Comparative analysis of crystal field effects and energy level scheme of six-fold coordinated Cr4+ ion in the pyrochlores, Y2B2O7 (B=Ti4+, Sn4+)

The electronic energy levels of the six-fold coordinated Cr4+ ion in the pyrochlores Y2B2O7 (B=Sn4+, Ti4+), have been computed using the exchange charge model of crystal field theory. The calculated Cr4+ energy levels and their trigonal splitting are in good agreement with experimental spectra. Calculations of the crystal field parameters show that the higher crystal field strength in Y2Sn2O7 (in comparison with Y2Ti2O7) arises from increased orbital overlap effects between the Cr4+ ion and the nearest oxygen ions, which are located at the 48f crystallographic position of the pyrochlore lattice. The increased overlap in Y2Sn2O7 occurs despite the fact that the Cr4+-O2- bond distance in Y2Sn2O7 is longer than in Y2Ti2O7. This is attributed to a lack of hybridization (covalent bonding) between the filled 2p orbital of oxygen ion occupying the 48f site of the pyrochlore lattice and the filled Sn4+ 4d10 orbital. As a result, a stronger crystal field is experienced by Cr4+ ions in Y2Sn2O7, even if the Cr4+-O2- distances are greater in this case, when compared to those in Y2Ti2O7.