Polymorphisme de l'hydrure de magnesium sous haute pression

The structural relationships in magnesium hydride have been studied under P-T conditions up to 80 kbar and 900°C, α-MgH₂ has a rutile structure under normal conditions and transforms into two new phases under high pressures: β-MgH₂ and γ-MgH₂ with a hexagonal (pseudocubic) and an orthorhombic unit-cell structure respectively. β-MgH₂ may be related to a fluorite type form (with an eight-coordination for Mg), transforming when the pressure is released into γ-MgH₂ having the αPbO₂ type structure (with a six-coordination for Mg). The density of the α-MgH₂ and γ-MgH₂ are very similar, but the density of the β-MgH₂ fluorite form is some 25% greater than that of the rutile type. Thus MgH₂ seems to exhibit under high pressure conditions the typical behavior of a number of rutile type compounds MX₂ such as fluorides or oxides.     

Synthese et etude de la conductivite anionique de nouveaux electrolytes MBiF4 (M  K,Rb, Tl)

New anionic electrolytes of formulation MBiF₄ (M  K, Rb, Tl) have been prepared. KBiF₄, RbBiF₄ and the high temperature β-TlBiF₄ variety have the fluorite type structure, low-temperature α-TlBiF₄ has an ordered α-PbF₂ structure. The materials have a conductivity of the same order of magnitude than β-alumina.     

Sur une variete haute pression de PdF2 de type fluorine

A fluorite-type form of PdF₂ has been obtained from the rutile-type variety at 25°C under 50 kb and quenched to room conditions. It is the first d-element fluoride presenting this structure so far isolated under normal conditions. The transition corresponds to a volume decrease $\text{δ}\text{V}\text{V}{}_{\mathrm{rutile}}$ of 11 %. The cell parameter $\text{a}\text{= 5.322 ± 0.002}\text{A}\text{?}$ supposes a PdF distance of 2.16 Å. The thermal variation of the magnetic susceptibility suggests an antiferromagnetic behavior with an ordering temperature close to 200 K. The rutile → fluorite transition has been followed by resistivity vs. pressure measurements. Fluorite-type PdF₂ presents at room conditions a resistivity of 10⁵Ω.cm, 10⁵ lower than the rutile form.     

Preparation de nouveaux oxydes de molybdene IV et de lanthanide de formule Ln2MoO5

Molybdenum IV compounds with formula Ln₂MoO₅ have been prepared under controlled atmosphere at 1473 K for all lanthanides with exception of cerium. The higher oxygen pressure limit of each Ln₂MoO₅ compound has been determined : Mo^IV is more strongly stabilized under very low oxygen partial pressures than in the MoO₂ oxide. A range of pressure where molybdenum is hexa- and pentavalent has been found with the lighter lanthanides from La to Gd. Ln₂MoO₅ compounds are of Yb₂ReO₅ type and indexed in a monoclinic cell. At high temperature they transform reversibly into a cubic fluorite type structure (Ln = Dy to Yb). Magnetic susceptibility measurements show that only Ln³⁺ ions contribute to the paramagnetic properties, like in the homologous rhénium IV compounds.     

Synthese hydrothermale a tres haute pression de deux borates de type olivine,AlMgBO4 et FeNiBO4

Two borates compounds with olivine type of structure have been prepared by hydrothermal synthesis under high pressure conditions: the mineral sinhalite with the formula AlMgBO₄ and a new dense form of the warwickite type FeNiBO₄. The density of this high pressure phase is about 5% higher than that of the warwickite form. This is the first example of dimorphism in a warwickite type of compound.     

Preparation and characterization of lithium aluminate

The synthesis of LiAlO₂ by reaction of alumina with mixtures of alkali carbonates or with alkali hydroxides was studied. At 600–700°C, α-LiAlO₂ was the major allotropic phase produced by the reaction of γ-Al₂O₃ with mixtures of Li₂CO₃ and K₂CO₃. Reaction of LiOH (or mixtures of LiOH and KOH) with γ-Al₂O₃ at 450°C produced only β-LiAlO₂ while reaction of LiOH with α-Al₂O₃ at 450°C produced mainly γ-LiAlO₂. The major product of the reaction with LiOH at 600°C was γ-LiAlO₂, regardless of the Al₂O₃ allotrope used. The α- and β- LiAlO₂ powders were observed by scanning electron microscopy to have three distinguishable particle morphologies, which were classified as clumps, platelets and rods. Both α-LiAlO₂ and β-LiAlO₂ particles were produced in the form of clumps and platelets while β-LiAlO₂ particles were also produced in the form of rods.     

