Tuning of phonon anharmonicity in pyrochlore titanates: temperature-dependent Raman studies of Sm₂Ti_2-xZr_xO₇ (x=0, ½, ¾, and 2) and stuffed spin-ice Ho_2+xTi_2-xO_7-x/2 (x=0, ⅓, and ⅔)

Abstract

Anomalous temperature dependence of Raman phonon wavenumbers attributed to phonon-phonon anharmonic interactions has been studied in two different families of pyrochlore titanates. We bring out the role of the ionic size of titanium and the inherent vacancies of pyrochlore in these anomalies by studying the effect of replacement of Ti⁴⁺ by Zr⁴⁺ in Sm₂Ti₂O₇ and by stuffing Ho³⁺ in place of Ti⁴⁺ in Ho₂Ti₂O₇ with appropriate oxygen stoichiometry. Our results show that an increase in the concentration of the larger ion, i.e. Zr⁴⁺ or Ho³⁺, reduces the phonon anomalies, thus implying a decrease in the phonon-phonon anharmonic interactions. In addition, we find signatures of coupling between a phonon and crystal field transition in Sm₂Ti₂O₇, manifested as an unusual increase in the phonon intensity with increasing temperature.