Comparative analysis of crystal field effects and optical spectroscopy of six-coordinated Mn⁴⁺ ion in the Y₂Ti₂O ₇ and Y₂Sn₂O₇ pyrochlores

The electronic energy levels of the six-coordinated Mn⁴⁺ ion in the pyrochlores Y₂B₂O₇ (B = Sn⁴⁺, Ti⁴⁺) have been computed using the exchange charge model of crystal field theory. The calculated Mn⁴⁺ energy levels and their trigonal splitting are in good agreement with the experimental spectra. The calculated crystal field parameters show that the higher crystal field strength in Y ₂Sn₂O₇ arises from an increased orbital overlap effect between the Mn⁴⁺ ion and the nearest oxygen ions, which are located at the 48f crystallographic position of the pyrochlore lattice. This increased overlap in Y₂Sn₂O₇ occurs despite the fact that the Mn⁴⁺-O^2- bond distance in Y ₂Sn₂O₇ is longer than in Y₂Ti ₂O₇ and 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. The low temperature emission spectrum of Mn⁴⁺ activated Y ₂Sn₂O₇ is analyzed in terms of a weak zero phonon line (R-line) with accompanying vibrational side bands.