A family of phosphates of langbeinite structure. Crystal-chemical aspect of radioactive waste immobilization

A systematic analysis of orthophosphates of framework (tetrahedral-octahedral) structure, belonging to langbeinite structural type (cubic cell, space group P2₁3), is performed. The langbeinite structure is considered as favorable for environmentally safe crystalline forms of radioactive waste solidification. Experimental data on the compositions of known phosphates of such structure, on isomorphism of cations in various crystallographic positions, and on specific features of distribution of various cations in them are summarized. Possible formula compositions of isostructural phosphates are calculated. Some of them have already been synthesized (data given in this paper), whereas other phosphates are yet unknown. The structural-chemical aspect was also applied to langbeinite-like phosphates of f elements, some of which were prepared in accordance with the “crystal-chemical prediction.” Phosphates of the rhombohedral [NaZr₂(PO₄)₃ type, NZP] and cubic (langbeinite type) tetrahedral-octahedral framework modifications, their cationic compositions, and structural features are compared. Properties of phosphates of langbeinite structure, useful in the development of new materials for radiochemical applications, and examples of possible use of crystal-chemical principles in solution of these problems are discussed.     

Anhydrous Lanthanide and Actinide(III) and (IV) Orthophosphates Me<Subscript>m</Subscript>(PO<Subscript>4</Subscript>)<Subscript>n</Subscript>. Synthesis,Crystallization, Structure,and Properties

Anhydrous orthophosphates Me_m(PO₄)_n, where Me is an element in the oxidation state +1 (A), +2 (B), +3 (R), +4 (M), or +5 (C), containing f elements(III) and (IV), are systematized. Their crystallographic characteristics are described, and the common and specific features of structure formation are discussed. The main structural (mineral) types realized in lanthanide and actinide phosphates are revealed: eulytite, zircon, monazite, NaZr₂(PO₄)₃ (NZP), NaTh₂(PO₄)₃ (NThP), rhabdophane, Sb_0.5Bi_1.5(PO₄)₃ (SbBi), whitlockite, arcanite, glaserite, langbeinite, scheelite, Sc₂(WO₄)₃ (ScW). The crystal-chemical factors determining the stability of structures, polymorphism of cations, morphotropy in the series, and possibility of formation of solid solutions on the basis of iso- and heterovalent isomorphism and isodimorphism are analyzed. The possibility of predicting new compounds of f elements of the expected structure is demonstrated.__________Translated from Radiokhimiya, Vol. 47, No. 1, 2005, pp. 15–30.Original Russian Text Copyright © 2005 by Orlova, Kitaev.     

Crystal-Chemical Approach to Predicting the Thermal Expansion of Compounds in the NZP Family

The general structural aspects of phosphates with {[L₂(PO₄)₃] ^p–}₃ frameworks (L = octahedral cation) are considered, and the possible isomorphous substitutions in NaZr₂(PO₄)₃ (NZP) phosphates are analyzed. The available data on the thermal expansion of NZP materials in the range 293–1273 K, together with crystal-chemical data on their structure, are used to identify the processes underlying the thermal expansion of these materials. The results provide basic guidelines in designing NZP-based materials with controlled (ultralow) thermal expansion and near-zero expansion anisotropy.     

Cesium and Its Analogs,Rubidium and Potassium,in Rhombohedral [NaZr<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript> Type] and Cubic (Langbeinite Type) Phosphates: 1. Crystal-Chemical Studies

The published crystallographic data on cesium, rubidium, and potassium phosphates crystallizing in the NaZr₂(PO₄)₃ (NZP) and langbeinite structural types are summarized and correlated. The existence of new phosphates, analogs of langbeinite mineral, is predicted. The phosphates of the suggested compositions are prepared and studied by X-ray and neutron diffraction and by IR spectroscopy. Phosphates of the formulas A₂RM(PO₄)₃, A₂B_0.5Zr_1.5(PO₄)₃, and ABR₂(PO₄)₃ have a cubic cell, space group P2₁3. The unit cell parameters of the phosphates in these series vary only slightly with variation of the cationic composition. Variations in the bond lengths and bond angles in the langbeinite structure depending on the cation are estimated from the results of structural studies. Cesium can be incorporated in cubic framework phosphates in an amount of up to 38 wt %. The langbeinite structure is characterized by wide possibilities of isomorphous substitutions involving large alkali and alkaline-earth metal cations arranged in the framework voids and small cations of p, d, and f elements in oxidation states 2+, 3+, and 4+, arranged in the framework positions. A specific role of lanthanides in formation of the langbeinite-type framework is noted.__________Translated from Radiokhimiya, Vol. 47, No. 3, 2005, pp. 203–212.Original Russian Text Copyright © 2005 by A. Orlova, V. Orlova, Buchirin, Beskrovnyi, Kurazhkovskaya.     

