A novel route to synthesize cubic ZrW2−xMoxO8 (x=0-1.3) solid solutions and their negative thermal expansion properties

Cubic ZrW_2−xMo_xO₈ (c-ZrW_2−xMo_xO₈) (x=0-1.3) solid solutions were prepared by a novel polymorphous precursor transition route. X-ray diffraction (XRD) analysis reveals that the solid solutions are single phase with α- and β-ZrW₂O₈ structure for 0?x?0.8 and 0.9?x?1.3, respectively. The optimum synthesis conditions of ZrWMoO₈ are obtained from differential scanning calorimetry-thermal gravimetric analysis (DSC-TGA), XRD and mass loss-temperature/time curves. Following the above experience, the stoichiometric solid solutions of c-ZrW_2−xMo_xO₈ (x=0-1) are obtained within 1 wt% of mass loss. The relationships of lattice parameters (a), phase transition temperatures (T_c) and instantaneous coefficients of thermal expansion (α_i) against the content x of Mo are discussed based on the variation of order degree parameters of ZrW_2−xMo_xO₈.     

Synthesis, thermal stability and magnetic properties of the Lu1−xLaxMn2O5 solid solution

Polycrystalline samples of the Lu_1−xLa_xMn₂O₅ solid solution system were synthesized under moderate conditions for compositions with x up to 0.815. Due to the large difference in ionic size between Lu³⁺ and La³⁺, significant changes in lattice parameters and severe lattice strains are present in the solid solution. This in turn leads to the composition dependent thermal stability and magnetic properties. It is found that the solid solution samples with x≤0.487 decompose at a single well defined temperature, while those with x≥0.634 decompose over a temperature range with the formation of intermediate phases. For the samples with x≤0.487, the primary magnetic transition occurs below 40 K, similar to LuMn₂O₅ and other individual RMn₂O₅ (R=Bi, Y, and rare earth) compounds. In contrast, a magnetic phase with a 200 K onset transition temperature is dominant in the samples with x≥0.634.     

A TEM, XRD, and crystal chemical investigation of oxygen/vacancy ordering in (Ba1−xLax)2In2O5+x, 0x0.6

A careful TEM and XRD study of the (Ba_1−xLa_x)₂In₂O_5+x, 0x0.6, ‘defect-perovskite’-type solid solution has been carried out. A well-defined structural phase transition is shown to occur between x=0.1 and 0.2 from the orthorhombic brownmillerite structure type on the low x side to a multiple twinned, tetragonal 1×1×2 perovskite-related superstructure phase on the high x side at x=0.2. This phase transition correlates with an important phase transition previously observed in electrical conductivity versus temperature measurements. The existence of additional satellite reflections close to the regions of reciprocal space was found to be typical of all (Ba_1−xLa_x)₂In₂O_5+x specimens, although their intensity relative to the parent Bragg reflections systematically reduces as x increases. As x increases beyond 0.2, the -type satellite reflections initially become weaker and rather more diffuse for x=0.3 before splitting into pairs of rather weak and somewhat diffuse incommensurate satellite reflections for x=0.4 and beyond. An interpretation in terms of oxygen vacancy ordering and associated structural relaxation is given. Additional structured diffuse scattering is also observed and a tentative explanation in terms of Ba/La ordering and associated local strain distortions put forward.     

Zr1−xLnxW2O8−x/2 (Ln=Eu, Er, Yb): Solid solutions of negative thermal expansion-synthesis, characterization and limited solid solubility

Zr_1−xLn_xW₂O_8−x/2 solid solutions (Ln=Eu, Er, Yb) of different substitution fractions x have been synthesized. Their X-ray diffraction (XRD) patterns have been indexed and lattice parameters calculated based on the α-ZrW₂O₈ structure. The coefficients of thermal expansion (CTEs) of these solid solutions were estimated to be −10.3×10⁻⁶ K⁻¹ in temperature range of 30-100 °C. The solubility of lanthanide ions in these solid solutions decreases linearly with the increase in the radius of substituted lanthanide ions. Based on the concentration dependence of phase transition temperatures, a novel method for determination of solubility of the lanthanide ions in Zr_1−xLn_xW₂O_8−x/2 solid solutions has been developed. This method seems to be more sensitive as compared with that based on XRD technique.     

