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.     

Thermal expansion in Cr:LiSrGaF6

High-temperature behaviour of LiSrGaF₆ doped with 1.5 at.% of Cr³⁺ was studied with high-resolution synchrotron angle-dispersive X-ray powder diffraction in the temperature range 298-539 K. No phase transitions were detected. The origin of negative thermal expansion along the c axis is discussed based on the temperature dependencies of structural parameters and octahedral distortions obtained with the Rietveld method. The SrF₆ slab contracts with increasing temperatures because of the diminishing F-Sr-F octahedral angles without any changes in the F-F octahedral edges not only around strontium but also around lithium and gallium. At the same time, the angular distortions of the SrF₆ octahedra are largely diminished. Such a behaviour is discussed in comparison with the thermal expansion of LiCaAlF₆ and LiSrAlF₆.     

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.     

Investigation of phases and stability of solid electrolytes in the NASICON system

In order to study the phase change and stability of the NASICON structure, sodium, lithium and magnesium ions were chosen to substitute the zirconium ion at octahedral sites in the NASICON network. It was found that the zirconium ion can not be replaced by these ions. All the synthesized products are Na_1+xZr₂Si_xP_3?xO₁₂ and phosphate salts. NASICON immersed in liquid sodium at 300°C also results phosphate salt and ZrO₂. It was found that an appropriate excess of sodium phosphate in NASICON will improve the chemical stability, corrosion against sodium and mechanical properties.     

Synthesis and characterization of zirconium titanium phosphate and its application in separation of metal ions

An advanced inorganic ion exchanger, zirconium titanium phosphate (ZTP) of the class of tetravalent bimetallic acid (TBMA) salt has been synthesized by sol–gel route. ZTP has been characterized for ICP-AES, TGA, FTIR and XRD. Chemical stability of the material in various media-acids, bases and organic solvents has been assessed. Cation exchange capacity (CEC) and effect of calcination (100–500 °C) on CEC has also been studied. Distribution behaviour of metal ions Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺ (d-block), Cd²⁺, Hg²⁺, Pb²⁺, Bi³⁺ (heavy) and La³⁺, Ce³⁺, Th⁴⁺, UO₂²⁺ (f-block) towards ZTP has been studied and distribution coefficient (K_d) determined in aqueous as well as various electrolyte media/concentrations. Based on the differential selectivity, breakthrough capacity (BTC) and elution behaviour of various metal ions towards ZTP, a few binary and ternary metal ion separations have been carried out.     

Sintering mechanism for high-density NZP ceramics

We have studied the shrinkage kinetics and mechanism of NaZr₂(PO₄)₃ containing inorganic additives. The sintering of NaZr₂(PO₄)₃ containing ZnO microadditives is shown to follow a liquid-phase mechanism. Effective sintering aids include oxides of metals in the oxidation states ²⁺ and ³⁺ that are capable of reacting with sodium zirconium phosphate to form solid solutions. Ceramics having a relative density of 96–99% and isostructural with NaZr₂(PO₄)₃ can be prepared by adding 0.75–2.0 wt % ZnO as a sintering aid capable of influencing the structure of grain boundaries in ceramic materials, pressing green bodies at 200–300 MPa, and sintering them at 1000–1100°C for 7–15 h.     

Synthesis and thermal properties of zirconium ortho- and diphosphates

Single phase sodium zirconium orthophosphates [NaZr₂(PO₄)₃ and Na₂Zr(PO₄)₂] and zirconium diphosphate (ZrP₂O₇) have been synthesized. The measured linear expansion coefficient of NaZr₂(PO₄)₃ was rather small only reaching a value of 0.16% at 800 °C. The expansion coefficients of Na₂Zr(PO₄)₂ and ZrP₂O₇ were also rather small and were 1.4 and 1.0% respectively at 800 °C. These phosphates showed different behaviour depending on the type of thermal pretreatment before the measurement of the expansion coefficient.     

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.     

Preparation of silylated α-zirconium phosphate and its thermal behavior

Porous zirconium phosphate was prepared by silylation with organic silanes. Octylamine was intercalated into α-Zr(HPO₄)₂·H₂O for expanding interlayer spacing and 1,2-bis(dimethylchlorosilyl)ethane (BMCE) was used for silylation of inorganic layers in toluene. The number of octhylamine molecules intercalated was similar to the number of phosphorus in the inorganic layers. On the other hand, that of BMCE molecule after silylating treatment was much smaller than that of phosphorus. The interlayer spacing of silylated compound was confirmed to expand via high-temperature XRD patterns and was preserved on thermal treatment up to around 500 °C. The maximum specific surface area of the silylated compound heated at 300 °C was around 70 m²/g, though that was smaller for silylated compounds heated at higher and lower temperature.     

