Infrared optical properties of (formula presented)

The (formula presented)-plane reflectance of a (formula presented) single crystal has been measured over a wide frequency range at a variety of temperatures, and the optical properties determined from a Kramers-Kronig analysis. Above (formula presented) the low-frequency conductivity increases quickly with temperature; (formula presented) follows the form (formula presented) where (formula presented) is much less than the inferred optical gap of (formula presented) Transport measurements show that at low temperature the resistivity deviates from activated behavior and follows the form (formula presented) indicating that the dc transport in this material is due to variable-range hopping between localized states in the gap. The four infrared-active (formula presented) modes dominate the infrared optical properties. Below (formula presented) a striking new feature appears near the low-frequency (formula presented) mode, and there is an additional new fine structure at high frequency. A normal coordinate analysis has been performed and the detailed nature of the zone-center vibrations determined. Only the low-frequency (formula presented) mode has a significant Pr-Cu interaction. Several possible mechanisms related to the antiferromagnetism in this material are proposed to explain the sudden appearance of this and other new spectral features at low temperature.