Pressure-induced structural transformations in lanthanide titanates: La2TiO5 and Nd2TiO5

The structure of orthorhombic rare earth titanates of La₂TiO₅ and Nd₂TiO₅, where Ti cations are in five-fold coordination with oxygen, has been studied at high pressures by X-ray diffraction (XRD), Raman scattering measurements, and quantum mechanical calculations. Both XRD and Raman results indicated two pressure-induced phase transitions during the process. An orthorhombic super cell (a×b×2c) formed at a pressure between 6 and 10 GPa, and then transformed to a hexagonal high-pressure phase accompanied by partial decomposition. The hexagonal high-pressure phase is quenchable. Detailed structural analysis indicated that the five-coordinated TiO₅ polyhedra remain during the formation of super cell, but the orthorhombic-to-hexagonal phase transition at high pressures is a reconstructive process, and the five-fold Ti-O coordination increased to more than 6. This phase transition sequence was verified by quantum mechanical calculations.     

Fe0.45S0.55-Ni0.66S0.34 section of the Fe-Ni-S phase diagram

The Fe_0.45S_0.55-Ni_0.66S_0.34 section of the Fe-Ni-S phase diagram was constructed using thermal analysis, microscopy, X-ray powder diffraction, and electron probe microanalysis data obtained for the samples prepared by isothermal annealing and thermal analysis and by directional solidification of Fe_0.2665Ni_0.2665S_0.467 melt under quasi-equilibrium conditions. Four invariant phase reactions are found in this section. The solid-phase mechanism of pentlandite formation is verified. Original Russian Text ? V.I. Kosyakov, E.F. Sinyakova, 2009, published in Zhurnal Neorganicheskoi Khimii, 2009, Vol. 54, No. 7, pp. 1212–1219.     

Thermodynamic study of the system NbO2Nb2O5 at high temperature

The partial pressure of oxygen in equilibrium with niobium oxides was determined from thermogravimetric gas-equilibrium measurements over the range 1000–1100°C. Hysteresis phenomena were shown, and a method has been perfected to eliminate these phenomena. Two nonstoichiometric stable phases, NbO_2.42 and NbO_2.47, were found. A new partial phase diagram is proposed.     

Solid solutions of perovskite in the LaO1.5-BaO-ScO1.5-ZrO2 system at 1600 °C

A partial pseudoternary phase diagram of the LaO_1.5-BaO-ScO_1.5 system was established at 1600 °C. According to the phase diagram, the solubility of barium into the cubic perovskite phase (LaScO₃) at 1600 °C is 0.24 in a mole fraction of barium oxide (X_BaO) on the composition line where the mole fraction of scandium oxide is 0.50. Another cubic perovskite phase (BaZrO₃) in the BaO-ZrO₂-ScO_1.5 system is also known. We investigated the phase relationships between the two cubic perovskites in the pseudoquaternary phase diagram of the LaO_1.5-BaO-ScO_1.5-ZrO₂ system. As a result, we found the existence of a wide solid solution region between the cubic perovskites at 1600 °C. The region was determined by X-ray diffraction (XRD) analysis and energy dispersive X-ray (EDX) microanalysis of samples with various compositions, and established the partial pseudoquaternary phase diagram.     

Mössbauer study of the thermal decomposition products of BaFeO4

A pure sample of a hexavalent iron compound, BaFeO₄, was decomposed at temperatures below 1200°C at oxygen pressures from 0.2 to 1500 atm. In addition to the already known BaFeO_x (2.5 ≦ x < 3.0) phases with hexagonal and triclinic symmetry, two new phases were obtained as decomposition products at low temperatures. One of the new phases, with composition BaFeO_{2.61 – 2.71}, has tetragonal symmetry; lattice constants are a₀ = 8.54 Å, c₀ = 7.29 Å. The phase is antiferromagnetic with Néel temperature estimated to be 225 ± 10 K. Two internal fields observed on its Mössbauer spectra correspond to Fe³⁺ and Fe⁴⁺. In the other new phase, with composition BaFeO_2.5, all Fe³⁺ ions had the same hyperfine field; it too is antiferromagnetic with a Néel temperature of 893 ± 10 K. Mössbauer data on the hexagonal phase coincided with earlier results of Gallagher, MacChesney, and Buchanan [J. Chem. Phys.43, 516 (1965)]. In the triclinic-I BaFeO_2.50 phase, internal magnetic fields were observed at room temperature, and it was supposed that there were four kinds of Fe³⁺ sites. The phase diagram of BaFeO_x system was determined as functions of temperature and oxygen pressure.     

