AN INSIGHT INTO CHARGE-TRANSFER TRANSITIONS AND THEIR BANDWIDTHS - ANALYSIS OF NA3IN2LI3F12-CR3+

Spin-polarized MS-X alpha and self-consistent charge extended Huckel c alculations 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 th e vacuum ultraviolet excitation spectrum of Na3In2Li3F12:Cr3+ associat ed with charge-transfer (CT) transitions as well as what electronic or bitals 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 tr ansitions involving at least one-orbital without a bonding would displ ay a smaller oscillator strength. It is shown that the two referred tr ansitions both exhibit a strong dependence on R (partial derivative E/ partial derivative R being about -200 meV pm(-1)) whose microscopic or igin is explained. Using this R dependence an explanation of the exper imental band width W of a CT transition is attempted for the first tim e. The present analysis indicates that W can be understood simply cons idering the coupling with the A(1g) mode, leading to a Huang-Rhys fact or 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 pres ent results together with experimental data on other 3d complexes indi cate that the gamma(L) down arrow--> e(g) down arrow transitions (whe re gamma(L) is t(1u)(sigma-pi) and t(1u)(sigma+pi)) are probably conta ined in the broad third band peaked at 12 eV. Also they shed some ligh t on the experimental results of other systems such as Al2O3:Ti4+ and Cs2NaYCl6:Fe3+ which are briefly discussed.