Dayong Tan

Large volume collapse observed in the phase transition in cubic PbCrO3 perovskite

When cubic PbCrO3 perovskite (Phase I) is squeezed up to ∼1.6 GPa at room temperature, a previously undetected phase (Phase II) has been observed with a 9.8% volume collapse. Because the structure of Phase II can also be indexed into a cubic perovskite as Phase I, the transition between Phases I and II is a cubic to cubic isostructural transition. Such a transition appears independent of the raw materials and synthesizing methods used for the cubic PbCrO3 perovskite sample. In contrast to the high-pressure isostructural electronic transition that appears in Ce and SmS, this transition seems not related with any change of electronic state, but it could be possibly related on the abnormally large volume and compressibility of the PbCrO3 Phase I. The physical mechanism behind this transition and the structural and electronic/magnetic properties of the condensed phases are the interesting issues for future studies.

Structural properties of PbVO3 perovskites under hydrostatic pressure conditions up to 10.6 GPa.

High-pressure synchrotron x-ray powder diffraction experiments were performed on PbVO(3) tetragonal perovskite in a diamond anvil cell under hydrostatic pressures of up to 10.6 GPa at room temperature. The compression behavior of the PbVO(3) tetragonal phase is highly anisotropic, with the c-axis being the soft direction. A reversible tetragonal to cubic perovskite structural phase transition was observed between 2.7 and 6.4 GPa in compression and below 2.2 GPa in decompression. This transition was accompanied by a large volume collapse of 10.6% at 2.7 GPa, which was mainly due to electronic structural changes of the V(4+) ion. The polar pyramidal coordination of the V(4+) ion in the tetragonal phase changed to an isotropic octahedral coordination in the cubic phase. Fitting the observed P-V data using the Birch-Murnaghan equation of state with a fixed [Formula: see text] of 4 yielded a bulk modulus K(0) = 61(2) GPa and a volume V(0) = 67.4(1) Å(3) for the tetragonal phase, and the values of K(0) = 155(3) GPa and V(0) = 58.67(4) Å(3) for the cubic phase. The first-principles calculated results were in good agreement with our experiments.

The effects of high temperature on the high-pressure behavior of CeO2

Raman spectrum and angle-dispersive x-ray diffraction (ADXD) measurements have been performed to investigate the effects of high temperature on the high-pressure behavior of bulk CeO 2 . A phase transformation of CeO 2 from fluorite to PbCl 2 -type structure occurs at 12.0 GPa and is completed at 14.2 GPa after the sample is heated, and the phase transition pressure decreases by nearly 20 GPa compared with that at room temperature. On decompression, the high-pressure phase of CeO 2 remains down to 2.2 GPa, and it changes back to a cubic structure at ambient conditions. At a pressure of 22.1 GPa, a 6.4% lattice volume difference between the fluorite and PbCl 2 -type structures was observed. The lattice volume of fluorite phase obtained in the areas that have been heated is about 1% less than that obtained in the areas that have not been heated. Besides prompting the phase transition of CeO 2 , high temperature also anneals the sample and leads a small reduction in lattice volume of the fluorite phase. The zero-pressure bulk modulus of the fluorite phase of CeO 2 after annealing is calculated to be about 200 GPa with an assumed pressure derivative of four, which is smaller than that of former x-ray diffraction experiments at room temperature.

Anomalous compression behavior of germanium during phase transformation

In this article, we present the abnormal compression and plastic behavior of germanium during the pressure-induced cubic diamond to β-tin structure transition. Between 8.6 GPa and 13.8 GPa, in which pressure range both phases are co-existing, first softening and followed by hardening for both phases were observed via synchrotron x-ray diffraction and Raman spectroscopy. These unusual behaviors can be interpreted as the volume misfit between different phases. Following Eshelby, the strain energy density reaches the maximum in the middle of the transition zone, where the switch happens from softening to hardening. Insight into these mechanical properties during phase transformation is relevant for the understanding of plasticity and compressibility of crystal materials when different phases coexist during a phase transition.

