Abstract: Magneto-optical measurements on ruby under high-pressure conditions provided direct determination of the trigonal crystal field acting at the t2g orbitals of Cr3+ in Al2O3
CrO6 and its dependence with pressure. The correlation study between the measured trigonal splitting and the trigonal distortion at the Al3+-substituted site indicates that the trigonal splitting increases with pressure whereas the trigonal distortion slightly reduces. The result is interpreted in terms of an enhancement of the electron-lattice coupling due to trigonal distortion upon reduction in the Al-O bond distance, i.e., the Cr-O bond distance R. The observed variations can be explained on the basis of empirical R dependence of the trigonal crystal field as
VtrR?n with n=6. It is shown that this exponent does not change when we consider the pressure variation of the local structure around Cr3+ obtained from ab initio calculations. By the way, we also demonstrate that a methanol-ethanol mixture is a good pressure transmitting medium at cryogenic temperatures

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