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.     

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.     

Isomorphism in Crystalline Phosphates of the NaZr2(PO4)3 Structural Type and Radiochemical Problems

The problems of isomorphism in phosphates that are structural analogs of sodium zirconiumorthophosphate NaZr₂(PO₄)₃ (NZP) are discussed. Data on the crystal chemistry of orthophosphates of theframework structure are systematized according to the suggested classification of their formula types. Thetheoretically possible, actually occurring isomorphism of cations in phosphates, in particular, isomorphisminvolving f-element cations, is discussed. The isomorphous capacity of the NZP structure toward variouscations was evaluated. Data on isomorphism in phosphates are necessary for choosing a suitable single-phase ceramic matrix for incorporation of radionuclides and adjusting its composition to actual nuclear waste.     

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).     

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.     

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.     

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.     

Fabrication of NaZr<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript>-type ceramic materials by spark plasma sintering

Ceramic materials based on Ca_0.5Zr₂(PO₄)₃ and NaFeNb(PO₄)₃, structural analogs of NaZr₂(PO₄)₃ (NZP), were prepared by spark plasma sintering. At sintering temperatures of 1100–1200 and 880°C and sintering times of 12 and 3 min, the relative densities reached were 99.1 and 99.9%, respectively. According to X-ray diffraction data, the sintering process caused no changes in phase composition. The ceramics had a dense, homogeneous microstructure and ranged in grain size from 0.5 to 2.5 μm.     

Thermal expansion studies on the Sodium Zirconium Phosphate family of compounds A1/2M2 (PO4)3: effect of interstitial and framework cations

Thermal expansion of the sodium zirconium phosphate (NZP) family of compounds A_1/2M₂(PO₄)₃ (A = Ca or Sr; M = Ti, Zr, Hf or Sn) has been measured in the temperature range 298–1273 K by high-temperature X-ray powder diffractometry. Some of the compounds in the series (calcium zirconium phosphate and calcium hafnium phosphate) display the typical thermal expansion behaviour of NZP compounds, namely expansion along the hexagonal c axis and contraction along the a axis. The other compounds, depending on their interstitial and framework composition, behave differently. The observed axial thermal expansion and contraction behaviour is explained on the basis of the crystal chemistry of the compounds. Low-expansion compounds in this series are identified and their expansion anisotropy examined. Infared spectra of the compounds are reported. Differential scanning calorimetry measurements on the tin compounds indicate the occurrence of a diffuse phase transformation at high temperatures.     

Comparison for the crystal structure,synthesis and microwave dielectric properties of alkaline earth orthophosphates

A₃(PO₄)₂ compounds have different crystal structures when A sites are occupied by Ca or heavy alkaline earth metal atoms (Sr or Ba). The compounds with isomorphous crystal structure were synthesized by solid-state reaction method when the A-site atoms were Sr or Ba, and their crystal structure and microstructure of the sintered ceramics were investigated by X-ray powder diffraction (XRD) and scanning electron microscope (SEM), respectively. The microwave dielectric properties were measured using a network analyzer. It was found that Ba₃(PO₄)₂ could be sintered at 1060 °C, while the α-Sr₃(PO₄)₂ ceramics that has a smaller Sr²⁺ ionic radius, could be sintered at 1200 °C, and higher relative densities were obtained. The dielectric constant (?) of the α-Sr₃(PO₄)₂ is higher than that of Ba₃(PO₄)₂, but Ba₃(PO₄)₂ has a higher Q × f value than that of β-Ca₃(PO₄)₂ and α-Sr₃(PO₄)₂, which could be interpreted by the differences in ionic polarizability and bond strength. The temperature coefficients of resonant frequency (τ_f) for all samples with isomorphous crystal structure have positive values, ranging between +11 and +66 ppm °C⁻¹.     

Thermal expansion studies of substituted CTP derivatives

Several new Na, Y and Zr substituted derivatives of Ca_0·5Ti₂(PO₄)₃ (CTP) have been synthesized. These derivatives retain the hexagonal structure of the parent (CTP) compound with minor changes in lattice parameters. Linear thermal expansion coefficients(α) have been obtained using a high sensitivity dilatometer.     

Thermal expansion of NASICON materials

Our and others’ results on the thermal expansion of compounds and the crystal-chemical information about their structure have been used to properly select compositions of new NASICON materials with small thermal expansion and low thermal expansion anisotropy: M _x M_1?x ′Ti₂(PO₄)₃ (M and M′ are alkali metals, 0 ≤ x ≤ 1), M″FeTi(PO₄)₃ (M″ is an alkaline-earth metal), and Ca_{0.5(1 + x)}Fe _x Ti_{2 ? x} (PO₄)₃ (0 ≤ x ≤ 1). Such materials have been synthesized through salt coprecipitation from aqueous solutions and their thermal expansion has been assessed by high-temperature X-ray diffraction. We have determined their unit-cell parameters as functions of temperature and correlated their thermal expansion parameters and composition. The K_0.5Rb_0.5Ti₂(PO₄)₃ and Rb_0.8Cs_0.2Ti₂(PO₄)₃ materials obtained combine small volumetric thermal expansion and near-zero thermal expansion anisotropy.     

