Metal (M) dopant centred local structures, high-pressure synthesis and bulk superconductivity in YBa2(Cu1-xMx)3O7-δ: M = Fe, Co, Ni

Depression of Tc by the dopants Fe or Co, but not Ni, is spectacularly reversed by synthesis of the titled cuprates at high P instead of ambient P of oxygen with Tc enhanced from 32 to 82 K for M=Fe at x=0.10. With Fe dopant, Moessbauer 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 dopant, and its consequences for superconducting behaviour discussed. (author). Letter-to-the-editor