Determination experimentale,calcul et prevision de certains diagrammes Sc2O3Ln2O3

A calculation, in the regular solution approximation, of the solidification diagram for some Sc₂O₃Ln₂O₃ (Ln: Ho, …, Lu, and Y) systems is presented. Experimental and calculated results obtained for these systems are compared.     

High pressure induced coordination evolution in chain compound Li2CuO2

Using diamond anvil cell technique with angle dispersive X-ray diffraction (ADXD) of synchrotron radiation and electrical conductivity measurements, we have observed that CuO₂ chain compound Li₂CuO₂ transforms from ambient orthorhombic symmetry into a new phase at above 5.4 GPa and room temperature. The new phase was found to be of monoclinic structure with an increased oxygen coordination number of Cu²⁺ from four at ambient to six at high pressure that provides a structural basis of the evolution of principle physical properties. The high pressure phase of Li₂CuO₂ is discussed in line with the first principle calculations.     

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.     

Phase equilibrium and electrical conductivity of SrCo0.8Fe0.2O3−δ

The phase diagram of the SrCo_0.8Fe_0.2O_3−δ compound has been determined at high temperatures (823?T?1223 K) and in the oxygen partial pressure range (10⁻⁵?pO₂?1 atm) by thermogravimetric measurements of the equilibrium pO₂, high temperature X-ray diffraction and electrical conductivity measurements. The cubic perovskite phase SrCo_0.8Fe_0.2O_3−δ is stable in a broad range of oxygen content, while the orthorhombic brownmillerite phase SrCo_0.8Fe_0.2O_2.5 stabilizes within a small range around 3−δ=2.5 at temperatures below 1073 K. Equilibrium pO₂ measurements under isothermal conditions show chemical hysteresis at the perovskite to brownmillerite transition. The hysteresis loop decreases its amplitude in pO₂ with decreasing temperature. This behavior is discussed considering the evolution from coherent intergrowth interfaces with elastic strain energy to incoherent interfaces without elastic strain energy as T decreases. The thermodynamic quantities h_O2^oxide and s_O2^oxide for the perovskite phase decrease when increasing the oxygen defects concentration. The electrical conductivity (σ) of the cubic phase exhibits a thermally activated behavior at high temperature. The variation of σ with the oxygen content is non-linear and the activation energy varies from 0.4 to 0.28 eV as the oxygen content increases from 2.4 to 2.6. These results are interpreted in the frame of the small polaron model.     

The relative stabilities of Bi2MoO6 polymorphs

The relative stability of Bi₂Mo₆ polymorphs was studied by isothermal heating at 250–635°C, and 100–1500 kg/cm². The results obtained follow: (1) A reversible transition was observed between two stable phases at low (L) and high (H) temperatures, γ(L)-Bi₂MoO₆ with the koechlinite structure and γ(H)-Bi₂MoO₆ (=γ′ labeled by Elman). (2) A pressure-temperature phase diagram of Bi₂MoO₆ was drawn and it showed that the γ(L)-form was more stable than the γ(H)-form in the low-temperature and high-pressure region. (3) The transition temperature of γ(L) ? γ(H) under atmospheric pressure was estimated to be about 570°C by extrapolation of the phase boundary. (4) A third modification, γ″-Bi₂MoO₆ (a metastable phase), was not detected in the experiments. A free-energy-temperature diagram for the three modifications, γ(L), γ(H), and γ″, is proposed on the basis of the present experimental results and previously published data.     