The solid solution and magnetic properties of (Mg,Cu)3 (PO4)2 phosphates

The powder synthesis of Mg_3−xCu_x(PO₄)₂ (0 ⩽ x ⩽ 3) between 1173 and 1273 K, shows a discontinuous solid solution. The X-ray diagrams of compositions 0 ⩽ x ⩽ 1.70 are similar to that of Mg₃(PO₄)₂. The compounds with 2.30 ⩽ x ⩽ 3 are isotypic with the stanskiite-related structure of Cu₃(PO₄)₂. Magnetic measurements (4.2–300K) achieved on powder samples with x = 0.50, 1.00, 1.50, 1.70 and 3 are reported. An antiferrodistorsive ordering induced by the cooperative Jahn — Teller effect of Cu²⁺ ions results in a one dimensional magnetic behaviour with the composition x = 3. The Bonner and Fisher's model for Heisenberg chains was used to determine the average value of the antiferromagnetic exchange.     

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).     

Bioceramics composition modulate resorption of human osteoclasts

Biomaterials used in bone regeneration are designed to be gradually resorbed by the osteoclast and replaced by new bone formed through osteoblastic activity. The aim of the present study is to analyze the role of osteoclasts in the resorption process. The attachment of human osteoclasts and the appearance of their resorption lacunae, when cultured on either the resorbable crystalline, calcium orthophosphate materials or on the long-term stable bioceramic material was investigated. The resorbable materials contain Ca₁₀[K,Na](PO₄)₇ (AW-Si) and Ca₂KNa(PO₄)₂ (GB14, GB9 & D9/25) as their main crystal phases, however they differ in their total solubility. These differences result from small variations in the composition. The long-term stable material consist of about 30% fluorapatite beside calcium zirconium phosphate (Ca₅(PO₄)3F + CaZr₄(PO₄)₆) and shows a very small solubility. AW-Si has an alkali containing crystalline phase, Ca₁₀[K,Na](PO₄). While GB14, GB9 and D9/25 contain the crystalline phase Ca₂KNa(PO₄)₂ with small additions of crystalline and amorphous diphosphates and/or magnesium potassium phosphate (GB14). D9/25 and AW-Si is less soluble compared to GB14, and GB9 among the resorbable materials. Resorbable and long-term stable materials vary in their chemical compositions, solubility, and surface morphology. Osteoclasts modified the surface in their attempts to resorb the materials irrespective of the differences in their physical and chemical properties. The depth and morphology of the resorption imprints were different depending on the type of material. These changes in the surface structure created by osteoclasts are likely to affect the way osteoblasts interact with the materials and how bone is subsequently formed.     

Cesium dizirconium phosphate: Synthesis and thermal properties

Crystalline CsZr₂(PO₄)₃ with the NZP [NaZr₂(PO₄)₃] structure was prepared by a sol gel procedure. The purity and composition of the sample were determined by scanning electron microscopy with energy-dispersive X-ray analysis as well as by X-ray phase analysis and IR spectroscopy. CsZr₂(PO₄)₃ is thermally stable in the range 7 K < T < 1553 K. Temperature dependences of the heat capacity C _p ⁰ = f(T) and thermal conductivity λ = f(T) of the phosphate in the range 320–650 K corresponding to thermal conditions of a nuclear waste repository were studied. The standard thermodynamic functions C _p ⁰ (T), H ⁰(T)–H ⁰(0), S ⁰(T), and G ⁰(T)–H ⁰(0) were calculated. The thermodynamic functions of formation of CsZr₂(PO₄)₃ were calculated. The possibility of decreasing the temperature of the synthesis-immobilization to 1000 K was experimentally confirmed.     

Structural features of AgCaCdMg2(PO4)3 and AgCd2Mg2(PO4)3, two new compounds with the alluaudite-type structure, and their catalytic activity in butan-2-ol conversion

AgCaCdMg₂(PO₄)₃ and AgCd₂Mg₂(PO₄)₃, two new compounds with the alluaudite-type structure, were synthesized by a solid state reaction in air at 750 °C. The X-ray powder diffraction pattern of AgCaCdMg₂(PO₄)₃ indicates the presence of small amounts of (Ca, Mg)₃(PO₄)₂ with the whitlockite structure, as impurity, whereas AgCd₂Mg₂(PO₄)₃ is constituted by pure alluaudite. The Rietveld refinements of the X-ray powder diffraction patterns indicate an ordered cationic distribution for AgCd₂Mg₂(PO₄)₃, with Ag on A(2)′, Cd on A(1) and M(1), and Mg on M(2), whereas a disordered distribution of Cd and Ca between the A(1) and M(1) sites is observed for AgCaCdMg₂(PO₄)₃. The catalytic properties of these compounds has been measured in reaction of butan-2-ol dehydrogenation. In the absence of oxygen, both samples exhibit poor dehydrogenation activity. All samples displayed no dehydration activity. Introduction of oxygen into the feed changed totally the catalytic behavior of the catalysts. The production of methyl ethyl ketone increases with time on stream and the reaction temperature. AgCaCdMg₂(PO₄)₃ is more efficient than AgCd₂Mg₂(PO₄)₃.     

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.     

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.     

Phase transitions of the NASICON-type mixed phosphates LiM<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript> (M = Ti,Zr) and LiInNb(PO<Subscript>4</Subscript>)<Subscript>3</Subscript>

The structural phase transitions of LiTi₂(PO₄)₃, LiInNb(PO₄)₃, and LiZr₂(PO₄)₃ have been studied by X-ray diffraction, impedance spectroscopy, ⁷Li NMR spectroscopy, and calorimetry. The results indicate that, as the temperature is raised, the lithium ions in the structure of LiTi₂(PO₄)₃ and LiInNb(PO₄)₃ redistribute between the M1 and M2 sites. The thermal expansion coefficients along the crystallographic axes of LiTi₂(PO₄)₃ and LiInNb(PO₄)₃ are estimated.