High pressure X-ray diffraction study on BaWO4-II

BaWO₄-II has been synthesized at 5 GPa and 610°C. Its high pressure behavior was studied by in situ synchrotron X-ray diffraction measurements at room temperature up to 17 GPa. BaWO₄-II retains its monoclinic structure. Bulk and axial moduli determined by fitting a third-order Birch–Murnaghan equation of state to lattice parameters are: K ₀=86.2±1.9 GPa, K ₀(a)=56.0±0.9 GPa, K ₀(b)=85.3±2.4 GPa, and K ₀(c)=146.1±3.2 GPa with a fixed K′=4. Analysis of axial compressible modulus shows that the a-axis is 2.61 times more compressible than the c-axis and 1.71 times more compressible than the b-axis. The beta angle decreases smoothly between room pressure and 17 GPa from 93.78° to 90.90°.     

A high pressure,high temperature study of 1,1-diamino-2,2-dinitro ethylene

We report a synchrotron energy-dispersive X-ray diffraction study of the novel high explosive 1,1-diamino-2,2-dinitroethylene at high pressures and high temperatures. Pressure was generated using a Paris–Edinburgh cell to employ larger sample volumes. High temperatures were created using a resistive graphite cylinder surrounding the sample. The PT phase diagram was explored in the 3.3 GPa pressure range and in the ～ 400°C temperature range. We believe that the sample commenced in the α-phase and then ended up in an amorphous phase when the temperature increased beyond 280°C near 2 GPa, which we believe to be the γ-phase. Further pressure and temperature cycling suggests that the sample transformed reversibly into and out of the amorphous phase near the phase line.     

Pressure evolution of two polymorphs of Tb 2(MoO 4)3

We have studied the high pressure behavior of the α and β^′-phases of Tb ₂(MoO ₄)₃ using a combination of powder X-ray diffraction and ab initio calculations. The α-Tb ₂(MoO ₄)₃ phase did not undergo any structural phase transition in the pressure range from 0 up to the maximum experimental pressure of 21 GPa. We observed line broadening of the diffraction patterns at pressures above 7 GPa, which may be due to non-hydrostatic conditions. The complete amorphization of the sample was not reached in the pressure range studied, as expected from previous Raman studies. The behavior under pressure of the β^′-Tb ₂(MoO ₄)₃ phase is similar to that of other rare-earths trimolybdates with the same structure at room temperature. A phase transition was observed at 2 GPa. The new phase, which can be identified as the δ-phase, has never been completely characterized by diffraction studies. A tentative indexation has been performed and good refined cell parameters were obtained. We detect indications of amorphization of the δ-Tb ₂(MoO ₄)₃ phase at 5 GPa.     

Grain evolution of nano-crystals ZnO under HP and HT

Grain evolution of nano-crystals ZnO under high temperature and pressure is studied using a cubic high pressure apparatus. The structure, grain sizes and morphology of the samples are characterized by X-ray diffraction and field emission scanning electron microscopy. The results show that the grain sizes of ZnO grow rapidly at temperature 200°C under pressure. At temperature lower than 300°C (including 300°C), the grain sizes of the samples first increase with the pressure increasing from 1 to 3 GPa and later decrease from 4 to 6 GPa. The activation volume from 1 to 3 GPa and from 4 to 6 GPa is calculated respectively using the phenomenological kinetic grain growth equation at temperature 300°C. At temperature higher than 400°C (including 400°C), the grain sizes of the samples increase with the pressure increasing from 1 to 6 GPa. ZnO nano-bulks with good quality can be obtained under the specific conditions.     

The baddeleyite-type high pressure phase of Ca(OH)2

Abstract

A phase transition from Ca(OH)₂ I (portlandite) to Ca(OH)₂ II at high pressure and temperature has been confirmed, using in situ x-ray diffraction in a multianvil high pressure device (DIA). The structure was determined at 9.5 GPa and room temperature from data collected after heating the sample at 300°C at 7.2 GPa in a diamond anvil cell. Both the Le Bail fit and preliminary Rietveld refinement suggest that the new phase, which reverts to Ca(OH), I during pressure release, has a structure related to that of baddeleyite (ZrO₁); it is monoclinic (P2₁/c) with a= 4.887(2), b= 5.834(2), c = 5.587(2), β = 99.74(2)°. The coordination number of Ca increases from six to seven (5 + 2) across the transition. At 500°C, the phase boundary is bracketed at 5.7 ± 0.4 GPa by reversal experiments performed in the DIA.     