Structural variations and hydrogen storage properties of Ca5Si3 with Cr5B3-type structure

The structure and phase variation of Ca₅Si₃ upon hydrogenation were systematically investigated using combined neutron powder diffraction (NPD), neutron vibrational spectroscopy (NVS), and first-principles calculations. The hydrogen absorption equilibrium was first attained with formation of Ca₅Si₃H(D)_0.53 (I4/mcm) with H exclusively located in Ca₄-tetrahedral sites. More hydrogen absorbed into the system under higher pressure leads to dissociations into CaH₂ (an amorphous hydride at higher pressures) and CaSi. The hydrogen-induced formation of an amorphous phase under higher pressures is very unusual in Cr₅B₃-type compounds and the observed formation of CaH₂ upon hydrogen absorption confirmed the proposed composition equilibrium between A₅Tt₃ (A = Ca, Sr; Tt = Si, Ge, Sn) and AH₂.     

Phase equilibria in the FeMnTiO system for the compositions Fe0.297Mn0.097Ti0.606O

This paper reports equilibrium-phase data for the FeMnTiO system at oxygen fugacities between 10^?18.2 and 10^?10.0 atm and at temperatures between 1249 and 1514K. An equilibration-and-quench method was used, and the oxygen fugacities were controlled with mixtures of hydrogen and carbon dioxide. The phase relations are presented on a log f_O2 vs $\text{1}\text{T}$ diagram and are discussed interms of the phase relations in the two systems FeTiO and MnTiO.     

Thermal analysis and thermal expansion studies on high density YBa2−xLaxCu3O7−δ, 0x0.2

The compositions in the YBa_2−xLa_xCu₃O_7−δ (0x0.2) system were prepared by the solid state reaction, employing a novel high-temperature oxygen sintering route. The modified sintering route yields dense slab like microstructures with large grains. The decomposition (incongruent melting) temperature of the YBa₂Cu₃O_7−δ (Y-123) phase was found to shift to higher temperatures with increasing oxygen partial pressure and lanthanum content. Structure remained orthorhombic up to x=0.2 with a decrease in the orthorhombic strain ((b−a)/b). Iodometric titration indicated a systematic increase in the oxygen content with increasing lanthanum content. Thermo-gravimetric studies in various oxygen partial pressures revealed that the oxygen diffusion in to the YBa₂Cu₃O_7−δ (δ>0.5) lattice is an exothermic event and takes place at temperatures not less than 573 K. High-temperature thermal-expansion measurements in air indicated that the nonlinearity in thermal expansion behaviour was reduced by the substitution of lanthanum.     

室温离子液体FeCl3-BPC体系的研究

利用差示扫描量热法(DSC)建立了无水三氯化铁和氯化正丁基吡啶(BPC)二元体系相图. 依据相图, FeCl₃和BPC形成室温离子液体的窗口是x＝0.26～0.58; 室温离子液体的深度是80 ℃. 利用UHF/6-31G^*对FeCl₃, FeCl₄^－, Fe₂Cl₇^－等配合物的几何结构、键长、能量和Raman频率进行优化, 从头算和Raman光谱证实了相图中FeCl₃摩尔分数x＝0.50处有稳定化合物存在, FeCl₄^－是主要阴离子; x＝0.67处, FeCl₄^－, Fe₂Cl₇^－是主要阴离子.     

Phase equilibrium in the Pb2SnSb2S6-SnS system

The Pb₂SnSb₂S₆-SnS system was studied over a wide range of concentrations using a set of physicochemical methods (powder X-ray diffraction, DTA, microstructure examination, and microhardness measurement). A phase diagram for the title quasi-binary join was constructed for the first time. The phase diagram is of the eutectic type; the eutectic coordinates are 25 mol % SnS and 775 K. The extents of solid solutions based on the terminal components were determined to be 10 mol % SnS and 5 mol % Pb₂SnSb₂S₆. Alloys having compositions in the SnS-based solid solution region are semiconductors.     

