25-K phase transition inNpO2fromNp237Mössbauer spectroscopy

${}_{}{}^{237}{}_{}{}^{}\mathrm{Np}$ Mössbauer spectroscopy in magnetic external fields up to 8 T and in the temperature range 1.5–160 K, demonstrates that ${\mathrm{NpO}}_2$ remains paramagnetic well below $T_c$=25 K. The specific-heat anomaly, the cusp in magnetic susceptibility, and the changes in the shape of the Mössbauer spectra observed at 25 K are attributed to a structural transition. Both the bulk susceptibility and the Mössbauer data can be satisfactorily reproduced under the assumption of an inhomogeneous compressional-expansional internal distortion of the oxygen sublattice. It is suggested that a magnetic ordering of ${\mathrm{NpO}}_2$ is prevented (or at least depressed) by this competitive structural transition.