Embedded-quantum-cluster study of local relaxations and optical properties of Cr3+ in MgO.

Abstract

Local distortions and optical properties induced by the substitutional ${\mathrm{Cr}}^{3+}$ impurity in the MgO host lattice are computed using the embedded-cluster approach implemented in the icecap code. Within this methodology, the impurity centered cluster is described quantum mechanically and is self-consistently coupled with the lattice environment by means of the shell-model treatment of the lattice polarization and distortion. Our calculations predict inward relaxations of the six ${\mathrm{O}}^{2\mathrm{\ensuremath{-}}}$ nearest neighbors surrounding the ${\mathrm{Cr}}^{3+}$ ion for both cubic and noncubic (tetragonal and orthorhombic) configurations in MgO. For the cubic configuration, selected low-lying excited states, including the 10Dq generator $^{4}$${\mathit{T}}_{2}$, are calculated at several Cr-O separations. After taking into account lattice relaxations and correlation corrections, the computed 10Dq value lies 0.19 eV lower than the experimental one. Finally, the response of the Dq parameter and the Cr-O separation to hydrostatic pressure is obtained by performing analogous cluster-in-the-lattice calculations with the lattice parameter of MgO varying according to its equation of state. Analysis of these results reveals that the compressibility at the host cation site is greater than that at the impurity site.