Influence of the valence states of atoms on conducting properties of PrBa2Cu3O6 + x

Pr3+ valent state only was discovered in PrBa2Cu3O6 + x within the oxygen content range 0.4 ≤ x ≤ 0.6. Out of this range, Pr4+ state appears also. All the samples obtained showed insulating behavior of resistivity r(T). In PrBa2Cu3O6 + x (0.4 ≤ x ≤ 0.6), the resistivity is Mott variable-range hopping conduction type and is connected with oxygen disorder only. Out of this range, the localization is much stronger; this increase is due to Pr4+/Pr3+ position disorder. Also, the localization decreases after annealing at room temperature in the results of ordering. Thus, the absence of metallic (and therefore superconducting) state in PrBa2Cu3O6 + x is due to the presence of both Pr4+ and Pr4+/Pr3+ disorder.