Comparative analysis of crystal field effects and energy level scheme of six-fold coordinated Cr~4 ion in the pyrochlores, Y_2B _2O_7 (B=Ti~4, Sn~4)

摘要

The electronic energy levels of the six-fold coordinated Cr~4 ion in the pyrochlores Y_2B_2O_7 (B=Sn~4, Ti~4), have been computed using the exchange charge model of crystal field theory. The calculated Cr~4 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 Y _2Sn_2O_7 (in comparison with Y_2Ti _2O_7) arises from increased orbital overlap effects between the Cr~4 ion and the nearest oxygen ions, which are located at the 48f crystallographic position of the pyrochlore lattice. The increased overlap in Y_2Sn_2O_7 occurs despite the fact that the Cr~4O~2 bond distance in Y_2Sn_2O _7 is longer than in Y_2Ti_2O_7. 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 Sn~4 4d~(10) orbital. As a result, a stronger crystal field is experienced by Cr~4 ions in Y_2Sn_2O _7, even if the Cr~4O~(2-) distances are greater in this case, when compared to those in Y_2Ti_2O_7.