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

Direct-current measurements were used to determine the electrical resi stivities of several manganese oxide materials having either layered o r tunnel structures. Birnessite (Na-OL-1, OL = octahedral layer) consi sts of layers of edge- and corner-sharing MnO6 octahedral units with N a+ in the interlayer regions. Todorokite and cryptomelane (Mg-OMS-1 an d K-OMS-2, respectively, OMS = octahedral molecular sieve) are similar ly built from MnO6 units, but in these systems the octahedra join to f orm 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 material s in which (a) cation exchange was carried out at layer and tunnel sit es or (b) isomorphous substitution for Mn was performed by doping smal l amounts of foreign cations into the manganese oxide framework. Four probe measurements on pressed pellets reveal that OL-1 and OMS-1 mater ials 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 patter n of increasing resistivity with decreasing temperature. However, betw een 153 and 293 K, OL-1 and OMS-1 materials do not obey a simple expon ential variation of resistivity and temperature. By contrast, OMS-2 sa mples follow the Arrhenius relationship over a comparable temperature range. Activation energies for the conductivity OMS-2 materials were c alculated to be in the range 0.5-0.6 eV. Solid-state voltammetry was u sed to determine the electrical resistance of OMS-1 and OMS-2 samples at higher temperatures from 298 to 673 K. A general exponential decrea se; in resistance with increasing temperature was observed for both cl asses of materials. Ac resistivity measurements show similar trends to de resistivity data.