A new high-pressure phase of LiAlO2

A new high-pressure phase of LiAlO₂ has been recovered through a shock recovery technique at pressures above 9 GPa. This new phase has been refined as a tetragonal structure with lattice parameters of a=0.38866(8) nm and c=0.83001(18) nm. Its calculated density is 3.51 g/cm³, about 34% denser than γ-LiAlO₂. The aluminum and lithium cations in this new phase are six-fold coordinated, as in α-LiAlO₂ and the structure of this new phase is similar to tetragonal LiFeO₂. This new high-pressure phase is stable at temperatures up to 773 K.     

High pressure behavior of α-NaVO3: A Raman scattering study

The reported pressure-induced amorphization in α-NaVO₃ has been re-investigated using Raman spectroscopy. Discontinuous changes are noted in the Raman spectrum above 5.6 GPa implying large structural changes across the transition. The decrease in frequency of the V-O stretching mode across the transition suggests that the vanadium atom may be in octahedral coordination in the high pressure phase. Excessive broadening of the internal modes is observed above 6 GPa. New peaks characteristic of a crystalline phase gain in intensity at higher pressures in the bending modes region; however, the transformation is not complete even at 13 GPa. Co-existence of phases is noted over a significant pressure range above the onset of transition. Pressure released spectrum is found to be a mixture of crystalline α-phase, traces of crystalline β-phase and highly disordered phase consisting of V-O units in five- and six-fold coordination.     

Structural evolution of (Ca0.35Sr0.65)TiO3 perovskite at high pressures

Lattice parameters of a synthetic powder sample of Ca_0.35Sr_0.65TiO₃ perovskite have been determined by the method of Le Bail refinement, using synchrotron X-ray diffraction patterns collected at pressures up to 15.5 GPa with a membrane-driven diamond anvil cell. At ambient conditions, diffraction data were consistent with the I4/mcm structure reported previously in the literature for the same composition. Diffraction data collected at high pressures were consistent with tetragonal (or, at least, pseudo-tetragonal) lattice geometry, and no evidence was found for the development of any of the orthorhombic structures identified in other studies of (Ca, Sr)TiO₃ perovskites. Additional weak reflections, which could not be accounted for by the normal I4/mcm perovskite structure, were detected in diffraction patterns collected at pressures of 0.9-2.5 GPa, and above ∼13.5 GPa, however. Small anomalies in the evolution of unit cell volume and tetragonal strain were observed near 3 GPa, coinciding approximately with breaks in slope with increasing pressure of bulk and shear moduli for a sample with the same composition which had previously been reported. The anomalies could be due either to new tetragonal↔tetragonal/pseudo-tetragonal phase transitions or to subtle changes in compression mechanism of the tetragonal perovskite structure.     

Structural changes of NaxCoO2 (x=0.74) at high pressures

Structural changes in the layered compound γ-Na_xCoO₂ (x=0.74) are studied by in situ Raman scattering and energy-dispersive X-ray diffraction methods at pressures up to 41 GPa. The pressure dependence of the lattice parameters indicate that γ-Na_xCoO₂ has a strong anisotropic compressibility before 15 GPa and the unit cell is easily compressed between layers. The discontinuity of the lattice parameters and Raman observations reveal that a phase transition occurred at pressures between 10 and 12 GPa. The high-pressure phase has the same hexagonal symmetry and the phase transition may be due to the pressure-induced rearrangement of one of the Na cations in the unit cell.     

Structural evolution of Na0.5K0.5NbO3 at high temperatures

Changes in structure and dielectric properties at elevated temperatures have been investigated on single-crystals of sodium potassium niobate, Na_0.5K_0.5NbO₃, grown by the flux method. Single-crystal X-ray diffraction studies revealed that the crystals underwent orthorhombic-tetragonal and tetragonal-cubic phase transitions at 465 and 671 K during heating and 446 and 666 K during cooling, respectively. Both transitions were accompanied by volumetric discontinuities of collapse upon heating and expansion upon cooling, suggesting that the transitions were of the first order. The coordination numbers of an Nb showed a decreasing tendency with decreasing temperature, i.e., 6 in cubic, 5+1 in tetragonal and 4+2 in orthorhombic. An Na atom occupied a slightly different position from the K atom in 12-fold coordination, resulting in fewer coordination numbers of 8+4 in cubic and tetragonal and 7+5 in orthorhombic. The spontaneous polarisation (P_s) estimated from the atom positions and formal charges were approximately 0.29 C m⁻² in orthorhombic and 0.18 C m⁻² in tetragonal. The contribution of the alkaline oxide components to P_s was estimated to be approximately 15% in both ferroelectric forms. The temperature-induced transitions were also confirmed through the dielectric constant and dielectric loss at various frequencies and the differential scanning calorimetry.     

