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Low-pressure clino- to high-pressure clinoenstatite phase transition: A phonon-related mechanism

Yonggang G. Yu^1,* and Renata M. Wentzcovitch^2,

¹ Department of Chemistry, Minnesota Supercomputing Institute, University of Minnesota, Minneapolis, Minnesota 55455, U.S.A.
² Department of Chemical Engineering and Materials Sciences, Minnesota Supercomputing Institute, University of Minnesota, Minneapolis, Minnesota 55455, U.S.A.

Correspondence: E-mail: wentzcov{at}cems.umn.edu

We have investigated by first principles the compressional behavior of low-pressure (LP) and high-pressure (HP) MgSiO₃ clinoenstatite. We have carefully examined unit-cell shapes, chain angles, and polyhedral volume responses, such as angle variances and quasi-elongations, under pressure at room temperature. We have observed opposite behavior of the tetrahedra in the S-rotated and O-rotated chains with pressure in the LP phase, with a slight increase (decrease) in angle variance and quasi-elongation in the former (latter). Inspection of zone center modes of both phases under pressure reveals a transition path that converts the S-rotated chain in the LP phase into the O-rotated chain in the HP phase. This conversion is related to a slight softening of an A_g "metastable" Raman mode under pressure.

Key Words: Phase transition mechanism • low-pressure clinoenstatite • high-pressure clinoenstatite • Raman • IR • phonon frequency