Phase behavior of Li2WO4 at high pressures and temperatures

The phase diagram of Li₂WO₄, previously studied by Yamaoka et al. (J. Solid State Chem.6, 280 (1973)) has been revised. Li₂WO₄ II is stable at atmospheric pressure below ～310°C. This phase appears to be a modified spinel, and is tetragonal, a, c = 11.941, 8.409Å, Z = 16, space group $\text{I4}{}_1\text{amd}$. The melting curve of phenacite-type Li₂WO₄ I rises with pressure with a slope of 0.9°C/kbar to the III/I/liquid triple point at 3.1 kbar, 743°C, beyond which the melting curve of orthorhombic Li₂WO₄ III rises steeply with pressure (initial slope 31°C/kbar). The Li₂WO₄$\text{I}\text{III}$ transition line at 3 kbar is almost independent of temperature, i.e., the $\text{I}\text{III}$ transition entropy is zero. Li₂WO₄ II is 21.3% denser than Li₂WO₄ I at ambient conditions.     

The preparation and characterization of CdVO3 prepared at ambient and high pressure

The solid state reaction of VO₂ and CdO yielded a phase of unknown structure, which transforms to CdVO₃(I) after treatment under 60–65 kbar pressure at 1200°C. The high-pressure product was characterized by crystallographic, electrical and magnetic properties. CdVO₃(I) is an orthorhombic perovskite-type compound and a metallic conductor, exhibiting Pauli paramagnetic behavior. In contrast, the ambient pressure phase displays Curie-Weiss magnetic behavior above 77°K.     

Melting curve and high-pressure polymorphism of NH4HF2

The melting curve of NH₄HF₂ I rises from 125.2°C at atmospheric pressure to a triple point II/I/liquid at 9.3 kbar, 220°C. The I/II phase boundary is terminated at a triple point III/I/II at ∼45 kbar, 295°C. The melting curve of the new phase NH₄HF₂ II passes through a broad maximum at ∼39 kbar, 306°C, and is terminated at a triple point III/II/liquid at 46.3 kbar, 301°C. The melting curve of NH₄HF₂ III rises with pressure. The NH₄HF₂ III may be a dense hydrogen-bonded phase. Liquid NH₄HF₂ appears to be anomalous in several respects, and has a high compressibility relative to the solid phases.     

Transformation of three-connected silicon in BaSi2

Solid-solid transitions in trimorphic BaSi₂ have been investigated up to 40 kbar and 1000°C by X-ray powder technique in quenched samples. All transformations between orthorhombic BaSi₂I with isolated Si-tetrahedra, trigonal BaSi₂II with corrugated Si-layers, and cubic BaSi₂III with a three-dimensional three-connected Si-net can be performed in approximately 5 min at high-pressure-high-temperature conditions. At ambient conditions the difference in molar volume between BaSI₂I and BaSi₂III is relatively large (ΔV_I–III = ?6.79 cm³/mole) and that between BaSi₂III and BaSi₂II very small (ΔV_III-II = ?0.05 cm³/mole). Consequently in the pressure-temperature phase diagram the boundary (I–III) shows a strong pressure dependence contrary to that of (III-II) which is less dependent on variation of pressure. The triple point between the three solid phases is near 11 kbar and 925°C. Substitution of divalent metal and quadrivalent metalloid can easily influence the phase relations in BaSi₂.     

Diagram of the PbF<Subscript>2</Subscript>–SnF<Subscript>2</Subscript> system

A phase diagram of the PbF₂–SnF₂ system has been studied by differential thermal analysis and X-ray powder diffraction. The system forms Pb_1–хSn_хF₂ (х ≤ 0.33) solid solution and three compounds. Pb₂SnF₆ decomposes in solid state by a peritectoid reaction at 350°С; Pb₃Sn₂F₁₀ and PbSnF₄ melt by peritectic reactions at 565 and 380°С, respectively. The eutectic coordinates are 180°С, 90 mol % SnF₂.     

Structural and electrical studies of the PbF2-BiOF system

An investigation of the PbF₂-BiOF system at 600°C has allowed isolation of a Pb_1?xBi_xO_xF_2?x solid solution (0<x? 0.80). For x ? 0.66 the structure is of fluorite type and for 0.66 <x? 0.80 it undergoes a fluorite derived rhombohedral distortion. The transport properties of Pb_1?xBi_xO_xF_2?x are for x ? 0.50 as good as those of the best fluorides.     