Composition-induced structural phase transitions in the (Ba1−xLax)2In2O5+x (0?x?0.6) system

Composition-induced structural phase changes across the high temperature, fast oxide ion conducting (Ba_1−xLa_x)₂In₂O_5+x, 0?x?0.6, system have been carefully analysed using hard mode infrared (IR) powder absorption spectroscopy, X-ray powder diffraction and electron diffraction. An orthorhombic brownmillerite to three-dimensionally disordered cubic perovskite phase transition in this system is signalled by a drastic change in slope of both wavenumber and average line widths of IR spectra as a function of composition. Some evidence is found for the existence of an intermediate tetragonal phase (previously reported to exist from electron diffraction data) around x∼0.2. The new spectroscopic data have been used to compare microscopic and macroscopic strain parameters arising from variation in composition. The strain and spectroscopic data are consistent with first-order character for the tetragonal→orthorhombic transition, while the cubic→tetragonal transition could be continuous. Differences between the variation with composition of spectral parameters and of macroscopic strain parameters are consistent with a substantial order/disorder component for the transitions. There is also evidence for precursor effects within the cubic structure before symmetry is broken.     

Phase separation in the spin-state transition system of La1−xBaxCoO3

The magnetic and electric transport properties of La_1−xBa_xCoO₃ (0<x≤0.50) have been studied systematically. Two effects of substitution divalent ions on the spin-state transition of Co³⁺ have been differentiated for the substitution of Ba²⁺ for La³⁺ in La_1−xBa_xCoO₃. The first is the transition from low-spin state to high-spin state due to lattice expansion, and the second is the transition from low-spin state to intermediate-spin state caused by the strong hybridization between ligand (oxygen) 2p and Co 3d orbital with introduction of holes in the oxygen 2p orbital. Based on the two different spin-state transition mechanisms and experimental results, a phase separation model has been developed and a very detailed magnetic and electric phase diagram of La_1−xBa_xCoO₃ has been constructed.     

Study of Ti substitution in ZrW2O8 negative thermal expansion materials

Powder XRD-analysis and thermo-mechanical analysis on sintered TiO₂-WO₃-ZrO₂ mixtures revealed the formation of Zr_1−xTi_xW₂O₈ solid solutions. A noticeable decrease in unit cell parameter ‘a’ and in the order-disorder transition temperature could be seen in the case of Zr_1−xTi_xW₂O₈ solid solutions.Studies performed on other ZrW₂O₈ solid solutions have attributed an increase in phase transition temperature to a decrease in free lattice volume, whereas a decrease in phase transition temperature was suggested to be due to the presence of a more disordered state. Our studies indicate that the phase transition temperature in our materials is strongly influenced by the bond dissociation energy of the substituting ion-oxygen bond. A decrease in bond strength may compensate for the effect of a decrease in lattice free volume, lowering the phase transition temperature as the degree of substitution by Ti⁴⁺ increases. This hypothesis is proved by differential scanning calorimetry.     

Solid solubility and phase transitions in the system LaNb1−xTaxo4

The solid solubility between LaNbO₄ and LaTaO₄ was investigated by X-ray diffraction, and a two-phase region was observed in the composition region LaNb_1−xTa_xO₄ where 0.4?x?0.8. Single-phase LaNb_1−xTa_xO₄ (0?x?0.4) with the monoclinic Fergusonite structure at ambient temperature, was observed to transform to a tetragonal Scheelite structure by in-situ high-temperature X-ray diffraction, and the phase transition temperature was shown to increase with increasing Ta-content. This ferroelastic to paraelastic second-order phase transition was described by Landau theory using spontaneous strain as an order parameter. The thermal expansion of LaNb_1−xTa_xO₄ (0?x0.4) was shown to be significantly higher below the phase transition than above. Single-phase LaNb_1−xTa_xO₄ (0.8?x?1) with another monoclinic crystal structure at ambient temperature was shown to transform to an orthorhombic crystal structure by X-ray diffraction and differential scanning calorimetry. The phase transition temperature was observed to decrease with decreasing Ta-content. Finally, orthorhombic LaTaO₄ could also be transformed to monoclinic LaTaO₄ at ambient temperature by applying a uniaxial pressure of 150-170 MPa, reflecting the lower molar volume of monoclinic LaTaO₄.     

