SINGLE-CRYSTAL MIXED-VALENT TRIPOTASSIUM MANGANATE(VI) MANGANATE(VII) - X-RAY-DIFFRACTION STRUCTURE AND ELECTRON-TRANSFER RATE BY DIELECTRIC RELAXOMETRY AND DC CONDUCTIMETRY

A structure study by XRD of single crystals of the key stoichiometric mixed-valent salt KMnO4 . K2MnO4 allows detailed consideration of the electron-transfer mechanisms in DC conduction and dielectric relaxation. A simple primitive monoclinic cell was established, with discrete MnO4- and MnO42- anions identifiable by Mn-O bond-lengths close to those of the congener single salts; the closest Mn(VII)-Mn(VI) distance is 3.98 angstrom (cf. 4.25 angstrom for Mn(VII)-Mn(VII) and 4.875 angstrom for Mn(VI)-Mn(VI) in the respective congeners). The closest manganate-permanganate tetrahedra have parallel opposing basal faces with the oxygens interpenetrating in a staggered opposed-equilateral-triangles projection allowing close contact. The electron-transfer rate studied by dielectric relaxometry (DR) was (1.06 +/- 0.02) x 10(5) Hz at 293 K with activation energy E = 49 +/- 5 kJ mol-1 yielding from an electron-hop model a calculated DC conductivity of 8.5 x 10(-7) S cm-1, cf. measured (1.0 +/- 0.3) X 10(-7) S cm-1 with E = 49 +/- 2 kJ mol-1, in satisfactory agreement. The high rate of electron transfer, combined with the crystallographic distinguishability of the anion centres, implies a rapidly reversed electron transfer in the relaxation event which possibly proceeds via a marginally different mechanism from that for the DC conduction, accounting for the observed slightly higher relaxation rate compared with the DC-derived value.