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1.3 Metal-insulator transitions The NdNiO3−δ oxygen deficient compounds have shown a great difference in temperature dependence of resistivity with respect to their non-stoichiometry(91). The almost stoichiometric NdNiO3 had the lowest MIT temperature (180 K) and a significant hysteresis in resistivity curve with a resistivity on heating larger than that on cooling. For deficiency of over 0.2 no transition was observed up to 300 K. The metallic regime was corresponding well to linear dependence of resistivity which was attributed to electron-phonon scattering mechanism. In relation to Zaanen, Sawatzky and Allen theory(99) the authors of (91) tried to explained the low temperature behaviour of deficient NdNiO3 compounds. While the stoichiometric NdNiO3 was said to show activated behaviour below 200 K (based on Zaanen), the deficient compound did not fit the theory. NdNiO3−δ did not fit well the activated transport equation: σ = σ∞exp[−T0 ]. (1.10) T where σ∞ and T0 are constants; neither Variable Range Hopping (VRH) con- ductivity σ = AT−1/2exp[−(T0 )0.25]. (1.11) T where A and T0 are constants. Instead, an empirical relation was found σ = σ(0) + ATm. (1.12) here σ(0) > 0, A > 0, m ≈ 0.3 − 0.4 to fit very good the low temperature exper- imental data. A non zero 0 K conductivity was attributed to be the result of an incomplete phase transition where a metallic phase exists down to very low tem- peratures and contributes to conductivity. An increase of this 0 K conductivity with the increase of oxygen concentration was found. Authors concluded that this is because of increase of residual metallic phase. The parameter ’m’ was found to be similar to observed in highly disordered metals, as if the insulating phase made the mixed in metallic phase disordered. Other models of electrical conductivity at low temperatures include: - thermally activated Arrhenius law σ(T) = σ(0)exp(−Ea/kBT) (1.13) where kB is Boltzmann constant and activation energy Ea is 22 meV for PrNiO3(35) (for low-T data below 100 K), and 25-28 meV for NdNiO3 (36). However at lower temperature this energy increases slightly. Which is corresponding to rapid in- crease of the semiconductor-like bandgap. 19PDF Image | Investigation of metal-insulator transition in magnetron sputtered samarium nickelate thin films
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