An insight into charge-transfer transitions and their bandwidths: Analysis of Na3In2Li3F12:Cr3+

Spin-polarized MS-X alpha and self-consistent charge extended Huckel calculations on CrF63- as a function of the Cr-F- distance R have been carried out in order to clarify the origin of the bands detected in the vacuum ultraviolet excitation spectrum of Na3In2Li3F12:Cr3+ associated with charge-transfer (CT) transitions as well as what electronic orbitals are involved in the jump. With the help of results on other 3d impurities it is explained that the two first transitions peaked at 8.0 and 8.7 eV are the t(1u)(sigma-pi) up arrow--> e(g)* up arrow and t(1u)(sigma+pi) up arrow--> e(g)* up arrow CT transitions while other transitions involving at least one-orbital without a bonding would display a smaller oscillator strength. It is shown that the two referred transitions both exhibit a strong dependence on R (partial derivative E/partial derivative R being about -200 meV pm(-1)) whose microscopic origin is explained. Using this R dependence an explanation of the experimental band width W of a CT transition is attempted for the first time. The present analysis indicates that W can be understood simply considering the coupling with the A(1g) mode, leading to a Huang-Rhys factor S = 20. Also it is pointed out that CT transitions involving the t(2g)* orbital would give rise to a significant decrement in S. The present results together with experimental data on other 3d complexes indicate that the gamma(L) down arrow--> e(g)* down arrow transitions (where gamma(L) is t(1u)(sigma-pi) and t(1u)(sigma+pi)) are probably contained in the broad third band peaked at 12 eV. Also they shed some light on the experimental results of other systems such as Al2O3:Ti4+ and Cs2NaYCl6:Fe3+ which are briefly discussed.