三種鉻酸鹽（MCrO4 M:Ba Pb Sr）之高壓X光繞射與拉曼光譜研究

Abstract

High-pressure Raman spectroscopic and X-ray diffraction experiments of three chromates were carried out in a diamond anvil cell up to 25 GPa. The goal is to determine the equation of states of the phases in this study. In addition, the mechanism of the phase transition is also inferred. On the basis of the changes in the X-ray diffraction patterns and the variation of the lattice parameters with pressure, it is inferred that three chromates undergo reversible phase transition. The phase transition pressure of barium chromate is the highest of three chromates and strontium chromate is the lowest. The low pressure phase and high pressure phase of three chromates coexisted after phase transition. The phase transition of barium chromate is first-order transformation, the volume decreases 9％ after phase transition. The phase transition of barium chromate is reflected by the splitting in the CrO4 vibration modes and the discontinuity in the slope of mode frequency verse pressure in the Raman observation. Strontium chromate has two discontinuities in the slope of mode frequency verse pressure at 9.5 GPa and 15 GPa, respectively. The mean Gruneisen parameter for barium chromate is 0.87. The mean Gruneisen parameter for strontium chromate is 0.84. The three chromates show anisotropic compressibility along three crystallographic axes. In barium chromate, a axis is the most uncompressible axis, which corresponding to the direction with edge share between Cr-O tetrahedral and Ba-O polyhedral. In lead chromate and strontium chromate, c axis is the most uncompressible axis which corresponding to the direction with edge share between Cr-O tetrahedral and Pb-O（Sr-O） polyhedral. The bulk modulus (K) of barium chromate is 53(1) GPa, the K value of lead chromate and strontium chromate is 62(1) GPa and 60(2) GPa, respectively.