MIXED-VALENT NICKEL AND MANGANESE OXIDE CERAMICS - MODEL SYSTEMS WITH SUPERCONDUCTING PROPERTIES

Various oxidic solids of the perovskite and K2NiF4 type with Mn(III) and low-spin Ni(III) have been synthesized with clear structural and spectroscopic evidence for strong vibronic Jahn-Teller interactions. In the latter case clustering seems to induce a change to the high-spin state. In mixed valent Mn(III)/Mn(IV) compounds the Jahn-Teller effect is suppressed by band-broadening effects above a critical Mn(IV) concentration. Mixed valence Ni(III)/Ni(IV) ceramics could also be prepared, in which the positive hole is presumably located predominantly on the oxygen ligands, however. Superconducting properties are not observed down to 4 K. The possible significance of a strongly Jahn-Teller unstable E ground state, which occurs in the cases of Cu2+, Mn3+, and low-spin Ni3+ in octahedral coordination, for the appearance of superconductivity is discussed. (C) 1995 Academic Press, Inc.