-nH₂0 (Va) were synthesized as follows:

Into 70 mL of boiling water were 33.0 g (100 mmol) Na₂W0₄2H₂0 and 0.8 g (4.05 mmol) As₂0₃ dissolved. The pH of the solution was decreased from 9.0 to 7.1 by the addition of 15.4 mL (115 mmol) 6M HC1. After 10 min. a solution of 4.69 g (10.8 mmol) Ce(N0₃)₃ 6H₂0 in 20 mL l .OM acetic acid was added with a pipette to the vigorously stirred hot (90C) starting solution. The color changed to orange and the pH decreased to 5.7. The mixture was heated for 30 m at 90° C in a water-bath. Addition of a warm (70°C) solution of 40g (748 mmol) NH₄C1 in 150 mL water to the still hot (90 °C) transparent solution yielded a yellow-orange precipitate which was recrystallized from 80 mL of warm water (70°C, resultant pH 6.2). The recrystallized material consisted of long orange needles and a yellowish powder (33.0 g). The solution was stirred with a spatula so that the needles gathered at the bottom of the beaker before the milky solution was decanted. Once the precipitate in the decantate had settled, a few mL of the clear supernatant was poured onto the needles to extract any remaining yellowish fine powder by decantation. This procedure was repeated a few times before the needles were finally filtered off (17.0 g). Recrystalhzation of the needles from 20 mL water at 75°C (pH 6.6) gave 10.0 g (yield: 36%) of the pure title compound. The lanthanum analogue of I was prepared in essentially the same way using lanthanum nitrate.

ffl₄)₂5-_zN^ A^₅M^I^(H₂0)4-4yW_39+2Y0_134+6yJnΗ₂α

M-Ce³⁺.La ⁺.Nd³⁺.Sm³⁺.Gd³⁺ y=0.25-0.50 The preparation of the Ce^ιπ containing anion is given as an example.

In a 90° C hot solution of 24 mL 4M NaCll 5 mL2M NaOAc and 1 mL 2M

HO Ac pH 5.6 1.74 g (4.0 mmol) Ce(N0₃)₃ 6H₂0 were dissolved (pH 4.9).

Next, 14.1 g (5.0 mmol) of solid Na9[(B~AsO3W9O30] nH₂0 were added.

To the resulting redish-brown solution additional 2mL 2M NaOAc were given (pH 6.5). The solution was refluxed for 30 min. before 20.0 g (374 mmol)

NH4C1 dissolved in 60 mL water (pH 3.6) were added to the still 60 °C warm solution. The mother of pearl-like precipitate was filtered off after 30 min.

(9.5 g, pH 6.4) and recrystallized twice from warm water 70 °C (20 and 10 mL, pH 6.9. Finally, 4.5 of brown hexagons were isolated, yield of about 35%.

A sample of 2.5 g (0.215 mmol) of Na₂₈[(B-α- A8O₃W₉O₃₀)₄(WO₂)₄]₂-nH₂O was dissolved in 200mL of water before 0.43 g (0.86 mmol) U0₂(CH₃COO)₂-2H₂0 were added, pH 5.4±0.2. Next, the solution was heated at 90 °C for two hours. To the still hot solution, pH 3.8 ± 0.2 at room temperature, 11.0 g NH₄C1 were added, a small amount of a yellow precipitate formed during cooling and was filtered off. The final product crystallized upon evaporation of water after several days. Yield: 1.5g, ( about 60%).

Nan[rW_;0₁₀) Ce]-nH₂0 (IVa)

A sample of 33.0 g Na₂W0₄-2H₂0 was dissolved in 60 mL water at 60 °C. The pH was adjusted to 7.3 with 6.2 mL glacial acetic acid before 4.34 g Ce(N0₃)₃-6H₂0 dissolved in 15 mL water (pH 1.7) were added over the course of 5 min. The resulting orange solution was heated for 30 min. at 90 °C, pH 5.9, 40.0 g NaCl were added at 70 °C. After 2 days 21.0 g of orange a crystalline solid were isolated, pH 5.7 which was recrystallized from 8 mL 60°C warm water, pH 7.5. Yield: 9.0 g, (about 30%).

