Tetragonal Structure of CaSiO3 perovskite at 20--46 GPa

CaSiO₃ perovskite is one of the major phases in the transition zone and lower mantle. Although its stability has been confirmed to 134 GPa, its crystal structure is less certain and several recent first-principles calculations predict a non-cubic structure of CaSiO₃ perovskite at mantle pressures. The low resolution of energy-dispersive X-ray diffraction that was previously used did not allow the detection of any deviation from cubic symmetry that involved a peak splittings (<0.7 %). We conducted high-resolution X-ray diffraction measurements using synchrotron radiation sources (CHESS and SSRL). Image plates were used to collect diffraction from monochromatic X-rays (17 keV, 25 keV). Pure wollastonite powder was mixed with platinum which serves as both a pressure standard and laser absorber. Argon was loaded as both pressure medium and thermal insulator. Ruby chips were also loaded to provide a secondary pressure calibration. In order to synthesize CaSiO₃ perovskite and anneal differential stresses, samples were heated to more than 1500 K using a continuous Nd:YAG laser before each X-ray diffraction measurement. We observed peak splittings for the 200 and 211 diffraction lines, attributable to a tetragonal distortion of CaSiO₃ perovskite at 24--46 GPa. The c axis of the tetragonal phase is shorter than the a axes by 0.3--0.7 %, and the bulk modulus is 255(5) GPa for K'=4. The observed diffraction patterns are consistent with the tetragonal distortion being induced by either a rigid octahedral rotation around the tetragonal a axes or an anisotropic distortion of SiO₆ octahedra, rather than a rigid octahedral rotation around the tetragonal c axis as proposed by some first-principles calculations. This discovery may have important implications for the elastic moduli and and anisotropy, and element partitioning in CaSiO₃ perovskite at lower-mantle pressures.