Hongyu Zhu

A comparative study of high pressure behaviors of pyrochlore-type and thortveitite-type In2Ge2O7

Polycrystalline In2Ge2O7 with a monoclinic structure (thortveitite-type, T-type) and a cubic structure (pyrochlore-type, P-type) have been synthesized by using different methods. The structural stabilities and electrical transport properties of these two polymorphs under high pressure have been investigated by angle-dispersive X-ray diffraction (ADXRD) and alternate current (AC) impedance spectra. An irreversible structural phase transition from monoclinic (C2/m) to another monoclinic (P21/c) phase has been found in the T-type In2Ge2O7 above 6.6 GPa. Furthermore, the pressure dependent electrical resistance of the T-type In2Ge2O7 shows a dramatic change at 5.3 GPa, where it can be attributed to the observed pressure-induced structural phase transition. On the contrary, the P-type In2Ge2O7 with the cubic (Fdm) structure at high pressure is much more stable up to 26.5 GPa.

Pressure-induced disordering and phase transformations in Eu2Zr2O7 pyrochlore

ABSTRACT The structural properties of pyrochlore Eu2Zr2O7 under high pressure have been studied by using Raman spectroscopy and in situ angle-dispersive X-ray diffraction (ADXRD). The results of Raman spectra indicate that Eu2Zr2O7 undergoes a reversible structural change around 21.2 GPa. The results of Rietveld refinements from in situ ADXRD data indicate that the ordered pyrochlore structure (Fd-3m) transforms to the defect-cotunnite structure (Pnma) at 26.5 GPa. The phase transition is irreversible and the transformation process is mainly induced by the accumulations of anti-site defects of the cation sublattice and Frenkel defects on the anion sublattice. Besides, the bonds should play a more important role than the bonds in the process of the phase transformation.

Effects of high pressure sintering on the microstructure and thermoelectric properties of BiCuSeO

ABSTRACT Polycrystalline BiCuSeO oxides are prepared by solid-state reaction followed by pressureless sintering (PLS) and high pressure sintering (HPS) methods. Both the experimental results and the density functional theory calculations indicate that the crystal defect concentrations of BiCuSeO can be reduced under the effect of high pressure. By comparing with the PLS sample, a larger power factor and a lower thermal conductivity can be obtained for the HPS sample. The maximum figure of merit ZT∼0.4 @ 800 K was obtained for the HPS sample, which is about 3 times higher than the PLS sample. These results indicate that the effect of high pressure is beneficial to modifying the microstructure and improving the thermoelectric performance of BiCuSeO oxyselenide.

Abnormal Pressure‐Induced Photoluminescence Enhancement and Phase Decomposition in Pyrochlore La2Sn2O7

La2 Sn2 O7 is a transparent conducting oxide (TCO) material and shows a strong near-infrared fluorescent at ambient pressure and room temperature. By in situ high-pressure research, pressure-induced visible photoluminescence (PL) above 2 GPa near 2 eV is observed. The emergence of unusual visible PL behavior is associated with the seriously trigonal lattice distortion of the SnO6 octehedra, under which the Sn-O1-Sn exchange angle θ is decreased below 22.1 GPa, thus enhancing the PL quantum yield leading to Sn 3 P1 → 1 S0 photons transition. Besides, bandgap closing followed by bandgap opening and the visible PL appearing at the point of the gap reversal, which is consistent with high-pressure phase decomposition, are discovered. The high-pressure PL results demonstrate a well-defined pressure window (7-17 GPa) with flat maximum PL yielding and sharp edges at both ends, which may provide a great calibration tool for pressure sensors for operation in the deep sea or at extreme conditions.