Insulator-metal transition by the substitution of Ho, Y or Ca for Pr in PrBa2Cu3O7-delta

Comparative studies of electrical transport in the non-superconducting part of the R1-xPrxBa2Cu3O7-delta system (R = Ho, Y; x > 0.60) and in the Pr1-x CaxBa2Cu3O7-delta system (x less than or equal to 0.5) have been carried ou t for ceramic samples. Electrical conduction of all samples at low temperat ures can be described by the formula sigma = sigma(0) + a(x)T(n), where n c ongruent to 1 for the Ho- and Y-systems and 1 less than or equal to n less than or equal to 1.88 for the Ca-system. For Ho1-xPrxBa2Cu3O7-delta sigma(0 ) not equal 0 and sigma(0) --> 0 as x --> 1, but for Pr1-xCaxBa2Cu3O7-delta sigma(0)= 0. Selected samples of the Ho1-xPrxBa2Cu3O7-delta system were su bstituted with gold, which blocks the conduction of chains. For gold substi tuted Ho1-xPrxBa2Cu2.92Au0.08O7-delta samples sigma(0) = 0. We conclude tha t the suppression of superconductivity in the Ho1-xPrxBa2Cu3O7-delta system is connected with a metal-insulator (M-I) transition in planes(at x approx imate to 0.6), but chains are still conducting and become insulating only i n the limit x = 1. The electrical conduction of chains is dominated by tunn eling, being possible due to the pronounced texture of samples. To get furt her insight into the mechanism of suppression, we studied also Sr- anal Ca- substituted, oxygenated and deoxygenated samples. No superconductivity was obtained although it is known that Pr loses its anomalous properties in deo xygenated samples and Ca induces superconductivity in the deoxygenated Y1-x CaxBa2Cu3O7-delta system. (C) 2000 Elsevier Science B.V. All rights reserve d.