Metal-insulator transition in Nd1-xEuxNiO3 probed by specific heat and anelastic measurements

Oxides RNiO3 (R - rare-earth, R not equal La) exhibit a metal-insulator (MI) transition at a temperature T-MI and an antiferromagnetic (AF) transition at T-N. Specific heat (C-P) and anelastic spectroscopy measurements were performed in samples of Nd1-xEuxNiO3, 0 <= x <= 0.35. For x - 0, a peak in C-P is observed upon cooling and warming at essentially the same temperature T-MI - T-N similar to 195 K, although the cooling peak is much smaller. For x >= 0.25, differences between the cooling and warming curves are negligible, and two well defined peaks are clearly observed: one at lower temperatures that define T-N, and the other one at T-MI. 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 C-P, and no peak is observed upon cooling. The elastic modulus hardens below T-MI much more sharply upon warming, while the cooling and warming curves are reproducible above T-MI. Conversely, 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.