Structural phase transformation of inorganic compounds under very high pressure studied by in-situ X-ray diffraction analysis

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An X-ray diffraction system has been combined with a 250 ton cubic-anvil type high pressure apparatus to make in-situ measurements of crystal structure, compressibility and phase equilibrium of pressure-induced phases of inorganic substance. The X-ray diffraction system consists of intense X-ray source of the rotating anode type (6OkV, 200mA), and solid state detector and pulse height analyzer system (2048 channels, 2627eV/channel). Both the X-ray source and goniometer carrying SSD are mounted on a common plate so as to make easier adjustment of optical path through pressure-medium composed of boron-epoxy lesin mixture. The d-specings of the diffraction peaks of NaCl which is used for pressure standard could be determined with an accuracy of <plus-minus> 0.1 %, which in turn warrants precision of (SY.+-.[) 0.1 GPa for the pressure calibration. B1-B2 transition of KCl was also detected at 2.1 GPa with a volume decrease of 13 %, in agreement with previous reports. Bulk moduli of B1 and B2 phase of KCl were also determined to be 18.2 <plus-minus> 0.2 and 21.5 <plus-minus> 1.1 GPa, respectively. Pressure-induced phase transition of SrF_2 was also studied up to 8GPa at room temperature. Fluorite structure was observed up to 5.8 GPa, where transformation to a PbCl_2 type took place. However, the high pressure phase remained down to 2.0 GPa with decreasing pressure, indicating a remarkable hysteresis. These observations suggest a martensitic mechanism of the fluorite-PbCl_2 type phase transition, which has been noted to be very sensitive to hydrostacity due to pressure medium. Bulk modili of SrF_2 with the fruorite and PbCl_2 type were determined to be 69.1 <plus-minus> 0.7 and 73.6 <plus-minus> 3.8 GPa respectively, in agreement with previous data. Along with the construction of insitu diffraction system, various investigations of phase transition in MO_2 and M_2O_3 were carried out at high pressure and temperatures, and crystallochemical implications were discussed.