Optical properties of pyrochlore oxides R2Mo2O7-δ (R:Sm, Gd, and Ho)

The temperature dependence of the reflectance of the spin glass materials R2Mo2O7-δ (R:Sm, Gd, and Ho) has been measured for frequencies from 40 to 40 000 cm-1. The AC conductivity of Sm2Mo2O7-δ derived from Kramers-Kronig analysis, indicates a Drude-like behaviour as the temperature is lowered. Between 150 and 40 K the scattering rate shows a sharp drop which is attributed to the scattering of conduction electrons by short-range ordered moments of the Mo ions. Below the spin glass transition temperature Tf (≅40 K) the scattering rate saturates due to the freezing of the moments. Gd2Mo2O7-δ behaves like a poor metal at room temperature, but at low temperatures shows a linear increase in the conductivity for frequencies up to 400 cm1 suggesting a localized hopping conductivity. The localization remains in the spin glass state for T<Tf (≅25 K). The conductivity of Ho2Mo2O7-δ is semiconductor-like with a small and slightly temperature-dependent gap around 0.25 eV. Our optical results support a qualitative band model proposed by Sleight and Bouchard for the pyrochlore oxides. (author)