摘要
Barium germanate (BaGeO3) was studied using double-sided laser-heating diamond anvil cell (LHDAC). At ambient conditions, BaGeO3 has a pseudowollastonite structure. At about 12 GPa, BaGeO3 crystal begin to translate into an amorphous phase. The amorphous BaGeO3 was further pressurized to about 22 GPa and then heated at (1800 ± 200) K conditions. Raman spectra shows the amorphous BaGeO3 transforms into a new high pressure phase, which has not been reported so far. The new high pressure phase of BaGeO3 was further measured with the synchrotron radiation X-ray diffraction in the pressure ranges of 0?17.4 GPa. The diffraction patterns can be indexed with a 6H-type hexagonal perovskite structure and this structure remained stable as the pressure unloading to ambient pressure. In order to obtain the structural parameters of the new high pressure phase of BaGeO3, the X-ray diffraction patterns of 17.4 GPa and ambient pressure were refined with a model structure of 6H-type perovskite using the Rietveld method. The experimental pressure-volume data was fitted with the second-order Birch-Murnaghan equation of state, and obtained the volume bulk modulus and zero-pressure unit-cell volume are K0 = 150(2) GPa and V0 = 373.0(3) A3 respectively. On the basis of the experimental results in this study, we also carried out the first-principle theoretical calculation on the 6H-type perovskite BaGeO3. The calculated lattice constants and volume with the corresponding pressures are good agreement with the experimental results. Furthermore, the calculated volume bulk modulus and zero-pressure unit-cell volume are K0 = 153(1) GPa, V0 = 374.2(1) A3 respectively. The calculated Raman spectra at 20.0 GPa is also well consistent with the experimental results. This study not only complements the structural phase transition of pseudowallastonite BaGeO3 at high temperature and high pressure, but also builds a solid foundation for further characterizing the physical and chemical properties of pseudowallastonite BaGeO3, and gives a chance to develop the perovskite structured germanate functional materials. In addition, this study has an important indicative significance for us to understand the phase transition rule and stability of silicate perovskite, the physical and chemical properties and changes of Earth's lower mantle.
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