Abstract
$^{237}\mathrm{Np}$ M\ossbauer 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 M\ossbauer spectra observed at 25 K are attributed to a structural transition. Both the bulk susceptibility and the M\ossbauer 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.
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