Crystallographic evaluation of sodium zirconium phosphate as a host structure for immobilization of cesium

Sodium zirconium phosphate (NZP) is a potential material for immobilization of nuclear effluents. The existence of cesium containing NZP structure was determined on the basis of crystal data of solid solution. It was found that up to ~9.0 wt% of cesium could be loaded into NZP formulations without significant changes of the three-dimensional framework structure. The crystal chemistry of Na_1−x Cs _x Zr₂P₃O₁₂ (x = 0.1–0.4) has been investigated using General Structure Analysis System programming. The CsNZP phases crystallize in the space group R-3c and Z = 6. Powder diffraction data have been subjected to Rietveld refinement to arrive at a satisfactory structural convergence of R-factors. The unit cell volume and polyhedral (ZrO₆ and PO₄) distortion increase with rise in the mole% of Cs⁺ in the NZP matrix. The PO₄ stretching and bending vibrations in the infrared region have been assigned. SEM, TEM, and EDAX analysis provide analytical evidence of cesium in the matrix.     

Lanthanides in phosphates with the structure of whitlockite mineral [analog of β-Ca3(PO4)2]

Data on phosphates with the structure of biogenic whitlockite mineral [analog of low-temperature modification of β-Ca₃(PO₄)₂] were analyzed. The possibility of isomorphic substitution of cations in phosphates with whitlockite structure was evaluated and formation of solid solutions of various compositions involving elements in oxidations states 1+, 2+, 3+, and 4+, including lanthanides and other elements and isotopes occurring in wastes from radiochemical plants, was studied. Phosphates of Ca, Mg, and lanthanides (Sm, Eu, Gd) were prepared by the sol-gel procedure, and their thermal, thermomechanical (thermal expansion), and hydrolytic properties were determined. The results of hydrothermal and thermal experiments were compared with published data for calcium phosphate and other matrices for immobilization of radioactive wastes (RAW).     

The crystal-chemical principle in designing mineral-like phosphate ceramics for immobilization of radioactive waste

The crystal-chemical principle of designing multicomponent compounds of the desired structure was applied to the development of solid matrices for immobilization of radioactive waste components. The approach to designing mineral-like crystalline materials was based on the structural features and isomorphism concept. The structural types of monazite, kosnarite (NZP), and langbeinite were considered as matrices for the incorporation of simulated wastes containing f elements and certain uni-, bi-, and trivalent elements involved in radiochemical processes. Phosphates of complex cationic compositions were prepared and studied by X-ray phase analysis and scanning electron microscopy. The main phases of the expected structures (monazite, kosnarite, langbeinite) and additional phases (Al and Fe phosphates, Zr pyrophosphate, Pd metal) were identified. The phase formation in the temperature range 80–1300°C was analyzed.     

Synthesis of castable sodium zirconium phosphate monoliths employing reactions between zirconyl nitrate hydrate and condensed phosphates

Reactions between zirconyl nitrate hydrate and condensed phosphates can be used to produce castable low CTE sodium zirconium phosphate (NZP) monoliths. Reaction between sodium nitrate, zirconyl nitrate hydrate and condensed phosphoric acid at room temperature (alkali nitrate method) produces monoliths having a heterogeneous microstructure, which are multiphasic in appearance. Except for the presence of crystalline sodium nitrate, they are X-ray amorphous. Differential thermal analysis revealed two distinct exothermic crystallization events when these materials are heated. The first event, with an onset temperature of 650°C, is the result of NZP and ZrO₂crystallization. The second is the result of ZrP₂O₇ crystallization. Reaction between zirconyl nitrate hydrate and condensed sodium phosphate (condensed alkali phosphate method) results in a more homogeneous microstructure in which crystalline zirconium hydrogen phosphate hydrate and sodium nitrate are present. Two exothermic peaks, with onset temperatures of approximately 570 and 860°C, are observed. The first exotherm is the result of NZP, ZrO₂ and ZrP₂O₇ crystallization; the second exotherm is the result of a further NZP formation. After heating materials made by these two methods at 940°C for 24 h, the condensed-alkali-phosphate-method-derived material converted to phase-pure NZP, while the alkali-nitrate-method-derived material contained ZrP₂O₇. The differences in phase evolution between the materials prepared by these two methods are attributable to the differences in chemical and microstructural homogeneity that result from the reactants used. This revised version was published online in September 2006 with corrections to the Cover Date.     

