Large anisotropic thermal expansion and magnetostriction in the mixed-valence compound CeNi

We present thermal-expansion and magnetostriction measurements performed at low temperature along the three symmetry axes of the orthorhombic intermediate-valence compound CeNi. The field effects are interpreted as the sum of a main contribution arising from the intermediate-valence character, which is temperature independent, and a smaller one whose temperature and field dependences are characteristic of ${\mathrm{Ce}}^{3+}$ impurities. The low-temperature thermal expansion and the main contribution to the magnetostriction are large and strongly anisotropic. The volume effects, which are larger than in the ${\mathrm{CeSn}}_3$ reference intermediate-valence compound, are interpreted within a T/$T_f$ scaling-law model ($T_f$ being the fluctuation temperature) which predicts the observed temperature dependence of the thermal expansion and field dependence of the magnetostriction. The strong anisotropy, especially the unusual negative thermal expansion and perpendicular magnetostriction along b^ must be related mainly to the local elastic properties in which the covalent character of the 4f-5d-3d hybridization between Ce and Ni could play a predominant role.