Yb-doped NaBi(WO4)2 fibre single crystals grown by the micro-pulling down technique and emission spectroscopic characterization

NaBi_1−xYb_x(WO₄)₂ fibres single crystals were successfully grown by micro-pulling down technology (MPD). The Yb³⁺-doped NaBi(WO₄)₂ fibres single crystals have been pulled using MPD technique with controlled diameter and stationary stable growth conditions corresponding to flat crystallization interface with meniscus length equal to the fibre radii and pulling rate range [6-48 mm h⁻¹]. We have determined the monophased field of NaBi_1−xYb_x(WO₄)₂ for x ≤ 0.3. The lattices parameters decrease as a function of Yb³⁺ substitution in Bi³⁺ sites. The melt behaviour has been study by DTA/TG analysis. We have found that the stoichiometric compounds NaBi(WO₄)₂ melt congruently at 935 °C. The fibre diameters varied from 0.5 to 1 mm depending on the capillary die diameter, pulling rate and the molten zone temperature. Complementary Yb³⁺ spectroscopic characterization in the NaBi(WO₄)₂ lattice has been done by IR emission measurements under laser pumping at room temperature.     

Effect of combined doping (y<Superscript>3 +</Superscript> + fe<Superscript>3 +</Superscript>) on structural features of nanodispersed zirconium oxide

Fully stabilized zirconium dioxide is widely used. One of the basic requirements to this material is the thermal stability of the structure. The most effective stabilizer for zirconium oxide is yttrium oxide. However, the structure of Y-ZrO₂ degraded at low temperature. Partial substitution of Fe^{3 +} for Y³⁺ decreases both the crystallization and sintering temperature of zirconia ceramic. The aim of present work is the investigation of structural peculiarities of zirconium oxide stabilized by combined dopant depending on chemical composition, synthesis conditions and heat treatment. The polymorphic composition of a ZrO₂-based materials has been determined in series of samples that correspond to the formula [1−(x+y)]ZrO₂⋅xY₂O₃⋅yFe₂O₃ in the temperature range 620–1570 K. It has been found that at the same molar ratio ZrO₂ : doping oxides, the degree of ZrO₂ stabilization increases, and the low-temperature degradation process is retarded by the partial substitution of Fe^{3 +} for Y³⁺. Nonequivalent sites of Fe^{3 +} ions have been identified: two with octahedral coordination for CPH and three with octa-, penta- and tetrahedral coordination for SPH. The possibility of cluster distribution of Fe³⁺ ions and the dependence of the number of vacancies on synthesis conditions have been shown.     

Thermal stability of the Mobil Five type metallosilicate molecular sieves—An in situ high temperature X-ray diffraction study

We have carried out in situ high temperature X-ray diffraction (HTXRD) studies of silicalite-1 (S-1) and metallosilicate molecular sieves containing iron, titanium and zirconium having Mobil Five (MFI) structure (iron silicalite-1 (FeS-1), titanium silicalite-1 (TS-1) and zirconium silicalite-1 (ZrS-1), respectively) in order to study the thermal stability of these materials. Isomorphous substitution of Si⁴⁺ by metal atoms is confirmed by the expansion of unit cell volume by X-ray diffraction (XRD) and the presence of Si-O-M stretching band at ∼960 cm⁻¹ by Fourier transform infrared (FTIR) spectroscopy. Appearance of cristobalite phase is seen at 1023 and 1173 K in S-1 and FeS-1 samples. While the samples S-1 and FeS-1 decompose completely to cristobalite at 1173 and 1323 K, respectively, the other two samples are thermally stable upto 1623 K. This transformation is irreversible. Although all materials show a negative lattice thermal expansion, their lattice thermal expansion coefficients vary. The thermal expansion behavior in all samples is anisotropic with relative strength of contraction along ‘a’ axes is more than along ‘b’ and ‘c’ axes in S-1, TS-1, ZrS-1 and vice versa in FeS-1. Lattice thermal expansion coefficients (α_v) in the temperature range 298-1023 K were −6.75 × 10⁻⁶ K⁻¹ for S-1, −12.91 × 10⁻⁶ K⁻¹ for FeS-1, −16.02 × 10⁻⁶ K⁻¹ for TS-1 and −17.92 × 10⁻⁶ K⁻¹ for ZrS-1. The highest lattice thermal expansion coefficients (α_v) obtained were −11.53 × 10⁻⁶ K⁻¹ for FeS-1 in temperature range 298-1173 K, −20.86 × 10⁻⁶ K⁻¹ for TS-1 and −25.54 × 10⁻⁶ K⁻¹ for ZrS-1, respectively, in the temperature range 298-1623 K. Tetravalent cation substitution for Si⁴⁺ in the lattice leads to a high thermal stability as compared to substitution by trivalent cations.     

