ELECTRICAL-RESISTIVITY MEASUREMENTS ON MANGANESE OXIDES WITH LAYER AND TUNNEL STRUCTURES - BIRNESSITES, TODOROKITES, AND CRYPTOMELANES

Direct-current measurements were used to determine the electrical resistivities of several manganese oxide materials having either layered or tunnel structures. Birnessite (Na-OL-1, OL = octahedral layer) consists of layers of edge- and corner-sharing MnO6 octahedral units with Na+ in the interlayer regions. Todorokite and cryptomelane (Mg-OMS-1 and K-OMS-2, respectively, OMS = octahedral molecular sieve) are similarly built from MnO6 units, but in these systems the octahedra join to form 6.9 and 4.6 Angstrom tunnels occupied by Mg2+ and K+, respectively. Resistivities were also measured for OL-1, OMS-1, and OMS-2 materials in which (a) cation exchange was carried out at layer and tunnel sites or (b) isomorphous substitution for Mn was performed by doping small amounts of foreign cations into the manganese oxide framework. Four probe measurements on pressed pellets reveal that OL-1 and OMS-1 materials have resistivities on the order of 10(5)-10(6) Omega cm at 298 K. OMS-2 materials have resistivities in the range 10(5)-10(6) Omega cm at 298 K. Variable-temperature measurements establish a general pattern of increasing resistivity with decreasing temperature. However, between 153 and 293 K, OL-1 and OMS-1 materials do not obey a simple exponential variation of resistivity and temperature. By contrast, OMS-2 samples follow the Arrhenius relationship over a comparable temperature range. Activation energies for the conductivity OMS-2 materials were calculated to be in the range 0.5-0.6 eV. Solid-state voltammetry was used to determine the electrical resistance of OMS-1 and OMS-2 samples at higher temperatures from 298 to 673 K. A general exponential decrease; in resistance with increasing temperature was observed for both classes of materials. Ac resistivity measurements show similar trends to de resistivity data.