Charge localization and successive magnetic phase transitions of mixed-valence manganese oxides K1.5(H3O)xMn8O16 (0<x<0.5)

Magnetism and transport properties are reported for hollandite-type manganese oxides K-1.5(H3O)(x)Mn8O16 (0<x<0.5) having a tunnel structure. Three phase transitions are observed at T-1 (varies from 180 to 250 K, very sensitive to the synthetic condition), at T-2 (52 K) and at T-3 (20 K). The conductivity and the thermoelectric power indicate that the electric conduction above T-1 is governed by the hopping of small polarons and that the polarons begin to be trapped at a part of Mn sites below T-1. The magnetic susceptibility suggests that the T-1 transition is a phase transition from a single-spin system (T>T-1) composed of only S approximate to 1.63 spins, to a multispin system composed of S=2 spins and S=3/2 spins. These observations strongly suggest that T-1 is a kind of a charge-localization transition of Mn4+ and Mn3+. Magnetic susceptibility also reveals that a weak ferromagnetism appears between T-2 and T-3 and disappears again below T-3. From the anisotropy of the magnetic susceptibility, the ground state is considered to be a helical magnetism. [S0163-1879(99)09219-X].