Na₈[(W₅0₁₈)₂Th]-nH₂0 (Va)

A sample of 33.0 g Na₂W0₄-2H₂0 was dissolved in 50 mL water at 80°C. The pH was adjusted to 7.3 with 6.2 mL glacial acetic acid before 5.5 g Th(N0₃)₄ dissolved in 10 mL water (pH 1.7) were added over the course of 2 min. The resulting white suspension was heated for 30 min. at 90°C, pH 5.8. The white precipitate was removed (1.5 g) and 25.0 g NaCl were added at 70°C. After 2 days 30.0 g of a white crystalline solid was isolated, pH 5.7 which was recrystallized from 15 mL 60 °C warm water, pH 6.1. Yield: 24.0g, (about 70%).

Na₁₀[(H₂O)ι cM'^IIrM^II1 OHyB-κ-Asθ₃Wo0_3?^rW0 ^]₂-nH₂0 M=La. Ce. Nd Sm. Eu and Gd A sample of 11.62g (lmmol) of Na₂₈[(B-α-AsO₃W₉O₃₀)₄]₂-nH₂O was dissolved in 75 ml 2M NaCl at 95°, pH ca. 5.0. In a separate beaker containing 15mL 2M NaOAc 3mmol of the appropriate lanthanide nitrate were brought into solution, pH 7.0-7.3, and then poured to the rapidly stirred solution of the tungstate. The resulting clear solution (pH 5.2 - 5.5) was heated (80 °C) and stirred for 10 min before 20g NaCl (0.34 mmol) were added. The precipitate was isolated (pH 5.2) and firstly from 50mL IM NaCl (70 °C) and then from 4mL (pH 5.8) water at 65 °C recrystallized. Two days later 3.2g (30%) of large parallelogram shaped crystals were collected at pH 6.3.

The sodium- free salts of I, II, IV and V were obtained by means of strong acid ion exchange resin DOWEX 50® in its NH₄ ⁺ form. The crystalline salts la, Ila, IVa and Va were dissolved in water (millipore®) at room temperature with the following amounts and pH values, la: 4.4g (0. lmmol) in 50mL H₂0, pH 6.7; Ila: 7.4g (0.6mmol) in 50mL H₂0, pH 7.0; IVa: 14.6g (4.5mmol) in 50mL H₂0, pH 7.7; Va: 3.5g (l.Ommol) in lOmL H₂0, pH 7.4 before passing onto the resin. The eluates were freeze-dried and the resulting yellow powders of ((NH₄)₇₆[As^πι ₁₂Ce^I"₁₆(H₂0)₃₆W₁₄₈0₅₂₄]-nH₂0 (lb), (NH₄)₂₅[As^m ₅Ce^ιπ ₄(H₂0)₄W₃₉0₁₄₃]-nH₂0 (II), (NH₄)₉[(W₅0₁₈)₂Ce]-nH₂0 (IVb), and (NH₄)₈[(W₅0₁₈)₂Th]-nH₂0 (Vb) kept overnight in a desiccator filled with P4O10. Sodium analysis revealed the following values: lb < 5ppm, lib 76ppm and IVM61 ppm.

EXAMPLE 3

Six compounds, with sample amounts of up to 3g were thermally treated according to the invention:

(NH₄)₇₀Na₆[As^πι ₁₂Ce^m ₁₆(H₂O)₃₆W₁₄₈O₅₂₄]-175H₂O (Ia), (NH₄)₇₆[As^m ₁₂Ce^m ₁₆(H₂0)₃₆W₁₄₈0₅₂₄]-nH₂0 (Ib),

(NΗ₄)₂₅[As^ι ₅Ce^ιπ ₄(H₂0)₄W₃₉0₁₄₃]-nH₂0 (IIb),

(NH₄)₁₅[Asⁿⁱ ₃(UO₂)₃(H₂O)₆W₃₀O₁₀₅]-25H₂O (IIIa),

(NH₄)₉[W₅0₁₈)₂Ce]-nH₂0 (IVb) and (NH₄)₈[W₅0₁₈)₂Th]-nH₂0 (Vb).

The structures of these compounds are shown in Figure 10. Thermal decomposition of these polyoxoanions in hydrogen up to 500° C and in argon or nitrogen up to 950 °C was studied by means of thermal analysis (TG, DSC), X-ray diffraction methods, chemical analysis and susceptibility measurements. X-ray analyses of the resulting deeply colored, arsenic-free powders, revealed the formation of cubic tungsten bronzes of cerium(III) Ce_xW0₃, x=0.066-0.162 with lattice parameters ranging from a=3.820(3) to 3.836(3)A, of thorium(IV) Th_xW0₃, x~0.1 with a=3.828(l)A, and of uranium(IV) U_xW0₃, x~0.1 with a=3.797(l)A as the main product respectively. These cubic bronzes were obtained in a one-step solid-state process at significantly lower temperatures than reported previously by using classical methods of synthesis.

