Local environment in Ba2In2−xWxO5+3x/2 oxide ion conductors

The actual oxygen environment of the tungsten dopant in the Ba₂In_2−xW_xO_5+3x/2 solid solution was revealed by combining X-ray absorption spectroscopy at the tungsten L_I and L_III edges and at the indium L_I edge. Whatever the substitution ratio, the tungsten atoms exhibit a regular octahedral environment. When the substitution ratio increases, the oxygen vacancies are progressively filled until their total occupancy for x=2/3. For x?0.3, the perovskite structure is stabilised; the tungsten atoms are randomly distributed in the structure. Although X-ray diffraction revealed a cubic symmetry for these compositions, a local distortion of the indium environment is observed when a tungsten atom is in its surrounding.     

Non-centrosymmetric Ba3Ti3O6(BO3)2

The compound previously reported as Ba₂Ti₂B₂O₉ has been reformulated as Ba₃Ti₃B₂O₁₂, or Ba₃Ti₃O₆(BO₃)₂, a new barium titanium oxoborate. Small single crystals have been recovered from a melt with a composition of BaTiO₃:BaTiB₂O₆ (molar ratio) cooled between 1100°C and 850°C. The crystal structure has been determined by X-ray diffraction: hexagonal system, non-centrosymmetric space group, a=8.7377(11) Å, c=3.9147(8) Å, Z=1, wR(F²)=0.039 for 504 unique reflections. Ba₃Ti₃O₆(BO₃)₂ is isostructural with K₃Ta₃O₆(BO₃)₂. Preliminary measurements of nonlinear optical properties on microcrystalline samples show that the second harmonic generation efficiency of Ba₃Ti₃O₆(BO₃)₂ is equal to 95% of that of LiNbO₃.     

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.     

The pyrochlore to ‘defect fluorite’ transition in the Y2(ZryTi1−y)2O7 system and its underlying crystal chemistry

A careful investigation of the previously reported single phase, pyrochlore structure type Y₂(Zr_yTi_1−y)₂O₇, 0?y?0.9, (YZT) solid solution has been carried out. Given the known slow rate of diffusion of cations in fluorite-related stabilized zirconia systems and the consequent difficulty in achieving equilibrium, careful attention was paid to synthesis procedures while Guinier XRD and electron diffraction were used to investigate the synthesized materials. As a consequence, a subtle but nonetheless clear two-phase region separating a pyrochlore-type solid solution field (from 0?y?∼0.54) from a “defect fluorite” type solid solution field (from ∼0.68?y?1) has been found. The underlying crystal chemistry of the system has been investigated using the bond valence sum approach. The dielectric properties have also been measured as a function of composition. The dielectric constant and dielectric loss of the lowest y compounds are potentially quite useful but deteriorate rapidly with increasing Zr content.     

Synthesis and nonlinear optical properties of BaTi(BO3)2 and Ba3Ti3O6(BO3)2 crystals in glasses with high TiO2 contents

The ternary BaO-TiO₂-B₂O₃ glasses containing a large amount of TiO₂ (20-40 mol%) are prepared, and their optical basicities (Λ), the formation, structural features and second-order optical nonlinearities of BaTi(BO₃)₂ and Ba₃Ti₃O₆(BO₃)₂ crystals are examined to develop new nonlinear optical materials. It is found that the glasses with high TiO₂ contents of 30-40 mol% show large optical basicities of Λ=0.81-0.87, suggesting the high polarizabity of TiO_n polyhedra (n=4-6) in the glasses. BaTi(BO₃)₂ and Ba₃Ti₃O₆(BO₃)₂ crystals are found to be formed as main crystalline phases in the glasses. It is found that BaTi(BO₃)₂ crystals tend to orient at the surface of crystallized glasses. The new XRD pattern for the Ba₃Ti₃O₆(BO₃)₂ phase is proposed through Rietvelt analysis. The second harmonic intensities of crystallized glasses were found to be 0.8 times as large as α-quartz powders, i.e., I^2ω(sample)/I^2ω(α-quartz)=0.8, for the sample with BaTi(BO₃)₂ crystals and to be I^2ω(sample)/I^2ω(α-quartz)=68 for the sample with Ba₃Ti₃O₆(BO₃)₂ crystals. The Raman scattering spectra for these two crystalline phases are measured for the first time and their structural features are discussed.     

