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

The ab initio model potential (AIMP) method has been proven to produce effe ctive one-electron operators that accurately represent the embedding effect s 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 theoret ical study of the structure and spectroscopy of promising laser materials l ike the Cr3+-doped KMgF3, KZnF3, and CsCaF3 crystals at the level of qualit y attainable in molecular quantum chemistry. The combination of the theoret ical results with available experimental data [electron paramagnetic resona nce (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 sop histicated spectroscopic data exist, mainly due to the difficulty associate d with the existence of cubic, trigonal, and tetragonal Cr3+ defects contri buting 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 prov ide new spectroscopic information, which shows that the laser emission of C r3+ defects in all three crystals should be free from excited state absorpt ion losses if the pumping process is done through selective excitation to t he T-4(2g) laser level, below the E-2(g) higher excited state. Since the Cr 3+ substitutional impurities create an excess positive charge, the effects of lattice site relaxation and dipole polarization on the local structure h ave been investigated and are presented here. (C) 2000 John Wiley & Sons, I nc.