Molybdenum Fixation in Crystalline NZP Matrices

A possibility of fixation of molybdenum present in spent nuclear fuel in ceramic matrices with the structure of the NaZr₂(PO₄)₃ (NZP) type was studied. The crystallochemical features of molybdenum incorporation into various crystallographic NZP structures depending on the synthesis conditions were considered. New molybdate-phosphates of variable composition Na _1-x ZrMo_xO₁₂ (0 x 0.6), crystallizing in the NZP crystal type, were prepared and characterized by X-ray diffraction and IR spectroscopy. The synthesis conditions and the concentration and temperature fields of stability of molybdate-phosphates in the system Na₂O-ZrO₂-MoO₂-P₂O₅ were studied. The crystallographic parameters of single-phase samples were evaluated. The results obtained suggest that the basic factors of formation of chemically stable single-phase NZP ceramics incorporating MoO₄ ^{2 -} anions are the composition of wastes and oxidative synthesis conditions.     

On the possibility of realization of monazite and langbeinite structural types in complex americium and plutonium phosphates. Synthesis and X-ray diffraction studies

The existence of phosphates containing Am + Pu with monazite-type structure and of those containing Am + Zr with langbeinite-type structure was substantiated on the basis of the main principles of isomorphism. The compounds were synthesized and examined by X-ray diffraction. Samples of the composition CdAmPu(PO₄)₃ crystallized in the monazite structural type. In the sample containing Am + Zr, in which the formation of a langbeinite-type compound was expected by analogy with lanthanides and Zr [isoformula phosphates K₂LnZr(PO₄)₃, where Ln = Ce-Yb, are known], a mixture of monazite and kosnarite phases was obtained from the chosen reactants under the chosen conditions.     

Erbium zirconium phosphate Er<Subscript>0.33</Subscript>Zr<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript> of the NaZr<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript> family. Structure contraction on heating

Erbium zirconium phosphate Er_0.33Zr₂(PO₄)₃, a member of the family of structural analogs of NaZr₂(PO₄)₃ (NZP), was prepared by the sol-gel process and studied by X-ray phase analysis, IR spectroscopy, and differential scanning calorimetry. The behavior of erbium zirconium phosphate on heating in the temperature interval from 25 to 625°C was studied by high-temperature X-ray diffraction. Expansion and contraction along different crystallographic directions and contraction of the structure as a whole were found. The overall contraction is due to higher contribution of the negative axial thermal expansion coefficients α _a and α _b to α_av and hence to the volume expansion of the phosphate. On heating to 900°C, the NZP structure is preserved.     

Synthesis,crystallographic characterization and ionic conductivity of iron substituted sodium zirconium phosphate Na<Subscript>1.2</Subscript>Zr<Subscript>1.8</Subscript>Fe<Subscript>0.2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript>