X-ray and neutron powder diffraction studies of Ba(NdxY2−x)CuO5

Ba(R,R′)₂CuO₅ (R,R′=lanthanides and Y) plays an important role as a flux-pinning agent in enhancing the superconducting properties of the Ba₂(R,R′)Cu₃O_6+x (R,R′=lanthanides and Y) coated conductors. Using X-ray diffraction and neutron diffraction, we found that the Ba(Nd_xY_2−x)CuO₅ solid solution adopts two structure types. In the Nd-rich region (1.8?x?2.0), the materials are of brown color (commonly referred to as the ‘brown phase’), and the structure is tetragonal with space group I4/mbm (no. 127). In the Y-rich region (0.0?x?1.4), the materials are green (commonly referred to as the ‘green phase’) and the structure is orthorhombic with space group Pnma (no. 62). A two-phase region (1.4<x<1.8) exists between the orthorhombic and tetragonal phases. The crystal chemistry and crystallography of the orthorhombic ‘green phase’ series, Ba(Nd_xY_2−x)CuO₅ (isostructural to BaY₂CuO₅), are discussed in this paper.     

Structural, spectroscopic and photoluminescence studies of LiEu(WO4)2−x(MoO4)x as a near-UV convertible phosphor

A series of lithium europium double tungsto-molybdate phosphors LiEu(WO₄)_2−x(MoO₄)_x (x=0, 0.4, 0.8, 1.2, 1.6, 2.0) have been synthesized by solid-state reactions and their crystal structure, optical and luminescent properties were studied. As the molybdate content increases, the intensity of the ⁵D₀→⁷F₂ emission of Eu³⁺ activated at wavelength of 396 nm was found to increase and reach a maximum when the relative ratio of Mo/W is 2:0. These changes were found to be accompanied with the changes in the spectral feature, which can be attributed to the crystal field splitting of the ⁵D₀→⁷F₂ transition. As the molybdate content increases the emission intensity of the 615 nm peak also increases. The intense red-emission of the tungstomolybdate phosphors under near-UV excitation suggests them to be potential candidate for white light generation by using near-UV LEDs. In this study the effect of chemical compositions and crystal structure on the photoluminescent properties of LiEu(WO₄)_2−x(MoO₄)_x is investigated and discussed.     

(ZrO2)0.92(Gd2O3)0.08纳米晶的水热合成及其烧结体的电性能

用新制备的(Gd，Zr)(OH)_x·yH₂O共沉淀作前驱体，在强碱性介质中用水热法合成了(ZrO₂)_0.92(Gd₂O₃)_0.08纳米立方晶，考察了反应温度、pH值等水热反应条件对纳米晶粒大小的影响。将(ZrO₂)_0.92(Gd₂O₃)_0.08     

Thermoelectric properties of Ag-doped n-type (Bi2Te3)0.9-(Bi2−xAgxSe3)0.1 (x=0-0.4) alloys prepared by spark plasma sintering

Ag-doped n-type (Bi₂Te₃)_0.9-(Bi_2−xAg_xSe₃)_0.1 (x=0-0.4) alloys were prepared by spark plasma sintering and their physical properties evaluated. When at low Ag content (x=0.05), the temperature dependence of the lattice thermal conductivity follows the trend of (Bi₂Te₃)_0.9-(Bi₂Se₃)_0.1; while at higher Ag content, a relatively rapid reduction above 400 K can be observed due possibly to the enhancement of scattering of phonons by the increased defects. The Seebeck coefficient increases with Ag content, with some loss of electrical conductivity, but the maximum dimensionless figure of merit ZT can be obtained to be 0.86 for the alloy with x=0.4 at 505 K, about 0.2 higher than that of the alloy (Bi₂Te₃)_0.9-(Bi₂Se₃)_0.1 without Ag-doping.     

Crystal Structure of Pseudorhombohedral InFe1−xTixO3+x/2 (x=2/3)

The structure of pseudorhombohedral-type InFe_1−xTi_xO_3−x/2 (x=2/3) was refined by Rietveld profile fitting. The crystal is a commensurate member of a series in a solution range on InFeO₃-In₂Ti₂O₇ including incommensurate structures. The structure with the unit cell of a=5.9188(1), b=10.1112(2), and c=6.3896(1) Å, β=108.018(2)°, and a space group P2₁/a is the alternate stacking of an edge-shared InO₆ octahedral layer and an Fe/Ti-O plane along c^*. Metal sites on the Fe/Ti-O plane are surrounded by four oxygen atoms on the Fe/Ti-O plane and two axial ones. Electric conductivities of the order 10⁻⁴ S/cm were observed for the samples at 1000 K, while the oxide ion transport number is almost zero as no electromotive force was detected by an oxygen concentration cell.     