Synthesis of amorphous supermicroporous zirconium phosphate materials by nonionic surfactant templating

Supermicroporous zirconium phosphate materials possessing wormhole-like pores in the size range 1.3-1.8 nm were synthesized by using nonionic poly(ethylene oxide) surfactant (e.g., C₁₆H₃₃(EO)₁₀, C₁₈H₃₅(EO)₁₀) as a structure directing agent. The textural and structural properties were characterized by powder X-ray diffraction, N₂ adsorption analysis, differential thermal analysis, scanning and transmission electron microscopy, ³¹P MAS NMR and infrared spectroscopy. The synthesized materials are amorphous, exhibiting high surface areas, narrow pore size distributions, excellent thermal stabilities (over 800 °C) and acidic properties. The supermicropore size of the synthesized zirconium phosphate may be tunable by the variation of alkyl chain length of the surfactant.     

Structure analysis on the Ba3Mg(Ta1−xNbx)2O9 ceramics: Coexistence of order and disorder

The Ba₃ZnTa₂O₉ (BZT) and Ba₃MgTa₂O₉ (BMT) ceramics, a family of A₃B²⁺B⁵⁺₂O₉ complex perovskites, are extensively utilized in mobile based technologies due to their intrinsic high unloaded quality factor, high dielectric constant and a low (near-zero) resonant frequency temperature coefficient at microwave frequencies. The preparation conditions as well as size and nature of B cations have a profound effect on the final dielectric properties. In this article, we report the effect of Nb⁵⁺ at the Ta⁵⁺ site on the BMT structure prepared at four synthesis temperatures (1300, 1400, 1500 and 1600 °C). The analysis has been carried out using the Rietveld technique on the X-ray powder diffraction data. Results suggest that both the preparation temperatures and Nb⁵⁺ content have significant effect on the ordering of B cations in the Ba₃Mg(Ta_1−xNb_x)₂O₉ solid solution. A disordered (cubic) structure is preferred by the 1300 °C compounds. The weight percentage of the ordered (trigonal) phase escalates, for a given composition, with increasing calcination temperature. A fully ordered trigonal arrangement exists only for x = 0.0 and 0.2 compounds calcined at 1600 °C, and the rest are biphasic (cubic and trigonal). The increase in the cubic fraction upon Nb⁵⁺ augmentation suggests that the solid solution leans more toward the disordered structural arrangement of B²⁺ and B⁵⁺ cations.     

Synthesis, magnetic and thermoelectric properties of Rh2MO6 (M = Mo, Te, and W) with rutile-related structure

The Rh₂MO₆ compounds with M = Mo, W and Te were synthesized by solid state reaction. The compounds all crystallize in a rutile-type structure. These compounds would be expected to be diamagnetic insulators if the oxidation states were Rh³⁺ and M⁶⁺. In fact, all show relatively high electronic conductivities with Rh₂TeO₆ showing the highest electronic conductivity of ∼500 S/cm at room temperature. Measurable magnetic moments also indicate valence degeneracy between Rh and the M cation. The measured Seebeck coefficients are relatively low and positive indicating hole-type conduction.     

Phase transitions and ionic mobility in hydrogen zirconium phosphates with the NASICON structure, H1±XZr2−XMX(PO4)3·H2O, M = Nb, Y

Phase transitions and the mobility of proton-containing groups in hydrogen zirconium phosphate HZr₂(PO₄)₃·nH₂O with the NASICON structure were studied by X-ray powder diffraction, ¹H, ³¹P NMR, IR spectroscopy and TG analysis. Heating HZr₂(PO₄)₃·H₂O above 420 K results in dehydration and in a rhombohedral-triclinic phase transition. Continued heating to about 490 K results in the thermal activation of cation disordering and phase transition of HZr₂(PO₄)₃ from triclinic to rhombohedral phase. Parameter “a” of HZr₂(PO₄)₃ lattice decreases during the heating. It is shown that oxonium ions in HZr₂(PO₄)₃·H₂O are characterized by high rotation and translation mobility. Rotation mobility of oxonium ions can be increased by the substitution of zirconium by yttrium or niobium.     

Insertion of trivalent cations in the layered MPS3 (Mn, Cd) materials

The Al_0.21Mn_0.78PS₃, Al_0.15Cd_0.83PS₃, In_0.20Cd_0.70PS₃ and Ga_0.28Cd_0.58PS₃ compounds have been synthesized by the cation-exchange process used for other cationic species of the same family. These compounds were characterized by X-ray diffraction , Fourier transform infrared spectroscopy, ICP plasma analyses, photoconductivity and electrochemical impedance spectroscopy. The compounds synthesized show electrical conductivities (σ) of the order of 10⁻⁹ to 10⁻¹⁰ S cm⁻¹ at 298 K and photoconductive effect. The physical properties of the new Al³⁺ materials reveal the same behavior as our previous report on In³⁺ and Ga³⁺ compounds.     

