INFLUENCE OF THE VALENCE STATES OF ATOMS ON CONDUCTING PROPERTIES OF PRBA2CU3O6+X

Pr3+ valent state only was discovered in PrBa2Cu3O6+x, within the oxyg en content range 0.4 less than or equal to x less than or equal to 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 less than or equal to x less than or equal to 0.6), the resistivity i s Mott variable-range hopping conduction type and is connected with ox ygen disorder only. Out of this range, the localization is much strong er this increase is due to Pr4+/Pr3+ position disorder. Also, the loca lization decreases after annealing at room temperature in the results of ordering. Thus, the absence of metallic (and therefore superconduct ing) state in PrBa2Cu3O6+x is due to the presence of both Pr4+ and Pr4 +/Pr3+ disorder. (C) 1997 Elsevier Science Ltd.