The Phase Relations in the System In2O3–TiO2–MgO at 1100 and 1350°C

The phase relations in the system In₂O₃–TiO₂–MgO at 1100 and 1350°C are determined by a classical quenching method. In this system, there are four pseudobinary compounds, In₂TiO₅, MgTi₂O₅ (pseudobrookite type), MgTiO₃ (ilmenite type), and Mg₂TiO₄ (spinel type) at 1100°C. At 1350°C, in addition to these compounds there exist a spinel-type solid solution Mg_2−xIn_2xTi_1−xO₄ (0≤x≤1) and a compound In₆Ti₆MgO₂₂ with lattice constants a=5.9236(7) Å, b=3.3862(4) Å, c=6.3609(7) Å, β=108.15(1)°, and q=0.369, which is isostructural with the monoclinic In₃Ti₂FeO₁₀ in the system In₂O₃–TiO₂–MgO. The relation between the lattice constants of the spinel phase and the composition nearly satisfies Vegard's law. In₆Ti₆MgO₂₂ extends a solid solution range to In₂₀Ti₁₇Mg₃O₆₇ with lattice constants of a=5.9230(5) Å, b=3.3823(3) Å, c=6.3698(6) Å, β=108.10(5)°, and q=0.360. The distributions of constituent cations in the solid solutions are discussed in terms of their ionic radius and site preference effect.     

固溶体Bi_2Mo_(1-x)W_xO_6的水热合成及光催化性能

采用水热法,在较低温度下合成了系列Bi2Mo1-xWxO6固溶体。结果表明,W的替代抑制了固溶体的晶粒生长,导致了较小的晶粒尺寸。随着x的增加,红外光谱中840cm-1处M-O键的振动频率νM-O有规律地向低频率方向移动,表明Mo6+离子逐步被W6+替代,生成了无限互溶的固溶体。光吸收性能研究表明,随着W6+逐步替代Mo6+,带隙出现了先降后升的趋势,x=0.4时带隙最小。而固溶体的光催化性能随着x的增加,出现了先增后减的趋势,x=0.4时光催化活性最高。此外,含W样品的光催化活性高于Bi2MoO6。这与固溶体的带隙、带结构和晶粒尺寸变化有关。     

Synthesis, optical properties and electronic structures of polyoxometalates K3P(Mo1−xWx)12O40 (0?x?1)

Various compositions of solid solutions K₃P(Mo_1−xW_x)₁₂O₄₀ (0?x?1) were prepared using two solid state synthetic routes. The crystallite size was determined by linewidth refinements of X-ray diffraction patterns using the Warren-Averbach method, and the grain size distribution by laser scattering experiments. Optical properties were determined by diffuse reflectance measurements in the UV-visible range. The optical gap E_g was found to increase exponentially from ∼2.5 to ∼3.30 eV with increasing x, and is systematically shifted to a higher energy when the grain size decreases. The relation between E_g and x was analyzed by calculating the HOMO-LUMO gaps of the [P(Mo_1−xW_x)₁₂O₄₀]³⁻ anions on the basis of tight-binding electronic structure calculations.     

Energetics of copper diphosphates – Cu2P2O7 and Cu3(P2O6OH)2

Differential scanning calorimetry and high temperature oxide melt solution calorimetry are used to study enthalpy of phase transition and enthalpies of formation of Cu₂P₂O₇ and Cu₃(P₂O₆OH)₂. α-Cu₂P₂O₇ is reversibly transformed to β-Cu₂P₂O₇ at 338–363 K with an enthalpy of phase transition of 0.15 ± 0.03 kJ mol⁻¹. Enthalpies of formation from oxides of α-Cu₂P₂O₇ and Cu₃(P₂O₆OH)₂ are −279.0 ± 1.4 kJ mol⁻¹ and −538.8 ± 2.7 kJ mol⁻¹, and their standard enthalpies of formation (enthalpy of formation from elements) are −2096.1 ± 4.3 kJ mol⁻¹ and −4302.7 ± 6.7 kJ mol⁻¹, respectively. The presence of hydrogen in diphosphate groups changes the geometry of Cu(II) and decreases acid–base interaction between oxide components in Cu₃(P₂O₆OH)₂, thus decreasing its thermodynamic stability.     