Sodium zirconium phosphate NaZr₂P₃O₁₂ (hereafter NZP) crystallizes in rhombohedral (hexagonal) symmetry with the space group R-3c. The NZP-related phase of synthetic iron substituted NZP has been prepared by partial substitution on zirconium site by Fe(III). The material has been synthesized by sintering the finely powdered oxide mixture in a muffle furnace at 1,050 °C. The polycrystalline phase of Na_1.2Zr_1.8Fe_0.2(PO₄)₃ has been characterized by its typical powder diffraction pattern. The powder diffraction data of 3,000 points have been subjected to general structural analysis system (GSAS) software to arrive at a satisfactory structural fit with R _p = 0.0623 and R _wp = 0.0915. The following unit cell parameters have been calculated: a = b = 8.83498(18) ?, c = 22.7821(8) ? and α = β = 90.0° γ = 120.0°. The structure of NZP consists of ZrO₆ octahedra and PO₄ tetrahedra linked by the corners to form a three-dimensional network. Each phosphate group is on a two-fold rotation axis and is linked to four ZrO₆ octahedra. Each zirconium octahedron lies on a threefold rotation axis and is connected to six PO₄ tetrahedra. AC conductivity of the solid solution has been measured between 303 and 773 K. The material exhibits temperature-dependent enhancement of ionic conduction by ≈400 times at elevated temperatures. The activation energies show significant change in slope at 1,000/T = 2.23(448 K).     

Crystal chemistry of the NaZr2(PO4)3, NZP or CTP,structure family

The NaZr₂(PO₄)₃ type structure (abbreviated as NZP or CTP, CaTi₄(PO₄)₆), has emerged as a new family, which has extraordinary technological utility in three fields: fast-ion conductors, radwaste solidification and zero expansion ceramics. NZP or CTP is formed by an extraordinary range of discrete compositions and crystalline solutions. In this paper these compositions are classified according to their crystal chemical substitution scheme, and some uncommon trends in the systematic variation of their lattice parameters are shown. Some of the major trends are explained by correlation with the rotation of polyhedra in the structure.     

Double Phosphates of Ce(IV) and Some Mono- and Bivalent Elements

A series of double Ce(IV) orthophosphates NaCe₂(PO₄)₃ and B_0.5Ce₂(PO₄)₃ (B = Mg, Ca, Sr, Cd) was prepared by heat treatment of gels with the stoichiometric content of the components. These phosphates were characterized by powder X-ray diffraction, IR spectroscopy, and electron spin resonance. The com- pounds have the monazite structure. The unit cell parameters of the phosphates were determined. The crystal structure of the phosphates is stable at temperatures from 20 to 1600°C. The crystallographic parameters of the double Ce(IV) orthophosphates were compared with those of simple Ce(III) phosphate CePO₄ which is artificial analog of monazite. Crystal chemical features of formation of double phosphates of tetravalent f elements and mono- and bivalent elements were discussed.     

Encapsulation of heterovalent ions of two simulated high-level nuclear wastes and crystallization into single-phase NZP-based wasteforms

Preliminary data were obtained on the immobilization of various heterovalent ions present in the two selected high-level nuclear waste compositions. The entire set of ions present in the waste compositions were immobilized into sodium zirconium phosphate (NZP) structure. The waste loading was in the range 5–25%. The two types of wasteforms loaded with the simulated high-level waste compositions were characterized by powder X-ray diffraction, FT-IR, TGA/DTA, and scanning electron microscopy. The difference in the compositions of the two simulated wastes was reflected in the waste loading percentage and the crystallization of the wasteforms into NZP structure. Up to 12% waste loading, single phase isostructural with NZP was obtained as a major product in the case of the first waste composition. An increase in the waste loading led to the segregation of ZrP₂O₇ as a secondary phase. With the second waste composition, an NZP-like phase was obtained as the major product even at 25% waste loading.     

Phosphors based on NaZr2(PO4)3-type calcium and strontium phosphates activated with Eu2+ and Sm3+

Ca_0.5Zr₂(PO₄)₃:Eu²⁺, Sr_0.5Zr₂(PO₄)₃:Eu²⁺, and Ca_0.5Zr₂(PO₄)₃:Eu²⁺, Sm³⁺ orthophosphates prepared through precipitation using sol-gel processes are analogs of NaZr₂(PO₄)₃ (NZP) and crystallize in space group R $\bar 3$ . Their crystallographic parameters determined by X-ray diffraction are consistent with the interatomic distances extracted from EXAFS data. Their luminescence spectra obtained under excitation in the range 300?C400 nm contain emission bands between 425 and 525 nm. Substitution of the larger sized cations Eu²⁺ and Sm³⁺ for Ca²⁺ shifts the emission bands to shorter wavelengths and reduces their width because of the decrease in the effect of the crystal field. Analysis of the spectra indicates that Eu²⁺ occupies two types of crystallographic sites (independent interstitial sites of different sizes and shapes in the NZP framework structure). Codoping with Eu and Sm has ensured luminescence with chromaticity coordinates approaching those of white light: (x = 0.27, y = 0.34).     