Effect of high pressure on diffusion reactions and phase diagram in Al–Mg system

Abstract

Diffusion reactions have been investigated under high pressures of up to 3·3 GPa in the Al–Mg system. The β-, γ-, and ε- phases appeared in the diffusion zone under vacuum, but only the γ- phase was found in the diffusion zone under high pressure. The occurrence of this phase in the diffusion zone is discussed using computed phase diagrams of the Al–Mg system under high pressure.

MST/402     

Characterization of plasma-sprayed titanium aluminates by electron spin resonance spectroscopy

The structures of plasma-sprayed titanium aluminates containing 13 wt.% and 40 wt.% TiO₂ were investigated using electron spin resonance (ESR) and X-ray diffraction. These sprayed materials were identified as mixtures of α-Al₂O₃, γ-Al₂O₃, β-Al₂TiO₅ and rutile phases. An ESR peak observed near g = 1.9 at room temperature and 77 K was assigned to Ti³⁺ ions in octahedral environments. The relations between the halfwidths of the α-Al₂O₃, γ-Al₂O₃ and β-Al₂TiO₅, the linewidth and the g value of the ESR peak suggest that the Ti³⁺ ions are in the β-Al₂TiO₅ lattice. Further, the degree of asymmetry of the environments of the Ti³⁺ ions in the β-Al₂TiO₅ phase depends on the crystallinity of β-Al₂TiO₅.     

Sur de nouveaux conducteurs anioniques de hautes performances

The study of ionic conduction in fluorides with fluorite type structure leads to new anionic superconductors of formulation Pb_1?xBi_xF_2+x (0 ? x ? 0.50). The strong polarizability of the Pb²⁺ and Bi³⁺ cations gives a highly conducting material. In particular Pb_0.75Bi_0.25F_2.25 has a conductivity close to that of β-alumina.     

A mild transformation of γ-FeOOH to γ-Fe2O3 using organic reagents

The layered compound, FeOCl is known to form stable intercalates with a large number of organic molecules. When the isostructural γ-FeOOH is treated with organic bases at temperatures of 120–140°C, it is quantitatively converted to the topotactically related γ-Fe₂O₃. The first step in this base assisted dehydration is presumed to be an intercalation. The second and possibly faster step could be a sequential elimination of water from the interlayer, the role of the base being catalytic. Slightly impure samples of γ-FeOOH that contain a small (5%) amount of α-FeOOH are transformed quantitatively into α-Fe₂O₃ under the above conditions but pure α-FeOOH is unaffected. A possible rationale for this observation is given.     

High pressure/low temperature sintering of nanocrystalline alumina

Bulknanocrystalline α-Al₂O₃samples with a relative density >98% and a grain size < 50 nm have been produced by high pressure/low temperature sintering, using a toroidal-type high pressure apparatus. Nanocrystalline (n-) alumina powder with metastable γphase was used as the starting material. During sintering, the γphase transforms to αphase. The transformation temperature decreases from ~1075 °C at ambient pressure to about 460 °C at 8 GPa. Grain growth is limited by the low sintering temperature, and a multiplicity of nucleation events in the parent γ phase at very high pressure creates a nanoscale α grain size. The average grain size of the α-Al₂O₃ increases from 18 nm in the original powder to only about 49 nm in the sintered compact (98.2% dense). In addition, we found that high pressure could increase the nucleation rate while reducing the growth rate of the transformed α phase so that its grain size decreased with sintering pressure under the same sintering temperature and time. Due to its high surface area, n-Al₂O₃ powder readily absorbs chemical species from the environment. Alumina hydrates, formed by the reaction of Al₂O₃ with chemisorbed OH⁻ species during sintering, had a profound influence on sintering and phase transformation behaviors of n-Al₂O₃. To control grain size of the transformed α phase, it is essential to eliminate the hydrates before sintering.     

