Comparative analysis of crystal field effects and energy level scheme of six-fold coordinated Cr⁴ ion in the pyrochlores, Y₂B ₂O₇ (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 Y ₂Sn₂O₇ (in comparison with Y₂Ti ₂O₇) 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 Cr⁴O² bond distance in Y₂Sn₂O ₇ 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 Y₂Sn₂O ₇, even if the Cr⁴O^2- distances are greater in this case, when compared to those in Y₂Ti₂O₇.