Comprehensive survey of Nd substitution In La2Mo2O9 oxide-ion conductor

Differential thermal analysis coupled to temperature-controlled diffraction have given evidence of a topological metastability phenomenon in an extended compositional range of the La_2−xNd_xMo₂O₉ solid solution. A metastable-stable phase diagram is proposed for this series of LAMOX-type fast oxide-ion conductors. In the Nd range 0<x?0.35, a freezing of the oxygen/vacancy disorder of the β-phase at ambient temperature can be achieved through a splat-quenching to water-ice mixture or/and shaping/sintering into pellet. In the intermediate 0.4?x?1.2 range, the amount of β-metastable phase grows upon substitution for powders. The negative impact of β-metastable to α phase transition on conductivity tends to disappear through the partial stabilization of the β phase by shaping/sintering.     

In situ X-ray diffraction study of reduction processes of Fe3O4- and Fe1−xO-based ammonia-synthesis catalysts

The temperature-programmed reduction process of two types of industrial ammonia-synthesis catalysts, A110 and ZA-5, which are, respectively, based on Fe₃O₄ and Fe_1−xO precursors, were studied by in situ X-ray power diffraction (XRD). It has been found that the ZA-5 has lower reduction temperature and faster reduction rate, and its active phase α-Fe possesses a higher value of lattice microstrain than A110. The simulation based on Rietveld refinement has also shown that the shape of α-Fe grain of ZA-5 has a mixed shape of cube and sphere with more exposing (111) and (211) planes, while that of A110 looks like a concave cube with more exposing (110) planes. Based on the results obtained, a growth model of α-Fe during the reduction of Fe₃O₄- and Fe_1−xO-based ammonia-synthesis catalysts is proposed, and the origins for the activity difference has been also discussed.     

The two-phase region in 2(ZnSe)x(CuInSe2)1−x alloys and structural relation between the tetragonal and cubic phases

The two-phase region in the system 2(ZnSe)_x(CuInSe₂)_1−x covers the chemical composition range 0.10<x?0.36, in which a tetragonal and a cubic phase are coexisting. The structural relation between both phases was determined by selected area diffraction (SAD) and transmission electron microscopy (TEM). Both crystal structures are very similar and the extremely small mismatch of the lattice constants of the tetragonal phase and the embedding cubic matrix phase allows for the grain boundaries to be virtually strain-free and, therefore, without notable dislocations. The tetragonal phase forms grains of flat discus-like shape in the ambient cubic matrix, with the short discus axis parallel to the tetragonal c-axis. TEM experiments proved that the discus-shaped tetragonal particles are collinear with the (100)_cub, (010)_cub and (001)_cub planes of the cubic phase. Cooling and annealing experiments revealed a near-equilibrium state only to be realized for small cooling rates less than 2 K/h and/or for a long-time annealing with subsequent rapid quenching. Only then there will be no cation ordering in both, the tetragonal domains and the parental cubic matrix phase. If, however, the samples are kept in a state far away from the equilibrium condition both phases reveal Stannite-type cation ordering. Within the composition range of 0?x?0.10 only tetragonal 2(ZnSe)_x(CuInSe₂)_1−x-alloys exist. At concentration rates above 36 mol% 2(ZnSe) only cubic structured solid solutions of ZnSe and CuInSe₂ are found to be stable. However, in the range 36 mol% to about 60 mol% 2(ZnSe) tiny precipitates with Stannite-like structure exist, too.     

Structural investigation of KxBa1−xGa2−xGe2+xO8 solid solutions using the X-ray Rietveld method

The K_xBa_1−xGa_2−xGe_2+xO₈ (x=0.6−1.0) solid solutions undergo a structural phase transition that has a significant effect on their sintering behavior and their microwave dielectric properties. The crystal structures of both phases within the solid-solution region were determined by the Rietveld method using powder X-ray diffraction data. We found that the low-temperature-stable phase is isostructural with the pseudo-orthorhombic KGaGe₃O₈ (space group P2₁/a), while the high-temperature-stable phase has a typical monoclinic feldspar structure (space group C2/m). Due to the topological differences between the two structures, the T-O bonds within the tetrahedra must be partially recombined to make a new framework, which causes an endothermic effect during the P2₁/a to C2/m phase transition. The correlation between the crystal structures, the microwave dielectric properties and the phase-transition behaviors were discussed in terms of the crystallographic features, the lattice parameters, and the strain-induced anisotropic peak-broadening.     

