Thermal Conductivity and Far-Infrared Absorption of UO2

Abstract

The thermal conductivity of single‐crystal UO₂ has been measured from liquid‐helium temperature up to room temperature. The sample temperature was determined, with a helium gas bulb thermometer, and the temperature gradient was measured with Au–Co vs Manganin differential thermo‐couples. In the vicinity of the Néel temperature, T_N=30.8°K, the conductivity is depressed by two orders of magnitude, from that expected for a similar diamagnetic crystal. The experimental results demonstrate that a strong phonon‐magnon coupling exists in this crystal. The far‐infrared transmission of the same UO₂ crystal has been measured from a frequency of 5–115 cm⁻¹ at 3°K. A grating monochromator and a Strong‐type lamellar interferometer have been used for these measurements. A sharp absorption line has been found at 17.63±0.1 cm⁻¹. Broader absorption lines have been observed at 19.2, 23, 79, and 100 cm⁻¹. Transmission measurements in which a 50‐kG magnetic field is applied to the sample or the sample temperature is varied indicate that impurity‐induced absorptions occur at 19.2 and 23 cm⁻¹, and the three remaining lines can be identified with the antiferromagnetic state of UO₂.