Comparative analysis of crystal field effects and optical spectroscopy of six-coordinated Mn4+ ion in the Y2Ti2O7 and Y2Sn2O7 pyrochlores

Abstract The electronic energy levels of the six-coordinated Mn 4+ ion in the pyrochlores Y 2 B 2 O 7 ( B  = Sn 4+ , Ti 4+ ) have been computed using the exchange charge model of crystal field theory. The calculated Mn 4+ 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 2 Sn 2 O 7 arises from an increased orbital overlap effect between the Mn 4+ ion and the nearest oxygen ions, which are located at the 48 f crystallographic position of the pyrochlore lattice. This increased overlap in Y 2 Sn 2 O 7 occurs despite the fact that the Mn 4+ –O 2− bond distance in Y 2 Sn 2 O 7 is longer than in Y 2 Ti 2 O 7 and is attributed to a lack of hybridization (covalent bonding) between the filled 2 p orbital of oxygen ion occupying the 48 f site of the pyrochlore lattice and the filled Sn 4+ 4 d 10 orbital. The low temperature emission spectrum of Mn 4+ activated Y 2 Sn 2 O 7 is analyzed in terms of a weak zero phonon line (R-line) with accompanying vibrational side bands.