This treatment led to intensely colored powders. X-ray analyses of these samples revealed the formation of a cubic tungsten bronze M_xW0₃, x=0.066-0.162 as the main product for all experiments. Table 1 lists the experiments and summarizes the results.

As an example, the powder diffractograms of the degradation products of la, lb and Ilia are given in Figures 11 and 12. The Figures reveal the characteristic reflection pattern of a cubic tungsten bronze. See for example the following documents, the contents of which are hereby incorporated by reference in their entirety: W. Ostertag, Inorg. Chem., 5, 758 (1966) and International Centre for Diffraction Data, Newtown Square PA, Powder Diffraction File (PDF) # 75-0241, Na0.39WO₃.

Detailed -high temperature X-ray analysis of the degradation process of la and lb in inert (Ar, N₂) as well as in reducing (H₂) atmospheres revealed the formation of the hexagonal tungsten bronze around 500 °C. Further heating up to 900 °C in argon led to the formation of the cubic cerium bronze Ce₀₀₈₇WO₃, starting at 600°C and of the tetragonal phase NaCe(W0₄)₂. For similar structures see, for example, the following documents, the contents of which are hereby incorporated by reference in their entirety: A. M. Golub and V. I. Maksin, Russ. JInorg. Chem., 22, 61 (1977) and PDF # 31-1244,

AgPr(W0₄)₂, isostmctural to NaCe(W0₄)₂. It should be noted that the thermal degradation of the analogous lanthanide-containing compound of type la in Ar up to 900 °C resulted in the formation of the bronze La_xW0₃ x~0.1.

It is believed that the formation of a scheelite-like phase during the treatment of la is due to the presence of sodium in the starting material. Upon preparation of the sodium- free sample lb, obtained from la via ion exchange and freeze drying, the thermal degradation in H₂ (RT→ 500 °C) and Ar (500 - 900 °C) led to the formation of Ce_{0 162}W0₃ and W0₂ (20). However, the decomposition of lb carried out exclusively in argon (RT→ 900 °C) quantitatively formed Ce₀₀₈₂WO₃. Reduction of lb in H₂ up to 500° C increased the cerium content in Ce_xW0₃, approaching the reported limit of x=0.186 as reported by J. J. Tully, G. J. Vogt, and R. J. Reuland, in Proc. Iowa Acad. Sci., 11, 354 (1970). The formation of W0₂ during the bronze preparation M_xW0₃ had been reported earlier and was observed for values of x > 0.19, M=La and x>0.15, M=Y, see B. Broyde, Inorg. Chem., 6,1588 (1967). The degradation of lib in Ar gave mainly the cubic bronze Ce₀₀₆₆WO₃. Two very minor phases were also observed, of which one was characterized as Ce₄W₉0₃₃, (see PDF # 25-0192, Ce₄W₉0₃₃).

The study of the uranyl-containing sample Ilia in N₂ led to the formation of the cubic uranium bronze U₀ ,W0₃, and accompanied by a minor phase <5% of U₅W₁₃O₅₀ at a remarkably low temperature of 725 °C as illustrated in Figure 13.

The degradation of the cerium-containing decatungstate IVb in Ar (from room temperature to 900 °C) resulted in the formation of Ce_{0 H8}W0₃ and W0₂, similar to the decomposition of lb in H₂ and Ar. The "molar" ratio between the phases was estimated as 85 to 15, based on comparative study of the intensities of the recorded diffractogram and the measured increase in weight upon oxidation of the powder in air. It is believed that this ratio is related to the structural arrangement in [(W₅0_]8)₂Ce]^9" (as seen in Figure 10). In this polyoxoanion, cerium obtains a nearest coordination to eight tungsten atoms, as it also occurs in the bronze, see Figures 10(d) and 10(e).

However, it must be noted that the decomposition of IVb in the DSC and TG cell in N₂ did not show the formation of W0₂. This effect might be the result of the smaller sample sizes (~20mg versus ~2g in the furnace experiments) resulting in a lower height of packing causing a shorter contact time of the reducing agent formed in situ in the powder.

