Structure and spectroscopy of Cr3+ defects in KMgF3, KZnF3, and CsCaF3 crystals. An ab initio model potential embedded cluster study

摘要

The ab initio model potential (AIMP) method has been proven to produce effective one-electron operators that accurately represent the embedding effects of the cations and anions that consititute crystalline lattices like the KMgF3, KZnF3, and CsCaF3 cubic fluoroperovskites. The combination of these quantum mechanical embedding potentials with highly sophisticated molecular quantum chemical calculations of the CrF63- defect cluster enables theoretical study of the structure and spectroscopy of promising laser materials like the Cr3+-doped KMgF3, KZnF3, and CsCaF3 crystals at the level of quality attainable in molecular quantum chemistry. The combination of the theoretical results with available experimental data [electron paramagnetic resonance (EPR), electron spin resonance (ESR), absorption, and emission spectra at ambient and high pressures] can contribute to clarifying the electronic structure of these systems, where different interpretations of the very sophisticated spectroscopic data exist, mainly due to the difficulty associated with the existence of cubic, trigonal, and tetragonal Cr3+ defects contributing to the spectra. The results of AIMP embedded-cluster studies of the cubic defects are presented here as one more useful and independent source of information that serves to clarify the divergent assignments and to provide new spectroscopic information, which shows that the laser emission of Cr3+ defects in all three crystals should be free from excited state absorption losses if the pumping process is done through selective excitation to the T-4(2g) laser level, below the E-2(g) higher excited state. Since the Cr3+ substitutional impurities create an excess positive charge, the effects of lattice site relaxation and dipole polarization on the local structure have been investigated and are presented here. (C) 2000 John Wiley & Sons, Inc. [References: 40]