Defect structure of Sb2−xMnxTe3 single crystals

Incorporation of the transition metal elements in the tetradymite structure of Sb₂Te₃ has a strong influence on electronic properties. Recent studies have indicated that Mn substitutes on the Sb sublattice increases the carrier concentration of holes. However, the doping efficiency of Mn appears rather low in comparison to what it should be based on the measurements of magnetization, structural analysis, and transport properties. In this paper we address this issue by making detailed studies of the Hall effect and electrical resistivity and we explain the results with the aid of a model that takes into account interactions of the Mn impurity with the native defects in antimony telluride. Specifically, we find that Mn atoms interact with antisite defects (antimony atoms located on the tellurium sublattice), a process that decreases the density of antisite centers and generates free electrons. These, in turn, recombine with holes and thus decrease their concentration and the apparent Mn doping efficiency.     

Modulated structure of the composite crystal InCr1−xTixO3+x/2

Incommensurately modulated structure of the composite crystal InCr_1−xTi_xO_3+x/2 was refined by the profile fitting of powder X-ray diffraction based on the four-dimensional superspace group. The crystal consists of two monoclinic subsystems mutually incommensurate in b. The first subsystem is the alternate stacking of an edge-shared InO₆ octahedral layer and a Cr/Ti triangle-lattice plane along c^*. A sheet of oxygen atoms constructing the second subsystem is also extending on the Cr/Ti plane. The whole structure is the alternate stacking of an edge-shared InO₆ octahedral layer and a Cr/Ti-O plane, where displacive modulation of O ions is prominent. Metal ions on the Cr/Ti-O plane are surrounded by three or four oxygen ions on the plane and, in addition, two axial ones.     

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.     

空气中合成固溶体荧光粉Ba2(Zn, Mg)Si2O7:Ce3+,Eu2+,Eu3+及其发光特性

采用高温固相法在空气中合成了Ba_1.97-yZn_1-xMg_xSi₂O₇:0.03Eu,yCe³⁺系列荧光粉。分别采用X-射线衍射和荧光光谱对所合成荧光粉的物相和发光性质进行了表征。在紫外光330~360 nm激发下,固溶体荧光粉Ba_1.97-yZn_1-xMg_xSi₂O₇:0.03Eu的发射光谱在350~725 nm范围内呈现多谱峰发射,360和500 nm处有强的宽带发射属于Eu²⁺离子的4f⁶5d¹-4f⁷跃迁,590~725 nm红光区窄带谱源于Eu³⁺的⁵D₀-⁷F_J (J=1,2,3,4)跃迁,这表明,在空气气氛中,部分Eu³⁺在Ba_1.97-yZn_1-xMg_xSi₂O₇基质中被还原成了Eu²⁺;当x=0.1时,荧光粉Ba_1.97Zn_0.9Mg_0.1Si₂O₇:0.03Eu的绿色发光最强,表明Eu³⁺被还原成Eu²⁺离子的程度最大。当共掺入Ce³⁺离子后,形成Ba_1.97-yZn_0.9Mg_0.1Si₂O₇:0.03Eu,yCe³⁺荧光粉体系,其发光随着Ce³⁺离子浓度的增大由蓝绿区经白光区到达橙红区;发现名义组成为Ba_1.96Zn_0.9Mg_0.1Si₂O₇:0.01Ce³⁺,0.03Eu的荧光粉的色坐标为(0.323,0.311),接近理想白光,是一种有潜在应用价值的白光荧光粉。讨论了稀土离子在Ba₂Zn_0.9Mg_0.1Si₂O₇基质中的能量传递与发光机理。     

Synthesis and properties of the Co7Se8−xSx and Ni7Se8−xSx solid solutions

We report the synthesis and elementary properties of the Co₇Se_8−xS_x (x=0-8) and Ni₇Se_8−xS_x (x=0-7) solid solutions. Both systems form a NiAs-type structure with metal vacancies. In general, the lattice parameters decrease with increasing x, but in the Ni₇Se_8−xS_x system c increases on going from x=5 to 7. Magnetic susceptibility measurements show that all samples exhibit temperature-independent paramagnetism from 25-250 K. Samples within the Co₇Se_8−xS_x system, as well as Ni₇Se₈ and Ni₇SeS₇, were found to be poor metals with resistivities of ∼0.20 and ∼0.06 mΩ cm at 300 K, respectively. The Sommerfeld constant (γ) was determined from specific heat measurements to be ∼13 mJ/mol_CoK² and ∼7 mJ/mol_NiK² for Co₇Se_8−xS_x and Ni₇Se_8−xS_x, respectively.     