The bronze phase Ce_xW0₃ of ca. 90% and a second component of ca. 10%, described as a high temperature phase (NH₄)₀₄₂WO₃, were found at 725 °C. On the other hand heating up to 950°C quantitatively formed the bronze Ce₀ )W0₃. Similar results were obtained for the thorium-analogous decatungstate Vb. The phases Th₀ ]W0₃ and (NH₄)₀₄₂WO₃ were found to be present at 725° C but only the bronze phase Th₀ ,W0₃ was detected at 950 °C. Figure 14 shows the variation of cubic cell parameter a₀ (A) as a function of the variable x in cubic bronzes Ce_xW0₃. The Figure reveals a fairly good linear relation between these numbers (r = 0.988).

Formation of the cubic bronze phase under inert conditions, starting at a temperature as low as 600 °C is noteworthy. The reduction of tungsten observed is interpreted as a result of the decomposition of ammonia. This decomposition is also believed to be responsible for the loss of arsenic, most likely in form of AsH₃. Thermal treatment in the temperature range up to 350°C causes the loss of H₂0 and NH₃. Characteristic weight losses of 2-4% at temperatures higher than 500 °C have been found for all studied compounds, see Figure 13 for example.

Susceptibility measurements were performed for the bronzes M_xW0₃ with M = Ce^m, U^IV and x -0.1 over the range room temperature to 4K. The Ce^m and U^IV samples have the magnetic moment expected for the isolated metal ions. The effective magnetic moment steadily decreases on decreasing temperature for both compounds confirming the results reported by F. Thomas and M. J. Sienko, in J Chem. Phys., 61, 3920 (1974).

TABLE 1

Experimental Conditions and Results for the Thermal Degradation of Compounds la

The results presented in Table 1 demonstrate that lanthanide- and actinide-containing bronzes can be prepared according to the present invention in a single-step solid-state process at temperatures significantly lower than those required by conventional synthesis processes. The results also illustrate that employing pure precursor polyoxometalate salts which contain the desired components of the bronzes dispersed at a molecular level and in the desired stoichiometry, can lead to materials of greater uniformity than is possible by heating of heterogeneous mixtures of components. While the invention has been described in terms of preferred embodiments, the skilled artisan will appreciate that various modifications, substitutions, omissions and changes may be made without departing from the spirit thereof. For example, While the foregoing is directed to tungsten based bronzes, the methods and compositions described bellow apply to the formation of bronzes based on other metal elements, such as the elements of groups IV to VII of the periodic table of the elements. It is believed that substitution of the tungsten with one or a combination of those elements can be achieved by one skilled in the art without undue experimentation. Such substitutions are therefore considered within the scope of the claims which follow.

Accordingly, it is intended that the scope of the present invention be limited solely by the scope of the following claims, including equivalents thereof.

Claims

WHAT IS CLAIMED IS:

1. A process for preparing a lanthanide or actinide-containing tungsten bronze comprising:

(a) heating a solid composition comprising a lanthanide or actinide-containing polyoxotungstate in the presence of a reducing agent until the temperature of said solid composition reaches a first predetermined temperature to form a reduced composition;

(b) heating said reduced composition in the presence of a inertgas until the temperature of said reduced composition reaches a second predetermined temperature; and

(c) maintaining said reduced composition at said second temperature for a predetermined period of time.

2. The process of claim 1, wherein said first temperature is about 500°C or less.

3. The process of claim 1, wherein said second temperature is about 1000°C or less.

4. The process of claim 1, wherein step (a) comprises increasing the temperature of said solid composition in steps separated by a temperature increment.

5. The process of claim 4, wherein said temperature increment is about 5 °C.

6. The process of claim 1, wherein step (b) comprises increasing the temperature of said reduced composition in steps separated by a temperature increment.

7. The process of claim 6, wherein said temperature increment is about 5 °C.

8. The process of claim 4, wherein step (a) comprises increasing the temperature of said solid composition from room temperature to said first predetermined temperature.

9. The process of claim 6, wherein step (b) comprises increasing the temperature of said reduced composition from room temperature to said first predetermined temperature.

10. The process of claim 1, wherein said predetermined period in step (c) is between about 3 and 6 hours.

11. The process of claim 1 , further comprising (d) cooling said reduced composition after step (c).

12. The process of claim 11, wherein the cooling in step (d) is conducted overnight.

13. The process of claim 1, wherein the reducing agent is hydrogen, ammonia or a mixture thereof.

14. The process of claim 1, wherein the polyoxoanion forms a salt with a cation.

15. The process of claim 14, wherein said cation is selected from monovalent metals, divalent metals, ammonium ion and mixtures thereof.

