S. Endo

Phase transformations of α-cristobalite GaPO4 at pressures up to 52 GPa

In situ x-ray diffraction studies on α-cristobalite GaPO 4 have found that the orthorhombic α-cristobalite GaPO 4 (I) transforms into a monoclinic phase (II) at about 2 GPa, then to the VCrO 4 -type orthorhombic phase (III) at 4 GPa, which persists to 52 GPa, and no further transformation was observed. The volume changes for (I)-(II) and (II)-(III) transitions are ∼<1% and 17%, respectively, at the transition pressure. Upon releasing the pressure to atmospheric pressure, the orthorhombic phase (III) either reverses back to the phase (I) or persists as the metastable phase, depending upon the loading process. By fitting the P-V data to the Mumaghan equation of state, we obtained values of the zero-pressure bulk modulus (K 0 ) and its pressure derivative (K 0 ) at 12.1 ± 2.1 GPa and 11.5 ± 3.5 for phase (I), and 144.1 ± 6.1 GPa and 5.2 ± 0.4 for phase (III).

High pressure raman spectroscopic study of low-cristobalite GaPO4

Gallium orthophosphate of low-cristobalite structure has been studied by Raman spectroscopy up to 28.5 GPa. Three phase transitions have been identified: 1.6, 3.1 and ~7 GPa. Raman spectra (200 to 700 cm-1) acquired above 3 GPa show signs of six-coordinated gallium ions, in agreement with earlier studies that the orthorhombic C2221 structure transforms into the orthorhombic Cmcm structure. As pressure increases above 8 GPa, the Raman peaks associated with the low-cristobalite framework structure diminish gradually and replaced by peaks that are possibly related to isolated phosphate units. The spectra also show that the structure of the quenched sample retains that of the high-pressure phase prior to decompression.

Pressure Dependence of the Magnetic Phase Transition Temperature in Mn2-xCrxSb

Pressure dependences of Ms-T (Ms: saturation moment) and magnetisation curves in Cr-modified Mn2Sb system were investigated in the pressure range up to 1 GPa and in the temperature range between 4.2 and 250 K. It was found that there are at least three intermediate magnetic states between the antiferromagnetic (AF) and perfect ferrimagnetic (FR) states. The AF-FR transition temperature decreased with increasing pressure, in contrast to the expectation from the conventional model.