Ab initio model potential study of environmental effects on the Jahn–Teller parameters of Cu2+ and Ag2+ impurities in MgO, CaO, and SrO hosts

Abstract

In this paper, the differential effects brought about by the MgO, CaO, and SrO family of hosts on the topology of the energy surface corresponding to the ground state of the clusters (CuO6)10− and (AgO6)10− along the a1g and eg,θ vibrational modes, are studied by means of the ab initio embedding model potential method (AIEMP) at the restricted open-shell Hartree–Fock level. For all six defect crystals, the equilibrium geometries, Jahn–Teller energies, vibrational frequencies, and relative energies between D4h elongated and compressed structures (energy barriers) have been calculated using (a) two different definitions of the defect cluster, namely (MeO6)10− and (MeO6M6)2+ (Me=Cu, Ag; M=Mg, Ca, Sr), (b) two different cluster LCAO expansions (including or not impurity second neighbor components), and (c) relativistic Cowan–Griffin ab initio (core and environment) model potentials. The results show the importance of using basis set functions located at lattice sites next to the (CuO6)10− and (AgO6)10− clusters in order to mimic the results of the larger clusters (CuO6M6)2+ and (AgO6M6)2+. The net relativistic effects calculated for SrO:Cu2+, Ag2+ and the individual contributions due to the impurity and to the environmental Sr2+ ions have been analyzed, all of them being rather small. The use of a simple point-charge model for the MgO, CaO, and SrO crystalline environments (Madelung potential) is shown to be inadequate and to lead to meaningless results, in agreement to what has been found in previous studies of bulk and surface defects in other ionic crystals. Given that the Ag2+ impurity in SrO, CaO, and MgO hosts is accepted to be an exceptionally clear example of transition from static to dynamic Jahn–Teller effect at low temperature, the reproduction of this trend can be considered to be a demanding test for an embedding method like the AIEMP: A joint analysis of our calculations and the available EPR experimental results is presented which reveals an agreement in the conclusion that the environmental effects promote stronger Jahn–Teller coupling from MgO to SrO. Also, the inadequacy of the strong linear Jahn–Teller coupling approximation for MgO:Cu2+ is suggested to be responsible for the present incomplete understanding of the features of its EPR spectrum, in contrast with the other systems.