La7O6(BO3)(PO4)2:Eu3+/Tb3+荧光材料的制备及其光致发光性能

采用优化的高温固相方法制备了稀土离子Eu³⁺和Tb³⁺掺杂的La₇O₆（BO₃）（PO₄）₂系荧光材料,并对其物相行为、晶体结构、光致发光性能和热稳定性进行了详细研究。结果表明,La₇O₆（BO₃）（PO₄）₂：Eu³⁺材料在紫外光激发下能够发射出红光,发射光谱中最强发射峰位于616 nm处,为⁵D₀→⁷F₂特征能级跃迁,Eu³⁺的最优掺杂浓度为0.08,对应的CIE坐标为（0.610 2,0.382 3）;La₇O₆（BO₃）（PO₄）₂：Tb³⁺材料在紫外光激发下能够发射出绿光,发射光谱中最强发射峰位于544 nm处,对应Tb³⁺的⁵D₄→⁷F₅能级跃迁,Tb³⁺离子的最优掺杂浓度为0.15,对应的CIE坐标为（0.317 7,0.535 2）。此外,对2种材料的变温光谱分析发现Eu³⁺和Tb³⁺掺杂的La₇O₆（BO₃）（PO₄）₂荧光材料均具有良好的热稳定性。     

Thermochemistry of Li1+xMn2−xO4 (0?x?1/3) spinel

Lithium substituted Li_1+xMn_2−xO₄ spinel samples in the entire solid solution range (0?x?1/3) were synthesized by solid-state reaction. The samples with x<0.25 are stoichiometric and those with x?0.25 are oxygen deficient. High-temperature oxide melt solution calorimetry in molten 3Na₂O·4MoO₃ at 974 K was performed to determine their enthalpies of formation from constituent binary oxides at 298 K. The cubic lattice parameter was determined from least-squares fitting of powder XRD data. The variations of the enthalpy of formation from oxides and the lattice parameter with x follow similar trends. The enthalpy of formation from oxides becomes more exothermic with x for stoichiometric compounds (x<0.25) and deviates endothermically from this trend for oxygen-deficient samples (x?0.25). This energetic trend is related to two competing substitution mechanisms of lithium for manganese (oxidation of Mn³⁺ to Mn⁴⁺ versus formation of oxygen vacancies). For stoichiometric spinels, the oxidation of Mn³⁺ to Mn⁴⁺ is dominant, whereas for oxygen-deficient compounds both mechanisms are operative. The endothermic deviation is ascribed to the large endothermic enthalpy of reduction.     

Evolution of structure and magnetic properties in electron-doped double perovskites, Sr2−xLaxMnWO6 (0?x?1)

Powder neutron and X-ray diffraction studies show that the double perovskites in the region 0?x?1 exhibit two crystallographic modifications at room temperature: monoclinic P2₁/n and tetragonal I4/m, with a boundary at 0.75<x<0.9. Magnetic susceptibility measurements indicate that for x=0 and 0.5 Sr_2−xLa_xMnWO₆ orders antiferromagnetically (AFM) at 15 and 25 K, respectively, for 0.75?x<1.0, a contribution of weak ferromagnetism (FM), probably due to canted-AFM order, increases with increasing x. The end point compound SrLaMnWO₆ shows the strongest FM cluster effect; however, no clear evidence of magnetic order is discernable down to 4.2 K. X-ray absorption spectroscopy (XAS) confirms Mn²⁺ and mixed-valent W^6+/5+ formal oxidation states in Sr_2−xLa_xMnWO₆.     

Dopant substitution and oxygen migration in the complex perovskite oxide Ba3CaNb2O9: A computational study

Atomistic simulation methods have been used to study the defect chemistry of the complex perovskite oxide Ba₃CaNb₂O₉. Calculations were carried out for the hexagonal (P-3m1) phase and the cubic (Fm-3m) phase. The hexagonal structure is predicted to be energetically more stable at room temperature. In both structures the most favourable dopant for Nb⁵⁺ is found to be Ca²⁺ rather than Mg²⁺, in contrast to the generally accepted rule that size similarities govern such processes. The diffusion of oxygen vacancies in the hexagonal and cubic phases occurs within different networks of corner-sharing NbO₆ and CaO₆ octahedra. Irrespective of the arrangement of octahedra, however, migration of oxygen vacancies around NbO₆ octahedra takes place with lower activation energies than around the CaO₆ octahedra.     

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.