Synthesis,Crystal Structure,and Properties of a New Lead Aluminum Fluoride Borate,Pb6AlB2O7F7

A new compound, Pb₆AlB₂O₇F₇, was synthesized from the PbF₂/B₂O₃ flux system, which is the first compound found in the PbF₂/Al₂O₃/B₂O₃ system. It crystallizes in the orthorhombic system, space group Cmca (No. 64) with unit cell parameters a = 11.649(7) Å, b = 18.300(11) Å, c = 6.394(4) Å, Z = 4. The crystal structure of Pb₆AlB₂O₇F₇ contains the group [Pb₆BO₁₁F₁₀] as basal building unit, which connects with each other forming two‐dimensional _∞[Pb₆BO₁₁F₁₀] layers, whereas aluminum atoms are filled in the interlayers. The IR spectrum further confirms the presence of BO₃ groups. The calculated band structures and the density of states of Pb₆AlB₂O₇F₇ suggest that its indirect gap is 2.968 eV. The DSC analysis and X‐ray diffraction technique prove that Pb₆AlB₂O₇F₇ is a congruent‐melting compound.     

Crystallographic aspects of the Polymorphism of Silver Chromate and Selenate at high pressures and temperatures

Ag₂CrO₄ II at 25 °C is orthorhombic, space group D-Pnma, with the chrysoberyl structure and a₀, b₀, c₀ = 10.01, 7.01, 5.56 Å. Precipitated Ag₂SeO₄ IV has the thenardite structure. Upon heating to ～600 °C under pressure it transforms sluggishly to Ag₂SeO₄ I, which, upon cooling, then transforms to Ag₂SeO₄ III directly above 8 kbar, or via Ag₂SeO₄ II below 8 kbar. Ag₂SeO₄ III does not easily reconvert to Ag₂SeO₄ IV, and can be quenched to room temperature. It has the same structure as Ag₂CrO₄ II, and the lattice constants a₀, b₀, c₀ = 10.10, 6.95, 5.52 Å. The previously observed transitions in Ag₂SeO₄ at 425 and 537 °C do not involve Ag₂SeO₄ IV and cannot be observed until the IV–I transformation has taken place.     

Structure and optical properties of CVD molybdenum oxide films for electrochromic application

Vibrational and optical properties of MoO₃ thin films have been studied by Raman and infrared spectroscopy. The films were deposited onto Si substrates at a temperature of 150 °C by chemical vapor deposition of Mo(CO)₆ at atmospheric pressure and different amounts of oxygen in the reactor. The Raman and IR spectral analyses show that the as-deposited films are in general amorphous. Post-deposition annealing at 300 and 400 °C leads to crystallization and the MoO₃ film structure is a mixture of orthorhombic and monoclinic MoO₃ modifications. Transformation of the monoclinic crystallographic modification to a thoroughly orthorhombic layered structure is observed for films heated at temperatures above 400 °C. Electronic Publication     

Mechanochemical synthesis,structure and properties of lead containing alkaline earth metal fluoride solid solutions MxPb1-xF2 (M = Ca,Sr, Ba)

The paper deals with the mechanochemical synthesis of lead containing alkaline earth metal fluoride solid solutions M_xPb_1-xF₂ (M = Ca, Sr, Ba) by high-energy ball milling. Several metal precursors and fluorinating agents were tested for synthesizing M_0.5Pb_0.5F₂. Metal acetates and ammonium fluoride as precursors show the most promising results and were therefore used for the formation of M_xPb_1-xF₂ with different metal cationic ratios. The characterization of the local fluorine coordination and the crystal structure was performed by ¹⁹F MAS NMR spectroscopy and X-ray diffraction. Additional calculations of ¹⁹F chemical shifts using the superposition model allow a deeper insight into the local structure of the compounds. The fluoride ion conductivity was followed by temperature dependent DC conductivity measurements. Significantly higher conductivities were found in comparison with those of the corresponding binary fluorides. The highest values were observed for samples with high lead content M_0.25Pb_0.75F₂, bearing in mind the much higher conductivity of PbF₂ compared to MF₂.     

