Anelastic spectroscopy study of the metal-insulator transition of Nd1-xEuxNiO3

Measurements are presented of the complex dynamic Young's modulus of NdNiO3 and Nd0.65Eu0.35NiO3 through themetal-insulator transition (MIT). Upon cooling, the modulus presents a narrowdip at theMIT followed by an abrupt stiffening of ~6%. The anomaly is reproducible between cooling and heating in Nd0.65Eu0.35NiO3 but appears only as a slow stiffening during cooling in undoped NdNiO3, in conformance with the fact that the MIT in RNiO3 changes from strongly first order to second order when the mean R size is decreased. The elastic anomaly seems not to be associated with the antiferromagnetic transition, which is distinct from the MIT in Nd0.65Eu0.35NiO3 . It is concluded that the steplike stiffening is due to the disappearance or freezing of dynamic Jahn-Teller (JT) distortions through theMIT, where the JT active Ni3+ is disproportionated into alternating Ni3+? and Ni3-? . The fluctuating octahedral JT distortion necessary to justify the observed jump in the elastic modulus is estimated as ~3% but does not have a role in determining the MIT, since the otherwise-expected precursor softening is not observed.