Oxygen deficiency and phase transitions in SrCo1– x – y Fe x Cr y O3–δ ( x =0.10–0.40, y =0–0.05)

Oxygen-deficient phases based on perovskite-like strontium cobaltites-ferrites are promising mixed conductors for high-temperature electrochemical applications. The p(O₂)-T-δ diagrams for the oxide systems SrCo_{1– x – y} Fe _x Cr _y O_{3– δ} (x=0.10–0.40; y=0–0.05) were studied at 500–1000 °C in the oxygen pressure range from 10^–5 to 0.21 atm using the coulometric titration technique and thermogravimetric analysis. Stability limits of the cubic perovskite phases having a high oxygen ionic conductivity were evaluated as functions of temperature, oxygen partial pressure and oxygen nonstoichiometry. It was found that doping with chromium and increasing the iron content in SrCo(Fe,Cr)O_{3– δ} both lead to a considerable enlargement of the cubic perovskite phase existence domain towards lower temperatures and reduced oxygen pressures. Electronic Publication     

Defect and dopant properties of the Aurivillius phase Bi4Ti3O12

Computer modelling techniques have been used to investigate the defect and oxygen transport properties of the Aurivillius phase Bi₄Ti₃O₁₂. A range of cation dopant substitutions has been considered including the incorporation of trivalent ions (M³⁺=Al, Ga and In). The substitution of In³⁺ onto the Bi site in the [Bi₂O₂] layer is predicted to be the most favourable. The calculations suggest that lanthanide (Ln³⁺) doping at the dilute limit preferentially occurs in the [Bi₂O₂] layer, with probable distribution over both the [Bi₂O₂] and the perovskite A-site at higher dopant levels. It is predicted that the reduction process involving Ti³⁺ and oxygen vacancy formation is energetically favourable. The energetics of oxide vacancy migration between various oxygen sites in the structure have been investigated.     

Pressure-induced structural changes of the tetragonal Bi2CuO4

The tetragonal compound Bi₂CuO₄ was investigated at high pressures by using in situ Raman scattering and X-ray diffraction (XRD) methods. A pressure-induced structural transition started at 20 GPa and completed at ∼37 GPa was found. The high pressure phase is in orthorhombic symmetry. Raman and XRD measurements revealed that the above phase transition is reversible.     

Structural change of layered perovskite La2Ti2O7 at high pressures

The perovskite-related layered structure of La₂Ti₂O₇ has been studied at pressures up to 30 GPa using synchrotron radiation powder X-ray diffraction (XRD) and Raman scattering. The XRD results indicate a pronounced anisotropy for the compressibility of the monoclinic unit cell. The ratio of the relative compressibilities along the [100], [010] and [001] directions is ∼1:3:5. The greatest compressibility is along the [001] direction, perpendicular to the interlayer. A pressure-induced phase transition occurs at 16.7 GPa. Both Raman and XRD measurements reveal that the pressure-induced phase transition is reversible. The high-pressure phase has a close structural relation to the low-pressure monoclinic phase and the phase transition may be due to the tilting of TiO₆ octahedra at high pressures.     

Disproportionation of stoichiometric LiMn2O4 on annealing in oxygen

The evolution of stoichiometric LiMn₂O₄ upon annealing under oxygen pressures in the range 0.2-5 atm at moderate temperature (450°C) was studied with a combination of thermogravimetry, X-ray and neutron diffraction. It is shown that such treatments result in a slight, but significant mass increase. Structural analyses show that the resulting spinel is a manganese-deficient spinel phase with lower cell parameter and higher manganese valence, and that the expelled manganese forms Mn₂O₃. The presence of this second phase, which was not identified in a recent study of oxygen annealing on this compound (Nakamura and Kajiyama, Solid State Ionics 133 (2000) 195), is compatible with the initial stoichiometry and does not require any oxygen vacancies in the initial LiMn₂O₄, as supposed earlier. The most likely formula of the resulting lithium-rich spinel with increased manganese valence is Li(Mn_2−ε□_ε)O₄ with ε in the range 0.02-0.03 at 5 atm O₂.