Oxydes de plomb: V. Etude de la texture des phases quadratique et orthorhombique de Pb3O4: Influence des defauts sur la transition de phase

Several samples of Pb₃O₄ have been prepared by oxidizing PbO in air at various temperatures in the range 705–815°K. A correlation is established between the nonstochiometry of the samples and their X-ray diffraction line profiles at 295°K which are characteristic of an orthorhombic distortion of the tetragonal lattice. In the high-temperature phase (T > 170°K), orthorhombic microdomains exist in the tetragonal matrix. The mean distortion increases with the nonstochiometry of the compound. Below 170°K Pb₃O₄ exhibits an orthorhombic phase with orthorhombic domains according to two orientation states, and para crystalline distortion. A model of texture is proposed and compared with the high-temperature one. The pretransitional effect which is observed between 250 and 170°K is correlated with the presence of orthorhombic microdomains in the high-temperature phase (tetragonal).     

Spectroscopy at very high pressures: Infrared study of the high pressure phase transitions in NaNO2 and KNO2

Infrared spectra of NaNO₂ and KNO₂ in the region of the anion internal modes have been obtained using a diamond anvil cell at pressures up to 65 kbar. At 39°C NaNO₂ undergoes a phase transition at ca. 10.0 kbar, not at 14 kbar as reported from an earlier IR study. A similar transition was found at 6.3 kbar for KNO₂. The symmetric modes lose intensity with increasing pressure, and all modes suffer blue-shifts. For KNO₂ ν(NO)₅ shifts much more than ν(NO)_a.     

Conductivité Anionique des Phases Isolées dans les Systèmes PbF2?SnF4 et PbF2?GeF4

Ionic Conductivity of the Phases Isolated in the Binary Systems PbF₂? SnF₄ and PbF₂? GeF₄ The study of PbF₂? MF₄ (M ?; Sn, Ge) systems reveals a restricted solid solution domain, with a fluorine structure and edfinite Pb₃GeF₁₀, Pb₃Ge₂F₁₄ and PbMF₆ phases. The ionic conductivity has been measured by complex impedance technic. It appears that, in the solid solutions the M⁴⁺ cations adopt a distorted octahedral environment.     

Pressure-products diagram of FexV1−xO2 system (0 ≤ x ≤ 0.5)

The pressure-products diagram of the Fe_xV_1?xO₂ system (0 ≤ x ≤ 0.5) was investigated under the conditions of 5 ～ 55 kbar and 800°C. In high-pressure synthesis, FeV₃O₈ phase (monoclinic), which is stable under ambient pressure, was converted to the O phase (orthorhombic) and the region of O phase was extended from x = 0.1 up to ～0.3. The O phase so obtained, showed the characteristic reversible temperature-induced phase transition to the high-temperature rutile phase under ambient pressure, prior to the decomposition to an ambient pressure form. At x = 0.5, successive transformations, FeVO₄-I (monoclinic) → -II (orthorhombic) → -III (α-PbO₂ type) → -IV (wolframite type), were observed with increasing pressure.     

Phase relation studies in Pb1−xM′xF2+x systems (0.0⩽x⩽1.0; M′=Nd3+, Eu3+ and Er3+)

In this communication we report the synthesis and characterization of a series of compounds with the general composition Pb_1−xM′_xF_2+x, (0.0⩽x⩽1.0; M′=Nd³⁺, Eu³⁺ and Er³⁺) to elucidate the detailed phase relations between PbF₂ and M′F₃. These three rare-earth fluorides were selected so as to delineate the effect of ionic size on the phase relations. In all the three systems, fluorite-type solid solutions are formed at the PbF₂ rich end. The solid solubility limits of NdF₃, EuF₃ and ErF₃ in the PbF₂ lattice, as observed from this study, are 30, 25 and 15 mol%, respectively. In PbF₂–NdF₃ system, beyond the fluorite-type solid solutions, NdF₃ phase is observed. However, in both PbF₂–EuF₃ and PbF₂–ErF₃ systems, certain fluorite related ordered phases, namely, a rhombohedral phase with about 40 mol% of EuF₃ or ErF₃ in PbF₂, and a tetragonal phase with 45–50 mol% of ErF₃ in PbF₂, are observed. In all the three systems, no solubility of the PbF₂ in the hexagonal or orthorhombic rare-earth fluoride lattice is observed. This is the first report on phase relation in these three systems under short annealed and slow cooled condition.     

