Computer Modeling of the Optical Properties of Transition-Metal Ions in Solids
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Storrs, C" {06269-3046, U.S.A, ABSIRACT Computational methods for modeling the optical properties of substitutional transition-metal impurities in insulating solids, potentially applicable to some scintillator and phosphor materials, are reviewed. Methods considered include crystal-field and semiempirical ligand-field models; SCF-Xa--SW, SC(-RlI; I.C(A.), SCYITtI--CA and (1 db hw&ýkimethods; and ICIEAP and IIAI)ISR embedded cluster methods with lattice relaxation. A detailed example of the application of the HAl)t{SR method to crystal-field spectra of Cr-3 ' in halide elpasolites is described. In this method, ab rnd,,molecular--orbital
calculations with effective core potentials are performed for selected ionic configurations. Simultaneous relaxation of the cluster and surrounding lattice, with mutual pair. potential interactions, is accomplished by a modified lattice statics program. Calculated properties include pressure-dependent optical transition energies, vibration frequencies and radiationless transition rates. IN'ROD)IJCI'ION The optical properties of transition-metal ions in ionic crystals have long been ol interest for a variety of applications, including phosphors and scintillators. 'fransition metal spectra can be classified in three categories,' all of which are manifest in (he characteristic colors of minerals: (1) Crystal-field spectra involve transitions within the open d-shell of the transitionmetal ion and typically occur in the visible. Since electric-dipole transitions between these states are parity-forbidden in the free ion, crystal-field spectra have low oscillator strengths and correspondingly long emission lifetimes. Thlley are exemplified by Cr 3' in octahedral coordination. (2) Oxygen-metal charge-transfer spectra involve the transfer of an electron between oxygen ligands and the d-shell of the transition-metal ion which is unoccupied in the ground state. 'Ilie corresponding broad absorption and emission bands peak in the ultraviolet. Absorption is strongly electric-dipole allowed but emission is multiplicity forbidden with correspondingly long lifetimes. T'hese spectra are of particular interest for phosphor and scintillator applications.2 They are exemplified by non-luminescent MnO 4 and by the isoelectronic luminescent complexes W0 42 and VO4 . (3) Intervalence charge-transfer spectra involve transitions between neighboring transition-metal ions of the same or different species. Broad absorption bands peak in the red or infrared, and recombination is often non-radiative. These spectra are exemplified by the transfer of an electron from Fe2- to Fe 3+ and from Fe 2' to Ti4 '. 343
Mat. Res. Soc. Symp. Proc. Vol. 348. 01994 Materials Research Society
A concise review of computational methods for investigating the optical properties
of transition-metal complexes in solids is presented in the next section, and a detailed example of the applicalion ofa particular method is provided( in the following section. RIV1EW OF COMPUTTIA
ONAl. MFTIIODS
Crystal. FieldTheory Crystal-field theory had
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