Phase change and pyroelectricity in natural hemimorphite

The phase change of a natural hemimorphite sample from Minas Gerais (Brazil) was investigated by two X-ray diffraction (XRD) methods and by near-infrared reflectance spectroscopy. Applying successive thermal treatments, the crystal structure undergoes two orientation conversions. The first one occurs at about 550 °C, and it was revealed by the Laue method. Below 500 °C, the water molecules were partially expelled without changing the crystal structure. A fact that supports this statement is the sequential disappearance of the water bands at 1400 and 1900 nm by thermal treatment. The second conversion takes place below 939 °C. Moreover, at 972 °C a phase change to the willemite mineral (α-Zn₂SiO₄) has been observed. This last conversion was confirmed by the power XRD. In addition, natural hemimorphite displayed a high pyroelectricity, which is related both to the absence of inverse centre and to the presence of molecular water and hydroxyl groups in the crystal structure.     

Equation of state and thermal expansivity of LiF and NaF

The high-pressure and high-temperature behaviors of LiF and NaF have been studied up to 37 GPa and 1000 K. No phase transformations have been observed for LiF up to the maximum pressure reached. The B1 to B2 transition of NaF at room temperature was observed at ～28 GPa, this transition pressure decreases with temperature. Unit-cell volumes of LiF and NaF B1 phase measured at various pressures and temperatures were fitted using a P–V–T Birch–Murnaghan equation of state. For LiF, the determined parameters are: α₀ = 1.05 (3)×10^?4 K^?1, dK/dT = ?0.025 (2) GPa/K, V ₀ = 65.7 (1) Å³, K ₀ = 73 (2) GPa, and K′ = 3.9 (2). For NaF, α₀ = 1.34 (4)×10^?4 K^?1, dK/dT = ?0.020 (1) GPa/K, V ₀ = 100.2 (2) Å³, K ₀ = 46 (1) GPa, and K′ = 4.5 (1).     

Synthesis and characterization of the new high pressure phases ACu3 V4O12(A=Gd,Tb, Er)

New ACu₃V₄O₁₂ (A=Gd, Tb, Er) phases have been prepared at high pressure and high-temperature conditions (P～8–9 GPa, T～1000°C) in a toroid-type high pressure cell. These compounds crystallize in the cubic symmetry with a perovskite-like structure. At ambient pressure, they are paramagnetic and have activation-type conductivity. The effect of high pressure (10–50 GPa) on the electrical properties of the materials was analyzed in the temperature range from 78 to 300 K. Pressure ranges of the transition from activation type to metallic conductivity have been determined. The crystal structure of ACu₃V₄O₁₂ (A=Gd, Tb, Er) was found to be stable up to 50 GPa.     

High pressure sintering behavior of polycrystalline diamond with tungsten

Polycrystalline diamond was investigated under high pressure and high temperature of 5.0 GPa and 1100–1500 °C in the presence of tungsten. In situ resistance measurements indicated that reactions between diamond and tungsten happened at about 960 °C. Phase analysis demonstrated that WC increased and meta-stability of W₂C decreased clearly at the higher temperature. It is clear from the characterization of the sintered body that the electrical resistance decreased and the density of specimens increased as the sintering temperature rose. The specimen sintered at 1500 °C has a homogeneous microstructure and good conductivity.     

Synthesis of nano-polycrystalline diamond from glassy carbon at pressures up to 25 GPa

ABSTRACT

Nano-polycrystalline diamond (NPD) with various grain sizes has been synthesized from glassy carbon at pressures 15–25?GPa and temperatures 1700–2300°C using multianvil apparatus. The minimum temperature for the synthesis of pure NPD, below which a small amount of compressed graphite was formed, significantly increased with pressure from ～1700°C at 15?GPa to ～1900°C at 25?GPa. The NPD having grain sizes less than ～50?nm was synthesized at temperatures below ～2000°C at 15?GPa and ～2300°C at 25?GPa, above which significant grain growth was observed. The grain size of NPD decreases with increasing pressure and decreasing temperature, and the pure NPD with grain sizes less than 10?nm is obtained in a limited temperature range around 1800–2000°C, depending on pressure. The pure NPD from glassy carbon is highly transparent and exhibits a granular nano-texture, whose grain size is tunable by selecting adequate pressure and temperature conditions.     

Pressure-dependent phase transition in the ordered BaBi0.7Nb0.3O3 perovskite

BaBi_0.7Nb_0.3O₃, an ordered perovskite, crystallizes in a centrosymmetric rhombohedral structure with the space group R3¯. The refined cell parameters obtained from synchrotron powder X-ray diffraction data for the rhombohedral phase at ambient pressure are a=6.109 (2) Å and α=60.3 (1)°. The pressure-dependent synchrotron powder X-ray diffraction studies show a phase transition around 8.44±1 GPa, where it transforms from rhombohedral structure to a monoclinic structure. The lattice parameters obtained for the monoclinic phase at a pressure of 15±1 GPa are a=5.91 (2) Å, b=6.25 (3) Å and c=8.22 (1) Å with monoclinic angle, β=88 (1)°.     

