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Lattice dynamics study of high pressure phase transition in SiO2

The pressure induced phase transition from the high pressure tetragonal structure of SiO2, stishovite (space group P42/mnm), to the highest observed form of orthorhombic structure (CaCl2) has been controversial for many years. With the advent of high pressure experimental techniques, the phase transition pressure was put in the range of 50 GPa. The Raman experiments at high pressures [1] give the pressure of 50 GPa, and the structural studies predict the transition pressure of 54 GPa [2]. The transition from stishovite (rutile structure) to CaCl2 structure is found to be accompanied with no or very small volume change (less than 1%), and experiments reveal that it is completely reversible. All previous attempts to explain the mechanism of the phase transition were based on the assumption that there is a soft optical mode driving the transition, but in previous experiments only partially soft mode was observed in Raman spectrum [1]. The previous structural experiments [2] put forward the idea that the order parameter of the transition is an orthorhombic distortion (a− b)/a of the tetragonal structure. It means that, at pressures higher than transition pressure, there is a distortion (a − b)/a different from zero, and that actual symmetry is orthorhombic (space group Pnnm). Below the transition pressure a = b, and the structure is tetragonal (rutile structure P42/mnm). The Landau theory of phase transitions, with primary order parameter being crystal strain [3], gives certain predictions about the evolution of physical values with temperature or pressure. In that case the fluctuations of the order parameter are acoustic modes [3]. We calculated the lattice dynamics of the tetragonal phase as a function of pressure, using density functional perturbation theory (DFPT) [4] as implemented in ABINIT program package [5]. The results of the lattice dynamics calculations point the phase transition pressure around 39 GPa. At that pressure the frequency of the long wave transversal acoustic mode, along (» ; , » ; , 0) direction in the Brillouin zone, becomes imaginary, i.e. the lattice is unstable with respect to such a distortion.

SiO2; high pressure; phase transitions; ABINIT; density functional theory

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