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.     

Thermal expansion behaviour of sodium zirconium phosphate structure type phosphates containing tin

Thermal expansion behaviour of sodium zirconium phosphate structure type phosphates of the formula AM³⁺SnP₃O₁₂ (A=Ca, Sr and Ba; M³⁺=Cr and Fe) was studied by high temperature X-ray diffraction and dilatometry in the temperature range 298-1073 K. The variation in the hexagonal lattice parameters of the Ca-containing compounds is in line with the ‘sodium zirconium phosphate behaviour’. However, the strontium- and barium-containing compounds display an altogether different behaviour of axial expansion. The results are explained based on the crystal chemistry of these compounds.     

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).     

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.     

Synthesis, microstructure and thermal expansion studies on Ca0·5+x/2Sr0·5+x/2Zr4P6−2x Si2x O24 system prepared by co-precipitation method

We report on the synthesis, microstructure and thermal expansion studies on Ca_0·5?+?x/2Sr_0·5?+?x/2Zr₄P_6???2x Si_2x O₂₄ (x = 0·00 to 1·00) system which belongs to NZP family of low thermal expansion ceramics. The ceramics synthesized by co-precipitation method at lower calcination and the sintering temperatures were in pure NZP phase up to x = 0·37. For x ≥ 0·5, in addition to NZP phase, ZrSiO₄ and Ca₂P₂O₇ form as secondary phases after sintering. The bulk thermal expansion behaviour of the members of this system was studied from 30 to 850 °C. The thermal expansion coefficient increases from a negative value to a positive value with the silicon substitution in place of phosphorous and a near zero thermal expansion was observed at x = 0·75. The amount of hysteresis between heating and cooling curves increases progressively from x = 0·00 to 0·37 and then decreases for x > 0·37. The results were analysed on the basis of formation of the silicon based glassy phase and increase in thermal expansion anisotropy with silicon substitution.     

Crystal chemistry of sodium zirconium phosphate based simulated ceramic waste forms of effluent cations (Ba(2+), Sn(4+), Fe(3+), Cr(3+), Ni(2+) and Si(4+)) from light water reactor fuel reprocessing plants

A novel concept of immobilization of light water reactor (LWR) fuel reprocessing waste effluent through interaction with sodium zirconium phosphate (NZP) has been established. Such conversion utilizes waste materials like zirconium and nickel alloys, stainless steel, spent solvent tri-butyl phosphate and concentrated solution of NaNO(3). The resultant multi component NZP material is a physically and chemically stable single phase crystalline product having good mechanical strength. The NZP matrix can also incorporate all types of fission product cations in a stable crystalline lattice structure; therefore, the resultant solid solutions deserve quantification of crystallographic data. In this communication, crystal chemistry of the two types of simulated waste forms (type I-Na(1.49)Zr(1.56)Sn(0.02)Fe(0).(28)Cr(0.07)Ni(0.07)P(3)O(12) and type II-Na(1.35)Ba(0.14)Zr(1.56)Sn(0.02)Fe(0).(28)Cr(0.07)Ni(0.07)P(2.86)Si(0.14)O(12)) has been investigated using General Structure Analysis System (GSAS) programming of the X-ray powder diffraction data. About 4001 data points of each have been subjected to Rietveld analysis to arrive at a satisfactory structural convergence of Rietveld parameters; R-pattern (R(p))=0.0821, R-weighted pattern (R(wp))=0.1266 for type I and R(p)=0.0686, R(wp)=0.0910 for type II. The structure of type I and type II waste forms consist of ZrO(6) octahedra and PO(4) 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 ZrO(6) octahedra while zirconium octahedra lies on a three-fold rotation axis and is connected to six PO(4) tetrahedra. Though the expansion along c-axis and shrinkage along a-axis with slight distortion of bond angles in the synthesized crystal indicate the flexibility of the structure, the waste forms are basically of NZP structure. Morphological examination by SEM reveals that the size of almost rectangular parallelepiped crystallites varies between 0.5 and 1.5 microm. The EDX analysis provides the analytical evidence of immobilization of effluent cations in the matrix. The particle size distributions of the material along selected reflecting planes have been calculated by Scherrer's formula.     

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.     

Structural model for thermal expansion in MZr2P3O12 (M=Li,Na, K,Rb, Cs)

A structural model is proposed to describe the highly anisotropic thermal expansion in the sodium zirconium phosphate NaZr₂P₃O₁₂ structure as a result of the thermal motion of the polyhedra in the structure. In the proposed model the rotations of the phosphate tetrahedra are coupled to the rotation of the zirconium octahedra. Of the two versions considered, the first one allows angular distortions to occur only in the ZrO₆ octahedra; the second one permits all polyhedra to be distorted.     

Thermal expansion behaviour of M′Ti2P3O12 (M′=Li,Na, K,Cs) and M″Ti4P6O24 (M″=Mg,Ca, Sr,Ba) compounds

The [NZP] family has been attracting considerable attention because of its potential in thermal shock-resistant applications. The compounds MTi₂P₃O₁₂ (M=Li, Na, K, Cs) and MTi₄P₆O₂₄ (M=Mg, Ca, Sr, Ba) belong to this new family of low-expansion materials. The results of a systematic study undertaken to investigate the thermal expansion behaviour of these materials are reported. A correlation between the ionic size and lattice expansion was also attempted, and compounds possessing lowest bulk thermal and low anisotropy were identified.     