Sb2O4 at high pressures and high temperatures

Investigations on Sb₂O₄ at high pressure and temperature have been performed up to 600 °C and up to 27.3 GPa. The so-called “high temperature” phase (β-Sb₂O₄) was obtained following pressure increase at ambient temperature and at relatively low temperatures. Thus, in contrast to previous perceptions, β-Sb₂O₄ is the modification more stable at high pressures, i.e., at low temperatures. The fact that the metastable α-form is typically obtained through the conventional way of preparation has to be attributed to kinetic effects. The pressure-induced phase transitions have been monitored by in-situ X-ray diffraction in a diamond anvil cell, and confirmed ex-situ, by X-ray diffraction at ambient conditions, following temperature decrease and decompression in large volume devices. Bulk modulus values have been derived from the pressure-induced volume changes at room temperature, and are 143 GPa for α-Sb₂O₄ and 105 GPa for the β-Sb₂O₄.     

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.     

Crystallite Nanosize Effect on the Structural Transitions of WO3 Studied by Raman Spectroscopy

Nanocrystallites of tungsten oxide samples of 2, 4, 16, 35 and 60 nm of diameter were prepared by cryosol and pyrosol techniques. The pressure- and temperature-induced phase transitions of these samples were monitored by Raman spectrometry from 0.1 MPa to 34 GPa and from 77 to 1200 K. The tetragonal (α)-orthorhombic (β)-monoclinic (γ) transitions in these nanometric samples are strongly downshifted in temperature by comparison with the bulk WO₃. For instance, the tetragonal phase which exists above 1171 K for the bulk tungsten oxide can be stabilized at 700 K for the 35 nm sample. In the same way, the monoclinic P2₁/n-monoclinic P2₁/c high-pressure-induced transition is slightly shifted from 0.1 GPa to a higher pressure (1.5 GPa). The discussion of these transition-line shifts is based on thermodynamic considerations in which the surface energy of crystallites plays an important role.     

The phase diagram of fullerene C60 at high temperatures and pressures

Experimental and literature data were used to calculate the Gibbs energies of polymerized C₆₀ phases and construct the equilibrium T-p phase diagram of fullerene C₆₀ at temperatures from 0 to 1000 K and pressures from 0 to 8 GPa. The diagram contains stability regions of the orthorhombic, tetragonal, and rhombohedral polymerized C₆₀ phases and primitive cubic (PC) and face-centered cubic (FCC) nonpolymerized C₆₀ phases. The orthorhombic phase (linear polymer) is an equilibrium phase at 298 K and 1 bar and in the adjacent region. The equilibrium line observed experimentally (FCC C₆₀—orthorhombic phase) is well described by the phase diagram. The optimum temperatures and pressures of the synthesis of polymerized phases are determined by kinetic rather than thermodynamic parameters.     

High-pressure synthesis, crystal structure and magnetic properties of a new cuprate (Nd,Ce)2+xCaCu2O6+y

New phase (Nd,Ce)_2+xCaCu₂O_6+y was prepared at a high-pressure/high-temperature condition of 6 GPa and 1300°C. It had a nonstoichiometric composition close to Nd_2.16Ce_0.225CaCu₂O_6+y. According to X-ray diffraction pattern, the Nd_2.16Ce_0.225CaCu₂O_6+y phase has a tetragonal lattice with a = 3.845(1) Å, c = 19.349(5) Å. However, electron microscopic observations revealed a complicated shear structure for this phase. Magnetic susceptibility and magnetic hysteresis measurements were performed for the Nd_2.16Ce_0.225CaCu₂O_6+y sample and it was found that the phase undergoes a weak ferromagnetic transition at 150 K. Below ≈40 K, complicated magnetic behavior was observed suggesting the presence of second weak ferromagnetic transition near 40 K.     

Pressure-induced phase transitions in Al2(WO4)3

The high pressure behavior of aluminum tungstate [Al₂(WO₄)₃] has been investigated up to ∼18 GPa with the help of Raman scattering studies. Our results confirm the recent observations of two reversible phase transitions below 3 GPa. In addition, we find that this compound undergoes two more phase transitions at ∼5.3 and ∼6 GPa before transforming irreversibly to an amorphous phase at ∼14 GPa.     

