Trigonal field acting at theCr3+E2states in ruby from magneto-optical measurements under high pressure

Abstract

Magneto-optical measurements on ruby under high-pressure conditions provided direct determination of the trigonal crystal field acting at the ${t}_{2g}$ orbitals of ${\text{Cr}}^{3+}$ in ${\text{Al}}_{2}{\text{O}}_{3}$ $({\text{CrO}}_{6})$ and its dependence with pressure. The correlation study between the measured trigonal splitting and the trigonal distortion at the ${\text{Al}}^{3+}$-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 $\ensuremath{\delta}{V}_{\text{tr}}\ensuremath{\propto}{R}^{\ensuremath{-}n}$ with $n=6$. It is shown that this exponent does not change when we consider the pressure variation of the local structure around ${\text{Cr}}^{3+}$ 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.