TEMPERATURE DEPENDENCE OF MAGNETIC EXCITATIONS OF URANIUM DIOXIDE

The magnetic excitations of UO2 have been measured by inelastic neutron- scattering techniques with the triple-axis crystal spectrometer at the C5 facility of NRU. On increasing the temperature, the neutron groups from the modes of higher (>1.5×1012 Hz) frequency decreased slightly in frequency, barely increased in width, but decreased markedly in intensity, and were not observable above TN[(30.5±0.5)°K for our specimen]. For example the results for the frequencies of the modes with scattering vector Q = (0.5, 0.5, 2.5)2π/a are (ν(T)×1012 Hz): ν(9) = 2.02±0.03, ν(21) = 2.00±0.04, ν(25.3) = 1.95±0.05, ν(27.0) = 1.75±0.05, ν(29.5) = 1.50±0.07. The neutron groups from the modes with lower frequencies decreased more markedly in frequency, but also showed little increase in width and were not observed above TN. The experiments indicate that the modes of lowest frequency which appeared to have almost zero frequency at TN are closely associated with the transition. These measurements, together with the strong magnon-phonon coupling in UO2 and the known temperature dependence of the elastic constants, suggest that the transition in UO2 is a Jahn-Teller distortion of the lowest-triplet state coupled to an antiferromagnetic ordering. © 1968 The American Institute of Physics.