  Oxides RNiO3 (R = rare-earth, R # La) exhibit a metal-insulator (MI)
transition at a temperature TMI and an antiferromagnetic (AF) transition at TN.
Specific heat (CP) and anelastic spectroscopy measurements were performed in
samples of Nd1-xEuxNiO3, 0 <= x <= 0.35. For x = 0, a peak in CP is observed
upon cooling and warming at essentially the same temperature TMI = TN ~ 195 K,
although the cooling peak is much smaller. For x >= 0.25, differences between
cooling and warming curves are negligible, and two well defined peaks are
clearly observed: one at lower temperatures, that define TN, and the other one
at TMI. An external magnetic field of 9 T had no significant effect on these
results. The elastic compliance (s) and the reciprocal of the mechanical
quality factor (Q^-1) of NdNiO3, measured upon warming, showed a very sharp
peak at essentially the same temperature obtained from CP, and no peak is
observed upon cooling. The elastic modulus hardens below TMI much more sharply
upon warming, while the cooling and warming curves are reproducible above TMI.
On the other hand, for the sample with x = 0.35, s and Q^-1 curves are very
similar upon warming and cooling. The results presented here give credence to
the proposition that the MI phase transition changes from first to second order
with increasing Eu doping.
