  Measurements are presented of the complex dynamic Young's modulus of NdNiO3
and Nd0.65Eu0.35NiO3 through the Metal-Insulator Transition (MIT). On cooling,
the modulus presents a narrow dip at the MIT followed by an abrupt stiffening
of ~6%. The anomaly is reproducible between cooling and heating in
Nd0:65Eu0:35NiO3 but only appears as a slow stiffening during cooling in
undoped NdNiO3, conformingly 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 the MIT, where the JT active Ni3+ is
disproportionated into alternating Ni3+d and Ni3-d. 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.