The structural and optical properties of ZnO bulk and nanocrystals under high pressure

The elastic properties of high-quality ZnO crystals and nanopowder of grain size of about 65 nm are studied for both wurtzite (low pressure) and rock-salt high pressure phases. The measured values of bulk moduli for wurtzite and rock-salt phases of bulk ZnO crystals are equal to 156±13 and 187±20 GPa, respectively, and considerably larger for ZnO nanocrystals. The phase transition begins at a pressure of about 9 GPa and it is completed at a pressure of about 13.8 GPa for bulk crystals, whereas the values of pressure at which the phase transition occurs are lower for nanocrystals. A carefull Rietveld analysis of the obtained data does not exhibit the presence of any intermediate phases between low pressure wurtzite and high pressure rock-salt phases of ZnO. The phase transition is accompanied by a strong decrease in the near-band-gap photoluminescence intensity. In addition, the pressure coefficient of the near-band-gap luminescence in ZnO nanocrystals exhibits strong deviation from the linearity observed in bulk crystals. An analysis of the results shows that defects present in the nanopowdered sample are responsible for the observed effects.     

High Pressure and High Temperature Studies on Manganese Oxides

High pressure and high temperature quench experiments on f -MnO 2 , Mn 2 O 3 and sol gel derived manganese oxides have been carried out to identify any new phases to which the materials may transform under high pressure and high temperature conditions. Results of ESR, DTA and TGA investigations on sol gel derived manganese oxide have shown it to be hausmannite Mn 3 O 4 , instead of n -Mn 2 O 3 as reported earlier in the literature. The sol gel derived manganese oxide transforms to n -Mn 2 O 3 when heated above 700°C. Sol gel derived Mn 3 O 4 , when quenched from 5 GPa and temperature range 800-1200°C, gives a mixture of Mn 3 O 4 (hausmannite) and a phase having CaMn 2 O 4 (marokite)-type structure. f -MnO 2 undergoes partial amorphization when pressure-quenched from 8 GPa at room temperature. The high pressure and high temperature quench experiments up to 5 GPa and 700°C showed that the decomposition temperature of f -MnO 2 increases with pressure. The new phase reported by Liu (1976) from diamond-anvil cell (DAC) experiments on pyrolusite MnO 2 is identified to be a low-density polymorph f -MnO 2 . This unusual result of formation of low-density f -MnO 2 , having an open structure at high pressure and high temperature, is probably due to quenching of a non-equilibrium phase in Liu's (1976) laser-heated DAC experiment.     

Bridging room-temperature and high-temperature plasticity in decagonal Al–Ni–Co quasicrystals by micro-thermomechanical testing

Ever since quasicrystals were first discovered, they have been found to possess many unusual and useful properties. A long-standing problem, however, significantly impedes their practical usage: steady-state plastic deformation has only been found at high temperatures or under confining hydrostatic pressures. At low and intermediate temperatures, they are very brittle, suffer from low ductility and formability and, consequently, their deformation mechanisms are still not clear. Here, we systematically study the deformation behaviour of decagonal Al–Ni–Co quasicrystals using a micro-thermomechanical technique over a range of temperatures (25–500 °C), strain rates and sample sizes accompanying microstructural analysis. We demonstrate three temperature regimes for the quasicrystal plasticity: at room temperature, cracking controls deformation; at 100–300 °C, dislocation activities control the plastic deformation exhibiting serrated flows and a constant flow stress; at 400–500 °C, diffusion enhances the plasticity showing homogenous deformation. The micrometer-sized quasicrystals exhibit both high strengths of ~2.5–3.5 GPa and enhanced ductility of over 15% strains between 100 and 500 °C. This study improves understanding of quasicrystal plasticity in their low- and intermediate-temperature regimes, which was poorly understood before, and sheds light on their applications as small-sized structural materials.     

High pressure X-ray diffraction and Raman spectroscopic studies of the phase change of D2O ice VII at approximately 11 GPa

High pressure experiments were performed on D₂O ice VII using a diamond anvil cell in a pressure range of 2.0–60 GPa at room temperature. In situ X-ray diffractometry revealed that the structure changed from cubic to a low symmetry phase at approximately 11 GPa, based on the observed splitting of the cubic structure's diffraction lines. Heating treatments were added for the samples to reduce the effect of non-hydrostatic stress. After heating, splitting diffraction lines became sharp and the splitting was clearly retained. Although symmetry and structure of the transformed phase have not been determined, change in volumes vs. pressure was calculated, assuming that the low-symmetry phase had a tetragonal structure. The bulk modulus calculated for the low-symmetry phase was slightly larger than that for the cubic structure. In Raman spectroscopy, the squared vibrational frequencies of ν₁ (A_1g), as a function of pressure, showed a clear change in the slope at 11–13 GPa. The full width at half maxima of the O-D modes decreased with increasing pressure, reaching a minimum at approximately 11 GPa, and increased again above 11 GPa. These results evidently support the existence of phase change at approximately 11 GPa for D₂O ice VII.     

