Comparative analysis of crystal field effects and optical spectroscopy of six-coordinated Mn4+ ion in the Y2Ti2O7 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 Me 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 Y2Sn2O7 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+-O2- bond distance in Y2Sn2O7 is longer than in Y2Ti2O7 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+ 4d(10) orbital. The low temperature emission spectrum of Mn4+ activated Y2Sn2O7 is analyzed in terms of a weak zero phonon line (R-line) with accompanying vibrational side bands. (C) 2011 Published by Elsevier B.V.