16. The process of claim 1, wherein the polyoxoanion is in the form of a sodium- free salt.

17. The process of claim 1, wherein said polyoxoanion is in the form of an ammonium containing salt and wherein said reducing agent comprises ammonia obtained by reducing the ammonium ion in the salt.

18. The process of claim 1, wherein the polyoxoanion comprises a lanthanide substituted polyoxometalate selected from the group consisting of

{M^m ₁₆(H₂O)₃₆As^m ₁₂W₁₄₈O₅₂₄}-⁷⁶, {M^III(H₂O)₁₁(M"¹ ₂OH)As^III ₄W₄₀O₁₄₀} {M' H.O^As¹¹¹^,^,^}-²⁵ and {(UO₂)₃(H₂O)₆As^m ₃W₃₀O₁₀₅}-¹⁵.

19. A tungsten bronze formed by the process of claim 1.

20. A cubic tungsten bronze formed by the process of claim 1.

21. A tungsten bronze of the formula M_xW0₃ formed by the process of claim 1.

22. A tungsten bronze of the formula M_xW0₃ formed by the process of claim 1, wherein x is controlled by adjusting the molar ratio M/W in the polyoxoanion.

23. A tungsten bronze of the formula M_xW0₃ formed by the process of claim 1, wherein M is a trivalent lanthanide or a tetravalent actinide and x is between about 0.06 and 0.17.

24. A tungsten bronze of formula M_x0₃, wherein M is selected from the group consisting of Sr, Cs, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Ac, Th, Pa, U, Np, Pu, Am and mixtures thereof.

25. A tungsten bronze formed by the process of claim 1 , wherein the polyoxoanion comprises a lanthanide substituted polyoxometalate selected from the group consisting of {M^III ₁₆(H₂0)₃₆As^II1 ₁₂W₁₄₈0₅₂₄}^"76, {M^m(H₂O)₁₁(M^m ₂OH)As"¹ ₄W₄₀O₁₄₀}-⁴⁰, {M^III ₄(H₂O)₄As^m ₅W₃₉O₁₄₃}-²⁵ and {(UO₂)₃(H₂O)₆As^ιπ ₃W₃₀O₁₀₅}^"15, wherein M^πι represents a trivalent lanthanide.

26. A method of preparing a plyoxoanion containing a lanthanide comprising:

(a) mixing, in an aqueous solution, a metal oxide, a tungstate, and a compound containing said lanthanide to form a reaction mixture containing an alkaline metal with an alkaline metal concentration of at least IM;

(b) adding a first acid solution to the mixture;

(c) heating said reaction mixture for a predetermined period of time to form a lanthanide-containing polyoxotungstate.

27. The method of claim 26, further comprising precipitating said polytungstate by forming a polytungstate salt.

28. The method of claim 27, further comprising:

(i) heating said polytungstate salt in the presence of a reducing agent until the temperature of said salt reaches a first predetermined temperature to form a reduced composition;

(b) heating said reduced composition in the presence of a inert gas until the temperature of said reduced composition reaches a second predetermined temperature; and (c) maintaining said reduced composition at said second temperature for a predetermined period of time.

29. A method for long term storage of radionuclides comprising :

(a) heating a reactor containing a solid composition comprising a tungsten oxopolyanion containing said radionuclide and a reducing agent until the temperature inside said reactor reaches a first predetermined temperature;

(b) introducing a inert gas into said reactor and heating said reactor until the temperature inside said reactor reaches a second predetermined temperature; and (c) maintaining said reactor at said second temperature for a predetermined period to form said radionuclide-containing tungsten bronze.

30. The method of claim 29, wherein said radionuclide is selected from the group consisting of Sr, Cs, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Ac, Th, Pa, U, Np, Pu, Am and mixtures thereof.

31. A lanthanide substituted polyoxometalate useful in the preparation of tungsten bronzes selected from the group consisting of

{M^m ₁₆(H₂O)₃₆As^III ₁₂W₁₄₈O₅₂₄}-⁷⁶, {M^m(H₂O)₁₁(M^III ₂OH)As^III ₄W₄₀O₁₄₀}-⁴⁰, {M^ιπ ₄(H₂0)₄As^m ₅W₃₉0₁₄₃}-²⁵ and {(UO₂)₃(H₂O)₆As ₃W₃₀O₁₀₅}-¹⁵, wherein M^πι represents a trivalent lanthanide.