MIXED-VALENT NICKEL AND MANGANESE OXIDE CERAMICS - MODEL SYSTEMS WITHSUPERCONDUCTING PROPERTIES

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