Synthesis and X-ray Diffraction Study of Mixed-Metal Orthophosphates with NaZr2(PO4)3 and CePO4 Structures

Mixed-metal orthophosphates with Ca^{2 +}, Cd^{2 +}, Gd^{3 +}, Ti^{4 +}, Hf^{4 +}, Ce^{4 +}, U^{4 +}, and Pu^{4 +} were synthesized and studied by X-ray diffraction. Crystalline compounds containing cerium or plutonium, or both these elements were prepared. Cerium was taken as an imitator for plutonium. The phosphates containing Ti^{4 +} and Hf^{4 +} formed two-phase systems. One of the phases was assigned to the Zr₂(PO₄)₃ structural type (NZP), and the other phase, to the CePO₄ type (monazite). In the other cases, single-phase products were formed. The effect of the cationic composition on the crystal structure of the complex orthophosphates, including the unit cell parameters of the crystalline phases, was studied.     

NZP: A new family of low-thermal expansion materials

A new structural family of low-expansion materials known as NZP has been recently discovered and has generated great interest for wide-ranging applications such as fast ionic conductors, devices requiring good thermal shock resistance, hosts for nuclear wastes, catalyst supports in automobiles, etc. This family is derived from the prototype composition NaZr₂P₂O₁₂ in which various ionic substitutions can be made leading to numerous new compositions. The bulk thermal expansion of these materials varies from low negative to low positive values and can be controlled and tailored to suit the needs for specific applications. In general, most of the NZP members demonstrate an anisotropy in their lattice thermal expansions, which is the main cause of the low-thermal expansion behavior of these materials. In CaZr₄P₆O₂₄ and SrZr₄P₆O₂₄ an opposite anisotropy has been observed which has led to the development of near-zero expansion crystalline solution composition. On the basis of the coupled rotations of the polyhedral network formed by ZrO₆ octahedra and PO₄ tetrahedra, a crystal structure model to interpret and explain the thermal expansion behavior has been discussed.Paper presented at the Tenth International Thermal Expansion Symposium, June 6–7, 1989, Boulder, Colorado, U.S.A.     

Immobilization of lanthanum,cerium, and selenium into ceramic matrix of sodium zirconium phosphate

The crystallographic nature of NaCe_0.2Zr_1.8P₃O₁₂, NaSe_0.2Zr_1.8P₃O₁₂, and NaLa_0.13Ce_0.14Se_0.15·Zr_1.58P₃O₁₂ phases has been investigated with the aim of developing methods for radionuclide immobilization into sodium zirconium phosphate (NZP) phase. The phases have the NZP structure, space group \(R\bar 3c\) , Z = 6. Powder diffraction data have been subjected to Rietveld refinement, and satisfactory structural convergence of R-factors was achieved. The PO₄ stretching and bending vibration bands in the IR region have been assigned.     

Study of barium feldspar polymorphism as a function of temperature and calcium content

A family of new glass-ceramic materials, of the general formula (25-x)CaO·xBaO·yMgO· zAl₂O₃·50SiO₂, where x = 1, 2, 5, 10, 15, 20, 25, y = 20 or 14 and z = 5 or 11 (mol%), has been prepared by melting raw materials in two parent glasses and performing heat treatments. The systematic substitution of BaO for CaO in the base glasses allows the effect of feldspars' isomorphism and polymorphism to be studied in a series of glass-ceramics where the structural environment around the bivalent cations, Ca²⁺ and Ba²⁺, is systematically altered. Ba²⁺ has a large effect on the glass transition temperature and dilatometric softening point, causing a decrease with increasing BaO. The crystalline phases have been identified and found to be dependent on the preparation conditions, which are the BaO and Al₂O₃ contents, the heating rate and the soaking temperature adopted for the crystallization treatments. The infrared spectroscopy technique helped to identify the different polymorphs of barium feldspar, that were not clearly distinguishable by X-ray powder diffractometry due to preferred orientations. The kinetic parameters for the formation of the different crystals have also been determined and correlated with their thermal stability resumed in the well-known time-temperature-transformation curves.