A new cubic perovskite in PbGeO3 at high pressures

Abstract A new cubic perovskite polymorph of PbGeO3 (Phase II) was synthesized by laser heating in the diamond-anvil cell (DAC) at the pressure of 36 GPa. Fitting the Birch-Murnaghan equation of state against its observed P-V data yields a bulk modulus K0 of 196(6) GPa and the volume V0 of 56.70(13) Å3 when K0′ is assumed being 4. After the pressure is released, the PbGeO3 Phase II changes gradually into an amorphous phase, which contains mainly fourfold-coordinated germanium. It indicates that the PbGeO3 Phase II with a GeO6 octahedron framework transforms to a GeO4 tetrahedron network during the amorphization. The existence of PbGeO3 cubic perovskite Phase II at high pressures indicates that the polarized character of the Pb2+ ion induced by its 6s2 lone pair electrons would be totally reduced in the environment of major silicate perovskites inside the lower mantle, and thus the Pb atom would substitute the Ca atom to enter the CaSiO3 perovskite.

Growth of magnesium aluminate nanocrystallites

Nanocrystalline magnesium aluminate was synthesized with the coprecipitation method. Its growing behaviors as a function of temperature were studied with synchrotron X-ray diffraction (XRD) and Raman spectroscopy. It is found that the particle growth was greatly inhibited at temperatures below 1000 °C due to the hydroxide precursor reactants. Above 1000 °C, magnesium aluminate nanoparticles start to grow fast. After two hours annealing at 1200 °C, the grain size changes by multiple folds, suggesting that oriented attachment may occur. Above 1200 °C, the grain size changes in various directions are much smaller than the average grain size, indicating the oriented attachment mechanisms become inactive in the growth of MgAl2O4 nanoparticles with sizes larger than 42 nm.

Interlayer-glide-driven isosymmetric phase transition in compressed In2Se3

We report an anomalous phase transition in compressed In2Se3. The high-pressure studies indicate that In2Se3 transforms to a new isosymmetric R-3m structure at 0.8 GPa whilst the volume collapses by ∼7%. This phase transition involves a pressure-induced interlayer shear glide with respect to one another. Consequently, the outer Se atoms of one sheet locate into the interstitial sites of three Se atoms in the neighboring sheets that are weakly connected by van der Waals interaction. Interestingly, this interlayer shear glide changes the stacking sequence significantly but leaves crystal symmetry unaffected. This study provides an insight to the mechanisms of the intriguing isosymmetric phase transition.

Cubic perovskite polymorph of strontium metasilicate at high pressures

Abstract By using a diamond-anvil cell (DAC) with laser heating technology, a cubic perovskite polymorph of SrSiO3 has been synthesized at - 38 GPa and 1500-2000 K for the first time. The P-V data of this new phase give ambient temperate elastic constants of V0 = 49.18(5) Å3, K0 = 211(3) GPa, respectively, when they are fitted against the Birch-Murnaghan equation of state with a fixed K0′ at 4. On decompression, the SrSiO3 cubic perovskite phase becomes unstable at - 6.2 GPa and disappears completely at - 4.7 GPa. The transformed product can be considered as an amorphous phase with a minor amount of small sized crystals in the amorphous matrix. First principle calculations predicted structural properties of both the cubic and the six-layer-repeated hexagonal perovskite polymorphs of SrSiO3 in good agreement with experimental results. The experimental and theoretical results indicate that the larger Sr2+ cation can substitute the Ca2+ cation and enter into the lattice of the cubic perovskite phase of CaSiO3 at lower mantle conditions with only a small lattice strain. These results indicate that Sr can be hosted in cubic perovskite CaSiO3 found as inclusions in diamonds originating from the lower mantle

Effects of pressure on PbWO4-III

In a hydrostatic pressure environment condition and in manual milling, respectively, investigations of PbWO4-III (P21/n) have been performed by X-ray diffraction and Raman scattering techniques. Experiments found that PbWO4-III keeps its monoclinic structure under hydrostatic pressures with the sample’s anisotropic compressibility up to 14.6 GPa, but transforms to PbWO4-I (I41/a) in a grinding process. The stability and variability of PbWO4-III depending on the strain states were also explored by first-principles calculations of elasticity. Calculations show PbWO4-III has an anisotropic compressibility and a ductile nature with increasing pressure up to 15 GPa.

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°.