Why pressure induces an abrupt structural rearrangement in PdTe2 but not in PtTe2

High-pressure X-ray diffraction measurements were carried out for polymeric CdI₂-type compounds MTe₂ (M=Pt, Pd) to investigate if they undergo a structural phase transition under pressure as does IrTe₂. Up to 27 GPa at room temperature PtTe₂ does not undergo any structural phase transition. In contrast, however, an abrupt change in the inter-atomic distances occurs in PdTe₂ above 15.7 GPa at room temperature, and above 5 GPa at 300 °C, but the volume vs. pressure curve exhibits no discontinuity. To account for the differences between the isostructural compounds PtTe₂, PdTe₂ and IrTe₂, their electronic structures and bonding were analyzed on the basis of first principles electronic band structure calculations.     

K2Mo4O13 phases prepared by hydrothermal synthesis

Hydrothermal synthesis in the K-Mo oxide system was investigated as a function of the pH of the reaction medium. Four compounds were formed, including two K₂Mo₄O₁₃ phases. One is a new low-temperature polymorph, which crystallizes in the orthorhombic, space group Pbca, with Z=8 and unit cell dimensions a=7.544(1) Å, b=15.394(2) Å, c=18.568(3) Å. The other is the known triclinic K₂Mo₄O₁₃, whose structure was re-determined from single crystal data; its cell parameters were determined as a=7.976(2) Å, b=8.345(2) Å, c=10.017(2) Å, α=107.104(3)°, β=102.885(3)°, γ=109.760(3)°, which are the standard settings of the crystal lattice. The orthorhombic phase converts endothermically into triclinic phase at ca. 730 K with a heat of transition of 8.31 kJ/mol.     

Energetics of formation of alkali and ammonium cobalt and zinc phosphate frameworks

Alkali and ammonium cobalt and zinc phosphates show extensive polymorphism. Thermal behavior, relative stabilities, and enthalpies of formation of KCoPO₄, RbCoPO₄, NH₄CoPO₄, and NH₄ZnPO₄ polymorphs are studied by differential scanning calorimetry, high-temperature oxide melt solution calorimetry, and acid solution calorimetry.α-KCoPO₄ and γ-KCoPO₄ are very similar in enthalpy. γ-KCoPO₄ slowly transforms to α-KCoPO₄ near 673 K. The high-temperature phase, β-KCoPO₄, is 5-7 kJ mol⁻¹ higher in enthalpy than α-KCoPO₄ and γ-KCoPO₄. HEX phases of NH₄CoPO₄ and NH₄ZnPO₄ are about 3 kJ mol⁻¹ lower in enthalpy than the corresponding ABW phases. There is a strong relationship between enthalpy of formation from oxides and acid-base interaction for cobalt and zinc phosphates and also for aluminosilicates with related frameworks. Cobalt and zinc phosphates exhibit similar trends in enthalpies of formation from oxides as aluminosilicates, but their enthalpies of formation from oxides are more exothermic because of their stronger acid-base interactions. Enthalpies of formation from ammonia and oxides of NH₄CoPO₄ and NH₄ZnPO₄ are similar, reflecting the similar basicity of CoO and ZnO.     

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.     

A high temperature superionic phase of CsSn2F5

The compound CsSn₂F₅ has been investigated over the temperature range from ambient to 545 K using differential scanning calorimetry, impedance spectroscopy and neutron powder diffraction methods. A first-order phase transition is observed from DSC measurements at 510(2) K, to a phase possessing a high ionic conductivity (σ∼2.5×10⁻² Ω⁻¹ cm⁻¹ at 520 K). The crystal structure of the high temperature superionic phase (labelled α) has been determined to be tetragonal (space group I4/mmm, a=4.2606(10) Å, c=19.739(5) Å and Z=2) in which the cations form layers perpendicular to the [001] direction, with a stacking sequence CsSnSnCsSnSn… All the anions are located in two partially occupied sites in the gap between the Cs and Sn layers, whilst the space between the Sn cations is empty, due to the orientation of the lone-pair electrons associated with the Sn²⁺. The structure of α-CsSn₂F₅ is discussed in relation to two other layered F⁻ conducting superionic phases containing Sn²⁺ cations, α-RbSn₂F₅ and α-PbSnF₄ and, to facilitate this comparison, an improved structural characterisation of the former is also presented. The wider issue of the role of lone-pair cations such as Sn²⁺ in promoting dynamic disorder within an anion substructure is also briefly addressed.     