Positron lifetime study of Pr1−xCaxMnO3 (x=0.5, 0.3) during magnetic transition

We measured the positron lifetime in perovskite manganites Pr_1−xCa_xMnO₃ (x=0.3, 0.5). Two lifetime components were observed for each compound; they were attributed to the annihilation of free positrons and positrons trapped at the A-site vacancies. The positron lifetime at the A-site vacancies changed significantly during the antiferromagnetic transition in both the compounds, whereas it was constant around the charge-ordering transition. This change indicates that the electron distribution at the vacancies changed possibly due to the change in the electron distribution of neighboring oxygen atoms. This result indicates that positron lifetime measurements can provide unique information on electronic states during a spin-related phase transition in various oxide materials.     

Crystal chemistry of Co-doped Zn7Sb2O12

Zn₇Sb₂O₁₂ forms a full range of Co-containing α solid solutions, Zn_7−xCo_xSb₂O₁₂, with an inverse-spinel structure at high temperature. At low temperatures for x<2, the solid solutions transform into the low temperature β-polymorph. For x=0, the β→α transition occurs at 1225±25 °C; the transition temperature decreases with increasing x. At high x and low temperatures, α solid solutions are formed but are non-stoichiometric; the (Zn+Co):Sb ratio is >7:2 and the compensation for the deficiency in Sb is attributed to the partial oxidation of Co²⁺ to Co³⁺. From Rietveld refinements using ND data, Co occupies both octahedral and tetrahedral sites at intermediate values of x, but an octahedral preference attributed to crystal field stabilisation, causes the lattice parameter plot to deviate negatively from the Vegard's law. Sub-solidus compatibility relations in the ternary system ZnO-Sb₂O₅-CoO have been determined at 1100 °C for the compositions containing ?50% Sb₂O₅.     

Thermoelectric properties of Cu-doped n-type (Bi2Te3)0.9–(Bi2−xCuxSe3)0.1(x=0–0.2) alloys

n-Type (Bi₂Te₃)_0.9–(Bi_2−xCu_xSe₃)_0.1 (x=0–0.2) alloys with Cu substitution for Bi were prepared by spark plasma-sintering technique and their structural and thermoelectric properties were evaluated. Rietveld analysis reveals that approximate 9.0% of Bi atomic sites are occupied by Cu atoms and less than 4.0 wt% second phase Cu_2.86Te₂ precipitated in the Cu-doped parent alloys. Measurements show that an introduction of a small amount of Cu (x0.1) can reduce the lattice thermal conductivity (κ_L), and improve the electrical conductivity and Seebeck coefficient. An optimal dimensionless figure of merit (ZT) value of 0.98 is obtained for x=0.1 at 417 K, which is obviously higher than those of Cu-free Bi₂Se_0.3Te_2.7 (ZT=0.66) and Ag-doped alloys (ZT=0.86) prepared by the same technologies.     

Dielectric and Raman studies on the solid solution (1−x)BaTiO3/xNaNbO3 ceramics

In order to elucidate the correlation between the relaxor type of phase transition and the percent of the A and B site substitution in the Ba_1−xNa_xTi_1−xNb_xO₃ solid solution, the dielectric permittivity was carried out in the temperature range 80–600 K. All ceramics of these solid solutions present a ferroelectric–paraelectric phase transition with relaxor and classical character depending on the value of x. With increasing x the three phase transition of pure BaTiO₃ are pinched into one rounded dielectric peak, and there is evidence for Vogel–Fulcher type relaxational freezing. Raman spectra of the x=0.3 and x=0.7 compositions taken at various temperatures and measured over the wavenumber range 100–1200 cm⁻¹ confirm that the first order scattering is dominant in phonon bands resulting from both ordered region and disordered matrix.