Magnetism and electron transport phenomena of manganese oxide ion exchanger with tunnel structure

Conductivity, thermoelectric power (TEP) and magnetic susceptibility of the tunnel-type manganese oxides K1.5Mn8O16 are reported. The conductivity and the TEP around room temperature suggest that the electrical conduction is governed by hopping of small-polarons. Three phase transitions are observed at T-1 (varies from 175 K to 215 K, batch-dependent), at T-2 (52 K) and at T-3 (20 K). The T-1 transition is considered to be a charge-ordering transition of Mn4+ and Mn3+ coupled with the ordering of the potassium ions in the tunnels. Weak ferromagnetism appears between T-2 and T-3 and disappears again below T-3. The anisotropic measurement of the susceptibility strongly suggests the occurrence of a helical magnetism below T-3. After the single crystals are soaked in cone. HNO3 for a few days, a part of the potassium ions in the tunnels are removed. Besides, the activation energy is drastically reduced and the T-1 transition is suppressed. This suggests that the charge-ordering of Mn4+ and Mn3+ is very sensitive to the state of the tunnel cations. (C) 1997 Elsevier Science S.A.