Chemical interaction in the B-BN system at high pressures and temperatures.: Synthesis of novel boron subnitrides

Chemical interaction and phase transformations in the B-BN system have been in situ studied by X-ray diffraction with synchrotron radiation at pressures up to 5.3 GPa and temperatures up to 2800 K using multianvil press. New rhombohedral boron subnitride B₁₃N₂ has been synthesized by crystallization from the B-BN melt at 5 GPa. The structure of B₁₃N₂ belongs to the R-3m space group (a=5.4455(2) Å, c=12.2649(9) Å) and represents a new structural type. The subnitride is an individual compound and not a solid solution, in contrast to boron carbide. Besides, the formation of two other boron-rich B-N phases denoted as “B₆N” and “B₅₀N₂” has been observed. Their structures seem to be much more sophisticated and have not been even resolved to present time.     

Single crystal X-ray diffraction study of a mixed-valence gold compound, Cs2AuAuCl6 under high pressures up to 18 GPa: Pressure-induced phase transition coupled with gold valence transition

We performed the single-crystal X-ray diffraction study of a perovskite-type gold mixed-valence compound, Cs₂Au^IAu^IIICl₆, under high pressures up to 18 GPa by using a diamond-anvil-cell with helium gas as an ideal hydrostatic pressure-transmitting medium. The lattice parameters and the variable atomic positional parameters were obtained with reasonable accuracy at various pressures. A structural phase transition at ca. 12.5 GPa from I4/mmm to Pm3m was found. The lattice parameters a₀ and c₀, denoted in the tetragonal cell setting, result in the relationship 2^1/2a₀=c₀, and the superstructure reflections h k l (l is odd), caused by the shift of the Cl ions from the midpoint of the Au ions, disappeared at pressures above the phase transition. Both elongated [Au^IIICl₆] and compressed [Au^ICl₆] octahedra in the low-pressure phase smoothly approach regular octahedra with increasing pressure. Above the structural phase transition at 12.5 GPa, all the [AuCl₆] octahedra are crystallographically equivalent, which shows that the tetragonal-to-cubic phase transition accompanies the valence transition from the Au^I/Au^III mixed-valence state to the Au^II single-valence state.     

Phase stability and ionic conductivity in substituted La2W2O9

Different substitutions, i.e. Sr²⁺, Ba²⁺, K⁺, Nb⁵⁺ and V⁵⁺, have been performed in the triclinic α-La₂W₂O₉ structure in order to stabilise the high temperature and better ionic conductor cubic β-phase. This approach has been used to try to obtain a new series of ionic conductors with LAMOX-type structure without molybdenum and presumably better redox stability compared to β-La₂Mo₂O₉. Nanocrystalline materials obtained by a freeze-drying precursor method at 600 °C exhibit mainly the β-La₂W₂O₉ structure, however, the triclinic α-form is stabilised as the firing temperature increases and the crystallite size grows. Only high levels of Ba²⁺ and V⁵⁺ substitutions retained the cubic form at room temperature after firing above 1100 °C. However, these phases are metastable above 700 °C, exhibiting an irreversible transformation to the low temperature triclinic α-phase. The synthesis, structure, phase stability, kinetic of phase transformation and electrical conductivity of these materials have been studied in the present report.     

Effect of capping and particle size on Raman laser-induced degradation of γ-Fe2O3 nanoparticles

Diol capped γ-Fe₂O₃ nanoparticles are prepared from ferric nitrate by refluxing in 1,4-butanediol (9.5 nm) and 1,5-pentanediol (15 nm) and uncapped particles are prepared by refluxing in 1,2-propanediol followed by sintering the alkoxide formed. X-ray diffraction (XRD) shows that all the samples have the spinel phase. Raman spectroscopy shows that the samples prepared in 1,4-butanediol and 1,5-pentanediol and 1,2-propanediol (sintered at 573 and 673 K) are γ-Fe₂O₃ and the 773 K-sintered sample is Fe₃O₄. Raman laser studies carried out at various laser powers show that all the samples undergo laser-induced degradation to α-Fe₂O₃ at higher laser power. The capped samples are however, found more stable to degradation than the uncapped samples. The stability of γ-Fe₂O₃ sample with large particle size (15.4 nm) is more than the sample with small particle size (10.2 nm). Fe₃O₄ having a particle size of 48 nm is however less stable than the smaller γ-Fe₂O₃ nanoparticles.