Phase transition of NpO2 at 25 K

We have measured the Np-237 Mossbauer spectra of NpO2 whose specimens have been stored for 12 years before and after annealing at 773, 973, 1373 K, The Mossbauer spectrum of NpO2 after annealing consists of a symmetrical absorption line, while the line shape of the sample before annealing is broad and asymmetric below 25 K. The change of line shape is attributed to the phase transition at 25 K which might be enhanced by the existence of the self-irradiation-induced defects. Irradiation-induced damage can be recovered by annealing above 973 K. (C) 2000 Published by Elsevier Science B.V. All rights reserved.