METAL (M) DOPANT CENTERED LOCAL STRUCTURES, HIGH-PRESSURE SYNTHESIS AND BULK SUPERCONDUCTIVITY IN YBA2(CU1-XMX)(3)O7-DELTA - M=FE, CO, NI

Depression of T-c by the dopants Fe or Co, but not Ni, is spectacularl y reversed by synthesis of the titled cuprates at high P instead of am bient P of oxygen with T-c enhanced from 32 to 82 K for M = Fe at x = 0.10. With Fe dopant, Mossbauer spectroscopy reveals a conversion of a tetrahedral (A) into a trigonal bipyramidal coordinated (C) Fe chain site by oxygen addition in the chains upon high-P processing. The A -- > C site transformation is elucidated and extended to the case of Co d opant, and its consequences for superconducting behaviour discussed.