Microstructure and microcracking behaviour of barium zirconium phosphate (BaZr4P6O24) ceramics

Barium zirconium phosphate (BaZr₄P₆O₂₄), a member of a new family of low-thermal-expansion materials known as NZP, was synthesized by the solution sol-gel method, and sintered ceramics were prepared at 1100–1600 °C. The effect of sintering parameters such as time and temperature on the microstructure and phase composition was studied. BaZr₄P₆O₂₄ is known to possess anisotropy in its axial thermal expansions, which usually causes microcracking in the sintered bodies when cooled. The microcracking activity of the sintered samples was examined by acoustic emission measurements.     

Thermal expansion behaviour of barium and strontium zirconium phosphates

Ba_1.5-xSr_xZr₄P₅SiO₂₄ compounds withx = 0, 0.25, 0.5, 0.75, 1.0, 1.25 and 1.5, belonging to the low thermal expansion NZP family were synthesized by the solid state reaction method. The XRD pattern could be completely indexed with respect to space group indicating the ordering of vacancy at the divalent cation octahedral sites. The microstructure and bulk thermal expansion coefficient from room temperature to 800°C of the sintered samples have been studied. All the samples show very low coefficient of thermal expansion (CTE), withx = 0 samples showing negative expansion. A small substitution of strontium in the pure barium compound changes the sign of CTE. Similarly,x = 1.5 sample (pure strontium) shows a positive CTE and a small substitution of barium changes its sign.X = 1.0 and 1.25 samples have almost constant CTE over the entire temperature range. The low thermal expansion of these samples can be attributed to the ordering of the ions in the crystal structure of these materials     

Diffusion of lanthanum into single-phase sodium zirconium phosphate matrix for nuclear waste immobilization

Sodium zirconium phosphate (NZP) is a potential material for immobilization of nuclear effluents. Up to ～2.17 mol % (10.71 wt %) lanthanum could be loaded into NZP formulations without significant changes in the three-dimensional framework structure. The crystal chemistry of Na_1+x Zr_2?x La _x P₃O₁₂ (x = 0.1?0.5) phases was investigated using General Structure Analysis System programming. The LaNZP phases crystallize in space group $R\bar 3c$ with Z = 6. Powder diffraction data were subjected to Rietveld refinement to arrive at a satisfactory structural convergence of R-factors. The PO₄ stretching and bending vibrations in the IR region were assigned. SEM and EDAX analysis provide evidence of La in the matrix.     

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.     

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.     

Strong anisotropic thermal expansion in oxides

The strong anisotropic thermal expansion behavior found for cordierite ((Mg₂Al₄Si₅O₁₅), β-eucryptite (LiAlSiO₄) and NZP (NaZr₂P₃O₁₂) is qualitatively rationalized using distance least squares (DLS) modeling. In this approach, the thermal expansion is driven by the ionic bonds of Mg²⁺, Li⁺ or Na⁺. Due to constraints imposed by shared polyhedra edges or faces, thermal expansion of the ionic bonds expands the lattice in only one or two dimensions. Due to the connectivity in these structures, this expansion in some directions causes contraction in the other directions. The thermal expansion of β-eucryptite was determined from powder neutron diffraction data over the temperature range 10–809 K. This revealed that the volume thermal expansion of β-eucryptite becomes substantially more negative below room temperature than it is above room temperature. The structure was refined by the Rietveld method from data collected at 12 different temperatures. DLS modeling studies suggest that Li–O bond expansion plus movement of Li from tetrahedral to octahedral sites can explain the thermal expansion behavior above room temperature. However, such an approach cannot explain the more pronounced low-temperature negative thermal expansion, which is most likely attributable to rocking motions of AlO₄ and SiO₄ tetrahedra.     

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.     

New [NZP] materials for protection coatings. Tailoring of thermal expansion

[NZP] (the NaZr₂P₃O₁₂-Family) materials can be selected for synthesizing new thermal shock resistant ceramic coatings with a thermal expansion that can be tailored to match that of the substrate, and to possess a low thermal expansion anisotropy. The tailoring technique will involve the selection of a suitable pair of compositions which, upon being mixed to form a crystalline solution, will possess the desired thermal expansion coefficient and will have negligible thermal expansion anisotropy. This can be done when the axial thermal expansion for one end member is larger in the a-direction than in the c-direction and vice versa for the other end member.     

Synthesis,sintering and thermal expansion of Ca1-x Sr x Zr4P6O24 — an ultra-low thermal expansion ceramic system

Ca_1-x Sr _x Zr₄P₆O₂₄ (O × 1.0) system which belongs to a new large family of low thermal expansion materials known as NZP or CTP, was synthesized by the solid state and the sol-gel methods. The conventional sol-gel method was modified by introducing a seeding step which resulted in significant improvement in the sintering characteristics and the microstructure of the sintered material. Sintering data were compared with those obtained by the powder mixing technique. Thermal expansion of the sintered samples was measured by classical dilatometry and by high-temperature X-ray diffractometry. It was found that CaZr₄P₆O₂₄ (x= 0) and SrZr₄P₆O₂₄ (x= 1) phases had opposite anisotropies in their respective axial thermal expansions. This behaviour led to the development of a crystalline solution composition of nearly zero expansion characteristic. Microstructures of the sintered specimens were examined by scanning electron microscopy.     

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.     

On the thermal expansion and crystallography of cubic (C) and monoclinic (B) forms of Gd2O3 in the temperature range 20 to 900°C

Thermal expansion coefficients of B-form (monoclinic) and C-form (cubic) Gd₂O₃ have been measured in the temperature range 20 to 900° C, by X-ray diffractometry. The thermal expansion coefficients of both cubic and monoclinic material are linear in the temperature range studied. The expansion of monoclinic material is, however, very anisotropic, and the minor axis of the thermal expansion ellipsoid is not parallel to the edge of the primitive cell to which Gd₂O₃ has been assigned. It is noted that the anisotropy in expansion behaviour of this material indicates that anisotropic growth probably occurs during irradiation.