Departamento de Física
URI permanente desta comunidadehttps://repositorio.fei.edu.br/handle/FEI/785
Navegar
2 resultados
Resultados da Pesquisa
- Metal-insulator transition in Nd1-xEuxNiO3: Entropy change and electronic delocalization(2015) Jardim R.F.; Barbeta V.B.; Andrade S.; Escote M.T.; Cordero F.; Torikachvili M.S.© 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.
- Metal-insulator transition in Nd1-x Eux NiO 3 probed by specific heat and anelastic measurements(2011) Barbeta V.B.; Jardim R.F.; Torikachvili M.S.; Escote M.T.; Cordero F.; Pontes F.M.; Trequattrini F.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-xEu xNiO3, 0≤x≤ 0.35. For x≥0, a peak in C P 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 the 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 T MI much more sharply upon warming, while the cooling and warming curves are reproducible above TMI. 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. © 2011 American Institute of Physics.