Bulk ac conductivity studies of lithium substituted layered sodium trititanates (Na<Subscript>2</Subscript>Ti<Subscript>3</Subscript>O<Subscript>7</Subscript>)

Lithium mixed sodium trititanates with 0.3, 0.5 and 1.0 M percentage of Li₂CO₃ (general formula Na_2−X Li _X Ti₃O₇) have prepared by a high temperature solid-state reaction route. EPR analysis, high temperature range (473–773 K) and variable frequency range (100 Hz–1 MHz) ac conductivity measurements were carried out on prepared sample. The lithium ions are accommodated with the sodium ions in the interlayer space. The EPR specta of lithium mixed sodium Trititanates confirm the partial reduction of Ti⁴⁺ ions to Ti³⁺. Four distinct regions have identified in the LnσT versus 1,000/T plots. Various conduction mechanisms which dependence on concentration, frequency and temperature are reported in this paper for lithium mixed layered sodium Trititanates. The dilation of interlayer space has further been proposed to occur due to inclusion of lithium ions in the interlayer space. The conductivity increases as the concentration of lithium increases. The increase of ionic conductivity in these compounds is due to accommodation of lithium ions with sodium ions in interlayer space.     

Preparation method and thermal properties of samarium and europium-doped alumino-phosphate glasses

The present work investigates alumino-phosphate glasses from Li₂O–BaO–Al₂O₃–La₂O₃–P₂O₅ system containing Sm³⁺ and Eu³⁺ ions, prepared by two different ways: a wet raw materials mixing route followed by evaporation and melt-quenching, and by remelting of shards. The linear thermal expansion coefficient measured by dilatometry is identical for both rare-earth-doped phosphate glasses. Comparatively to undoped phosphate glass the linear thermal expansion coefficient increases with 2 × 10⁻⁷ K⁻¹ when dopants are added. The characteristic temperatures very slowly decrease but can be considered constant with atomic weight, atomic number and f electrons number of the doping ions in the case of T_g (vitreous transition temperature) and T_sr (high annealing temperature) but slowly increase in the case of T_ir (low annealing temperature–strain point) and very slowly increase, being practically constant in the case of T_D (dilatometric softening temperature). Comparatively to undoped phosphate glass the characteristic temperatures of Sm and Eu-doped glasses present lower values. The higher values of electrical conductance for both doped glasses, comparatively to usual soda-lime-silicate glass, indicate a slightly reduced stability against water. The viscosity measurements, showed a quasi-linear variation with temperature the mean square deviation (R²) being ranged between 0.872% and 0.996%. The viscosity of doped glasses comparatively to the undoped one is lower at the same temperature. Thermogravimetric analysis did not show notable mass change for any of doped samples. DSC curves for both rare-earth-doped phosphate glasses, as bulk and powdered samples, showed T_g values in the range 435–450 °C. Bulk samples exhibited a very weak exothermic peak at about 685 °C, while powdered samples showed two weak exothermic peaks at about 555 °C and 685 °C due to devitrification of the glasses. Using designed melting and annealing programs, the doped glasses were improved regarding bubbles and cords content and strain elimination.     

Crystal structure, magnetic and thermal properties of LaFeTeO6

LaFeTeO₆ was prepared by solid state reaction of La₂O₃, Fe₂O₃ and TeO₂ in 1:1:2 molar ratios and characterized by powder X-ray diffraction, thermogravimetry and magnetometry. The detailed crystal structure analysis was carried out by Rietveld refinement. LaFeTeO₆ crystallizes in a trigonal lattice with unit cell parameters: a = 5.2049(1) Å and c = 10.3457(2) Å, V = 242.73(2) Å³. The crystal structure is built from sheets of the edge shared FeO₆ and TeO₆ octahedra stacked along the c-axis. These sheets are connected together by La³⁺ ions. Thermogravimetric analysis of the compound showed it to be thermally stable up to 1323 K and continuous loss of TeO₂ was observed above 1323 K leading to the formation of LaFeO₃. High temperature XRD studies revealed a normal expansion behavior of the compound. Temperature and field dependent magnetization of LaFeTeO₆ showed paramagnetic behavior in the temperature range of 3-300 K. The effective magnetic moment per Fe³⁺ ion (5.14 μB) indicates the high spin d⁵ state of Fe³⁺ ion.