Synthesis and Characterization of Indium-borate Glass-ceramics Containing Ho0.01Ce0.74Zr0.25O1.995 Nanorods via Incorporation Method

Glasses in the system 5In₂O₃·94Na₂B₄O₇ were fabricated via melt quenching technique. The amorphous nature of the quenched glasses was confirmed by X‐ray powder diffraction studies, and the infrared spectra of the glasses show no boroxol ring formation in the structure of these glasses. Differential thermal analysis is shown glass transition temperature 696°C and crystallization temperature 1151°C. A cerium‐zirconium mixed oxide Ce_0.75Zr_0.25O₂ and Ho‐doped cerium‐zirconium mixed oxide were obtained by solid‐state method. Then glass powder and Ho‐doped cerium‐zirconium mixed oxide were mixed. The mixture was heated in a crucible. The glass‐ceramic sample was obtained by pouring the melts on stainless steel. Obtained samples were annealed at 450°C for 1 h to remove thermal strain. Differential thermal analysis for glass‐ceramic sample is shown glass transition temperature 668°C and crystallization temperature 1159°C. The scanning electron microscopy study for glass‐ceramic indicates that the crystallized glass consists of rod‐like crystals with average diameter of about 38 nm dispersed in the glassy regions.     

Compound formation in the System PbGeO3?Pb5Ge3O11

Compound formation in the system PbGeO₃? Pb₅Ge₃O₁₁ was studied by thermal analysis and high-temperature X-ray diffraction. New modifications of PbGeO₃ and Pb₅Ge₃O₁₁ were prepared by the simultaneous hydrolysis of lead and germanium alkoxides, followed by washing and drying; the former has a hexagonal unit cell with a = 15.573 Å and c = 7.240 Å, and the latter has an orthorhombic crystal structure with a = 5.081 Å, b = 7.301 Å and c = 8.817 Å. They are transformed to the known monoclinic and hexagonal modifications at 575 to 610°C and 410 to 450°C, respectively. No compound of Pb₃Ge₂O₇ was confirmed. The structures of germanate groups in the lead germanate compounds are discussed on the basis of the infrared spectral data.     

Zur Neuuntersuchung des Phasendiagramms RbCl/PbCl2

Redetermination of the Phase Diagram RbCl/PbCl₂ A reinvestigation of the phase diagram RbCl/PbCl₂ revealed the existence of two not yet known phases with the formula Rb₆Pb₅Cl₁₆ and Rb₃PbCl₅. Both ternary compounds, which decompose peritectoidally at 350 °C for Rb₆Pb₅Cl₁₆ and 305 °C for Rb₃PbCl₅, crystallize in new structure types. The points of congruent melting of the 1 : 2‐phase (RbPb₂Cl₅) and of the 1 : 1‐phase (RbPbCl₃) at 420 °C and 440 °C respectively agree well with former specifications. But the transition of RbPbCl₃ at 310 °C, described by Treis [1] could not be verified. RbPbCl₃ is only stable at temperatures above 320 °C. For Rb₂PbCl₄ a point of incongruent melting was found at 440 °C and in addition a polymorphic transition may be inferred at lower temperatures.     

Effect of high pressure on the electronic properties of VO2

Samples of VO₂ were prepared at 50 kbar and 800°C in a belt apparatus. The products were then annealed in evacuated silica tubes at 800°C for 1 week. The X-ray diffraction patterns of these samples, before and after annealing, were compared with polycrystalline and ground single crystals of VO₂. In addition, the resistivity above and below 67°C was measured. The effect of high pressure on the metal-semiconducting transition is discussed in terms of the relative positions of the t₆ and π1 bands.     

Verfeinerung der Kristallstruktur von SrO2

Refinement of the Crystal Structure of SrO₂ Single crystals of SrO₂ have been obtained after high pressure/high temperature reaction of a SrO/KClO₃ mixture at 20 kbar, 1 400°C. The crystal structure was refined using 154 unique diffractometer data (I4/mmm; a = 3.5626(3), c = 6.6159(6) Å; Z = 2; R = 0.033, R_w = 0.022, S = 0.932). The O O distance (1.493(4) Å) is significantly longer than the one previously assumed. From a refinement of the site occupation factor for oxygen a composition SrO_1.95(2) has been found for the crystal investigated.