Departamento de Física
URI permanente desta comunidadehttps://repositorio.fei.edu.br/handle/FEI/785
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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.
- Anelastic spectroscopy study of the metal-insulator transition of Nd 1-xEuxNiO3(2011) Cordero F.; Trequattrini F.; Barbeta V.B.; Jardim R.F.; Torikachvili M.S.Measurements are presented of the complex dynamic Young's modulus of NdNiO3 and Nd0.65Eu0.35NiO3 through the metal-insulator transition (MIT). Upon 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 Nd0.65Eu 0.35NiO3 but appears only as a slow stiffening during cooling in undoped NdNiO3, in conformance 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 Nd 0.65Eu0.35NiO3. 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+δ and Ni3-δ. 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. © 2011 American Physical Society.
- 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.