Isothermal compressibility and phase transformation studies on CuGeO₃ under high pressures and high temperatures using synchrotron radiation

Abstract

Energy-dispersive X-ray diffraction measurements were carried out under high pressure and temperature (to 9.5 GPa and up to 800°C) on CuGeO₃ phase (I) in a large-volume apparatus (DIA-6 type). These experiments were conducted with synchrotron radiation at the National Synchrotron Light Source (NSLS). The results obtained at high pressures and room temperature are generally in good agreement with previous Raman, optical, and X-ray diffraction studies. Diffraction data collected under truly hydrostatic conditions show that the orthorhombic phase (I) to phase (II) transition starts near 7.5 GPa and is completed by 9 GPa. Upon decompression phase (II) transforms to a different orthorhombic phase (I′) at 4.5 GPa, which can be retained under ambient conditions. High temperature data obtained at about 9 GPa show that under truly hydrostatic conditions, phase (II) of CuGeO₃ first changes into a monoclinic phase (II-A) at 200°C and then breaks down completely into GeO₂ (rutile)+Cu near 300°C, indicating that a reaction between the sample and the pressure medium has occurred. Under quasihydrostatic conditions, the previously reported phase (IV) remains stable up to 600°C. At higher temperatures, two irreversible phase transitions take place, one near 700°C and another near 750°C. They seem to be the high P–T phases of CuGeO₃ that can be quenched to ambient conditions. However, decomposition cannot be completely ruled out at the present time. Fitting the more precisely determined new P–V data for phase (I) of CuGeO₃ using the Birch–Murnaghan equation of state (EOS), we obtain 40.0±1.8 GPa for the bulk modulus (K₀) and of 6.5±0.9 for the pressure derivative (K₀′). Similar fitting of the lattice parameters of phase (I) to a modified Murnaghan EOS yields the linear incompressibilities of 198±14, 57.5±3.7, and 450±95 GPa, for the a, b, and c axes, respectively. These results are in excellent agreement with those obtained in a recent X-ray diffraction and neutron diffraction study. Based on the limited data available for the phase (II) at pressure between 6 and 9.4 GPa, the linear compressibility of phase (II) is calculated as 0.00166, 0.00176, and 0.00168 GPa⁻¹, respectively, for the a, b, and c axes, indicating that its compression behavior is almost isotropic, in contrast to the highly anisotropic behavior of phase (I).