Non-cubic crystal symmetry of CaSiO 3 perovskite up to 18 GPa and 1600 K

In situ synchrotron X-ray diffraction experiments have been conducted on CaSiO ₃ perovskite (CaPv) in a double-stage multianvil apparatus up to 18 GPa and 1600 K using a newly developed step-scan diffraction technique, which utilizes an energy-dispersive setup with a solid-state detector and collimator assemblies to collect angle-dispersive diffraction data over a wide range of photon energies. Superlattice reflections were resolved throughout the pressure ( P) and temperature ( T) range of the experiments, confirming that the crystal symmetry of CaPv is neither cubic nor tetragonal. A combination of analyses of the complete two-dimensional intensity datasets (photon energy from 20 to 160 keV and 2 θ angle from 3.0° to 9.0°) and Rietveld refinements at selected wavelengths revealed that the most likely space group of CaPv in the experimental P- T range, and therefore in the Earth's transition zone, is either Pbnm or Cmcm. The difference between these two space groups was too small to resolve with our technique.