Excited states of ZnS:Mn and ZnSe:Mn stabilized by the Jahn–Teller active E mode: Calculations by semiempirical self-consistent field methods

Abstract

The splitting of the quartet excited states of Mn2+ doped in ZnS and ZnSe due to coupling with the normal E vibration of tetrahedral MnS4 and MnSe4 clusters is studied by semiempirical methods which neglect the differential overlap between atomic orbitals. Stable excited state geometries of the impurity centers are found only if (i) the second neighbor Zn ions are included in the treated clusters and (ii) the collective E vibration of all the atoms neighboring the central Mn ion is taken into account.

On the Jahn–Teller effect on Mn2+ in zinc-blende ZnS crystal

“…We should note, however, that such extreme structures are improbable since the surrounding lattice would hinder them, as previously discussed elsewhere. 12 For this reason, we constrain the presentation of our results in Fig. 1͑a͒ to angle distortions of Ϯ40°although these limits might also be considered too great.…”

“…No stabilization has been observed in this case, as determined previously. 12 A slight minimum can be seen at the T d geometry for which the a 4 T 1 term is of lowest energy. This minimum must result from the lattice model that we are using and not allowing the cluster to distort.…”

“…These values are obtained for small second neighbor displacements, typically 0.005 Å, and angle distortions up to Ϯ20°produced by the JT active e-mode. 12 Some of the higher excited states that are reached from the first excited state ͑ESA͒ might be similarly distorted, and the errors cancel, others might not distort or distort along a different symmetry, leading us to underestimated transition energies. In this latter case, the maximum error might be estimated at 0.16 -0.20 eV.…”

“…As shown in previous studies, 4,12 the inclusion of the surrounding lattice is necessary to correctly assess the transition energies 4 and the JT splittings. 12 Toward this end, we have considered an external potential modeled by 12 point charges, each having ϩ0.5 charge, situated at the crystallographic positions corresponding to Zn lattice sites in ZnS-3.82 Å away from the central Mn 2ϩ ion. The addition of 12 point charges creates a neutral cluster of overall T d symmetry and forms an external potential that simulates the rest of the lattice.…”

“…In previous papers 11, 12 semiempirical methods have been used to elucidate problems relating to the JT effect of Mn 2ϩ in group II-IV compounds. In this work we apply the intermediate neglect of differential overlap model parametrized for spectroscopy ͑INDO/S͒ within the restricted open-shell Hartree-Fock ͑ROHF͒ approximation to the excited states of Mn 2ϩ in ZnS, and follow these calculations with a configuration interaction ͑CI͒ treatment.…”