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 Mn{sub 6} octahedral units with Na{sup +} in the interlayer regions. Todorokite and cryptomelane (Mg-OMS-1 and K-OMS-2, respectively, OMS = octahedral molecular sieve) are similarly built from MnO{sub 6} units, but in these systems the octahedral join to form 6.9 and 4.6 {Angstrom} tunnels occupied by Mg{sup 2+} and K{sup +}, 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 subsititution 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{sup 5}-10{sup 6} {Omega}cm at 298 K. OMS-2 materials have resistivities on the order of 10{sup 5}-10{sup 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 dc resistivity data. 36 refs., 8 figs., 3 tabs.