Metal-insulator transition in Nd1-xEuxNiO3: Entropy change and electronic delocalization

Abstract

© 2015 AIP Publishing LLC.The metal-insulator (MI) phase transition in Nd1-xEuxNiO3, 0 ≤ x ≤ 0.35, has been investigated through the pressure dependence of the electrical resistivity ρ(P, T) and measurements of specific heat CP(T). The MI transition temperature (TMI) increases with increasing Eu substitution and decreases with increasing pressure. Two distinct regions for the Eu dependence of dTMI/dP were found: (i) for x ≤ 0.15, dTMI/dP is nearly constant and ∼-4.3 K/kbar; (ii) for x ≤ 0.15, dTMI/dP increases with x and it seems to reach a saturation value ∼-6.2 K/kbar for the x = 0.35 sample. This change is accompanied with a strong decrease in the thermal hysteresis in ρ(P, T) between the cooling and warming cycles, observed in the vicinity of TMI. The entropy change (ΔS) at TMI for the sample x = 0, estimated by using the dTMI/dP data and the Clausius-Clapeyron equation, resulted in ΔS ∼ 1.2 J/mol K, a value in line with specific heat measurements. When the Eu concentration is increased, the antiferromagnetic (AF) and the MI transitions are separated in temperature, permitting that an estimate of the entropy change due to the AF/Paramagnetic transition be carried out, yielding ΔSM ∼ 200 mJ/mol K. This value is much smaller than that expected for a s = 1/2 spin system. The analysis of ρ(P, T) and CP(T) data indicates that the entropy change at TMI is mainly due to the electronic delocalization and not related to the AF transition.