The orthorhombic phase of CaSiO 3 perovskite
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The orthorhombic phase of CaSiO3 perovskite Blanka Magyari-K¨ope1 , Levente Vitos2,3 , G¨oran Grimvall1, B¨orje Johansson2, and J´anos Koll´ar3 1 Theory of Materials, Physics Department, Royal Institute of Technology, Stockholm Center for Physics, Astronomy and Biotechnology, SE-106 91, Stockholm, Sweden 2 Applied Materials Physics, Department of Materials Science and Engineering, Royal Institute of Technology, SE-100 44, Stockholm, Sweden 3 Research Institute for Solid State Physics and Optics, H-1525 Budapest, P.O.Box 49, Hungary ABSTRACT Ab initio total energy calculations, combined with the global parametrization method of perovskite structures, are used to investigate the stability of cubic CaSiO 3 against octahedral rotations. We propose an equilibrium crystal structure of orthorhombic Pbnm symmetry. The larger compressibility of the SiO6 octahedra relative to the CaO12 polyhedra is reflected in gradual reduction of the orthorhombic distortion with hydrostatic pressure. INTRODUCTION The structural stability of the mineral perovskites at pressure conditions corresponding to the lower mantle of the Earth (24 − 136 GPa) is among the most important issues addressed by ab initio methods based on the density functional theory [1]. Structural phase transitions, for instance, may provide explanations for the discontinuities observed in the lower mantle and their study facilitates the better understanding of the Earth’s dynamics and evolution. Magnesium silicate, the most abundant mineral in the lower mantle, has an orthorhombic structure of Pbnm symmetry for a wide range of temperatures and pressures. The second most significant mineral is the calcium silicate. The crystal structure of CaSiO 3 has not yet been resolved. There is a controversy regarding the low temperature structure [2, 3, 4, 5, 6, 7, 8, 9] and, therefore, the study of this material presents major importance in material science. The perovskite structure of CaSiO3 is metastable below 10 GPa, and transforms to an amorphous phase below 1 GPa. In the stability region, and above 300 K, the earlier experimental studies [4, 5, 6], found that the crystal structure has the cubic symmetry, while the most recent measurements [9] identify a tetragonal phase. From the theory side, pseudopotential methods reported a cubic ground state [2, 7] while all-electron linear augmented plane wave (LAPW) calculations [3, 8] indicated the presence of lattice instabilities in the cubic structure, i.e. instabilities at M and R points in the Brillouin zone. In the latest studies a structure of tetragonal symmetry was considered as the ground state structure (I4/mcm, P4/mbm). However, in order to suppress simultaneously all the instabilities, the common symmetry elements from the R point of the Imma and from the M point of the P4/mbm space groups should be considered, which gives the Pbnm orthorhombic symmetry (e.g. MgSiO 3 ). It is worth noting that the instability at the M point in cubic CaSiO 3 is considerably smaller than the R point instability, which indicates a less distort
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