A preparation and polymorphic relations of sodium iron oxide (NaFeO2)

The formation of α-NaFeO₂ and β-NaFeO₂ below 700°C was found to depend strongly on the structure of starting iron oxide; α-Fe₂O₃ with hcp of oxygen yielded β-NaFeO₂ with hcp and γ-Fe₂O₃ with ccp of oxygen yielded α-NaFeO₂ with ccp by a reaction either with Na₂CO₃ or Na₂O₂. The doping of Na-ions into the γ-Fe₂O₃ structure and its rapid change to α-NaFeO₂ were observed. The transformation from the metastable β-NaFeO₂ to the stable α-form was very much sluggish in the solid state, but it was accelerated by the presence of the liquid Na₂O₂.     

Influence of different annealing processes under various atmospheres on the oxidation behaviour of γ-TiAl

Abstract

A detailed study of the oxidation behaviour of bare γ-TiAl based alloy Ti–45Al–8Nb under various conditions, such as different atmospheres, pressures, temperatures (900°C, 1000°C) and times (100–200 h) is presented. Under high vacuum conditions (10^–6 mbar) a continuous zone of α-₂Ti₃Al was formed at the surface with an oxygen-enriched phase on top. No oxide scale formation was obvious. During thermal treatment under Ar-atmosphere at low vacuum pressure (approximately 50 mbar) mainly nitrides (TiN, Ti₂AlN) and Al₂O₃ particles were formed at the surface with an α-₂layer below. Annealing γ-TiAl in hydrogen atmosphere (about 1040 mbar) led to the formation of a thick reaction zone. A TiO₂ layer was formed on top, followed by a mixed oxide scale. Beneath that scale a thick region with alumina, σ–Nb₂Al and α-₂Ti₃Al was observed.

Moreover, the oxidation behaviour of several thermally pre-treated samples was tested by cyclic oxidation at 900°C in air. The microstructure of the oxide scale formed after testing can be compared with that of non pre-treated material. γ-TiAl annealed under high vacuum conditions exhibits the lowest oxidation rate, while the mass gain of specimens pre-treated under Ar-atmosphere increased rapidly in the first cycles. All pre-treated specimens exceeded a lifetime of 600 cycles at minimum. The reference material failed after 520 cycles.     

Polymorphisme de l'hexahydroaluminate trisodique Na3AlH6

Structural investigations on the cryolite-like compound Na₃AlH₆ have been carried out using high-temperature diffractometry. α-Na₃AlH₆ which crystallises in a monoclinic pseudo-cubic system under room-temperature conditions exhibits at 525 K a polymorphic transition into a f.c.c. form β-Na₃AlH₆. This result has been established by high-pressure-high-temperature studies. From structural point of view Na₃AlH₆-β is to be related to the high temperature modification of cryolite Na₃AlF₆     

Synthesis and photoelectrochemical properties of thin bismuth molybdates film with various crystal phases

Bismuth molybdate films with various phase structures including α-Bi₂Mo₃O₁₂, β-Bi₂Mo₂O₉, γ-Bi₂MoO₆, and γ′-Bi₂MoO₆ are fabricated on the indium-tin oxide glass substrates from an amorphous heteronuclear complex via the dip-coating method by appropriate adjustment of the reaction conditions. α-Bi₂Mo₃O₁₂, β-Bi₂Mo₂O₉, and γ-Bi₂MoO₆ film can be obtained at 400 °C, 500 °C, and 500 °C for 1 h, respectively. At 500 °C, γ′-Bi₂MoO₆ can be obtained for 4 h. Film formation process is proposed based on the experimental results. Thin γ-Bi₂MoO₆ films exhibit high photoresponse under visible light irradiation. Incident photon to current conversion efficiency of thin γ-Bi₂MoO₆ film starts to increase near 450 nm. And, it can reach 4.1% at 400 nm. The top of the valence band and bottom of the conduction band are roughly estimated to be − 0.71 and 1.69 eV, respectively. In contrast, γ′-Bi₂MoO₆ generated weak photocurrent; α-Bi₂Mo₃O₁₂ and β-Bi₂Mo₂O₉ film has no photoresponse under visible light irradiation. The reason for the difference in the visible light response was discussed.     