Microstructural evolution and thermal stability of Fe–Zr metastable alloys developed by mechanical alloying followed by annealing

The effect of Zr (up to 1 at.%) addition on the formation of Fe–Zr metastable alloys and their thermal stability were investigated for their possible nuclear applications. Fe–xZr (x = 0.25, 0.5, 1%) alloys were synthesised by mechanical alloying under a high-purity argon atmosphere using stainless steel grinding media in a SPEX 8000M high energy mill. The milling was conducted for 20 h with a ball-to-powder weight ratio of 10:1. The formation of metastable solid solutions after milling was confirmed from the change in the Gibbs free energy analysis as per Miedema’s model. The microstructural characterisation was carried out by analysis of X-ray diffraction, atomic force microscopy and transmission electron microscopy. The effect of Zr on the thermal stability of Fe–Zr alloys was investigated by extensive annealing experiments followed by microstructural analysis and microhardness measurements. The stabilisation was found to occur at 800 °C and thereafter, no significant change in the crystallite size was observed for the samples annealed between 800 and 1200 °C. The supersaturated solid solution, especially 1% Zr alloy, found to be highly stable up to 800 °C and the microhardness value of the same measured to be as high as 8.8 GPa corresponding to a crystallite size of 57 nm. The stabilisation effect has been discussed in the light of both the thermodynamic and kinetic mechanisms and the grain size stabilisation is attributed to the grain boundary segregation of Zr atoms and/or Zener pinning by nanoscale precipitation of the Fe₂Zr phase.     

Effect of Al doping on structural,optical and electrical properties of SnO2 thin films synthesized by pulsed laser deposition

Aluminium-doped (Al = 0–5?wt.%) SnO₂ thin films with low-electrical resistivity and high optical transparency have been successfully synthesized by pulsed laser deposition technique at 500 °C. Structural, optical and electrical properties of the as-deposited and post-annealed thin films were investigated. X-ray diffraction patterns suggest that the films transform from crystalline to amorphous state with increasing aluminium content. The root mean square (R_q) surface roughness parameter, determined by atomic force microscopy decreases upon annealing of the as-deposited film. While resistivity of the film is the lowest (9.49 × 10^?4 Ω-cm) at a critical doping level of 1?wt.% Al, optical transparency is the highest (nearly 90%) in the as-deposited condition. Temperature dependence of the electrical resistivity suggests that the Mott’s variable range hopping process is the dominant carrier transport mechanism in the lower temperature range (40–135 K) for all the films whereas, thermally activated band conduction mechanism seems to account for conduction in the higher temperature region (200–300 K).     

Phase transition of ZnS at high pressures and temperatures

The high-pressure behaviour of zinc sulphide, ZnS, has been investigated, using an in situ X-ray powder diffraction technique in a diamond anvil cell, at pressures and temperatures up to 35 GPa and 1000 K, respectively. The pressure-induced phase transition from a zincblende (B3) to a rocksalt (B1) structure was observed. This transition occurred at 13.4 GPa and at room temperature, and a negative dependence on temperature for this transition was confirmed. The transition boundary was determined to be P (GPa) = 14.4 ? 0.0033 × T (K).     

The influence of pressure on spontaneous magnetization of Fe60Mn20B20 amorphous alloy at temperature range 77.4–300 K

Abstract

Amorphous, ferromagnetic, invar like, Fe₆₀ Mn₂₀ B₂₀ alloy has been investigated. Two kinds of experiments were carried out for this alloy. The first, using high pressure technique, revealed the influence of pressure on B(H) dependencies within the wide range of temperature under pressure of 0.5 GPa. From the magnetization curves obtained during these experiments the decrease of spontaneous magnetization caused by applied pressure 0.5 GPa at temperature -180°C has been calculated at the rate about 7 10^?11T/Pa.

In the second kind of experiments the measurements of volume magnetostriction up to 720 kA/m magnetic field intensity have been done. Volume magnetostriction coefficient at temperature 77.4 K has been determined to be about 2 10^?11 [A/m]^?1.     

Determination of the phase boundary of GaP using in situ high pressure and high-temperature X-ray diffraction

The high pressure behavior of gallium phosphide, GaP, has been examined using the synchrotron X-ray diffraction technique in a diamond anvil cell up to 27?GPa and 900?K. The transition from a semiconducting to a metallic phase was observed. This transition occurred at 22.2?GPa and room temperature, and a negative dependence of temperature of this transition was found. The transition boundary was determined to be P (GPa)?=?22.6???0.0014?×?T (K).