Microstructure evolution of Al-Mg alloy during solidification under high pressure

The microstructure of Al-21.6Mg alloy solidified under high pressure was investigated. The results show that the amount of β-Al₃Mg₂ phase decreases with increasing pressure and a supersaturated Al(Mg) solid solution is formed under 2 GPa. The distribution of Mg in the solid solution is inhomogeneous, causing peak asymmetry of the XRD patterns under high pressures. The Mg concentration in the interdendritic region extends up to about 30 at.%, which is higher than that in the dendrite. Besides solution treatment, mechanical alloying, mechanical deformation of pre-alloy ingots and rapid solidification, solidification under high pressure is proved to be a new way to prepare supersaturated Al(Mg) solid solution.     

The crystal structure of epitaxially grown SnO2 thin films

It is shown by computer simulation of the transmission electron diffraction patterns and by structural considerations that the recently reported thin film form β-SnO₂ is equivalent to the high pressure form SnO₂-II. It crystallizes in a columbite-like structure either under the influence of the substrate in the case of epitaxial oxidation of α-SnO to SnO₂ or under the influence of volume contraction conditioned by enhanced pressure and/or temperature during the cassiterite- columbite transformation. The columbite-brookite-cassiterite structural relationship is given and the possibility of the existence of a thin film fluorite-like polymorph is discussed.     

Precipitation in Al–Mg solid solution prepared by solidification under high pressure

The precipitation in Al–Mg solid solution containing 21.6 at.% Mg prepared by solidification under 2 GPa was investigated. The results show that the γ-Al₁₂Mg₁₇ phase is formed and the β′ phase cannot be observed in the solid solution during ageing process. The precipitation of γ and β phases takes place in a non-uniform manner during heating process, i.e. the γ and β phases are first formed in the interdendritic region, which is caused by the inhomogeneous distribution of Mg atoms in the solid solution solidified under high pressure. Peak splitting of X-ray diffraction patterns of Al(Mg) solid solution appears, and then disappears when the samples are aged at 423 K for different times, due to the non-uniform precipitation in Al–Mg solid solution. The direct transformation from the γ to β phase is observed after ageing at 423 K for 24 h. It is considered that the β phase is formed through a peritectoid reaction of α + γ → β which needs the diffusion of Mg atoms across the interface of α/γ phases.     

The crystal structures of lead dioxides from the positive plate of the lead/acid battery

Full profile neutron powder diffraction structure refinements have been performed on electrolytically- and chemically-prepared varieties of the dimorphs of lead dioxide, the primary constituents in the charged positive plate of the lead/acid battery. Electrolytic samples of α-PbO₂ (columbite related structure) contain significant amounts of noncrystalline material, have severe cation disorder, and display evidence of severe lattice strain and unit cell incoherency in the [010] direction. On the other hand, chemically-prepared varieties of α-PbO₂ are more highly crystalline, are essentially stoichiometric, and are well ordered. Electrolytic samples of β-PbO₂ (rutile structure) from both new and used battery plates also contain more noncrystalline material than chemically-prepared forms, but all varieties are near-stoichiometric, well ordered, and structurally indistinguishable insofar as coherent neutron diffraction is concerned. Given that positive plates cycled under normal conditions for long periods of time consist primarily of the beta form of lead dioxide, the onset of battery failure is therefore not manifest in the long range crystal structure of this dioxide. In view of the high proportion of noncrystalline material present in electrolytic samples of PbO₂, property changes previously associated with loss of crystalline dioxide activity may relate instead to the “amorphous” component of the positive plate mass.     

First principles investigation of electronic, optical and transport properties of α- and β-phase of arsenic telluride

Crystalline arsenic telluride exists in two stable phases. The monoclinic α-phase transforms to rhombohedral β-phase under high pressure. The electronic, optical and transport properties of the two phases has been investigated using full potential linear augmented plane wave (LAPW) + local orbitals (lo) scheme, in the framework of DFT with generalized gradient approximation (GGA). We present the energy bands, density of states and optical properties like the complex dielectric functions and absorption coefficients. From the dynamic dielectric constant, the structural anisotropy for the monoclinic α-phase is clearly observed, whereas the longitudinal and transverse components are almost identical for the β-phase. The optical absorption profiles clearly indicate that β-phase has possibility of greater multiple direct and indirect interband transitions in the infrared and visible regions compared to the α-phase. The rhomohedral phase which has the Bi₂Te₃ type structure has the possibility of thermoelectric properties, therefore transport properties like electrical and thermal conductivities, Seebeck and Hall coefficients etc. are also calculated. Good agreements are found with the available experimental results.