Single-crystal mixed-valent tripotassium manganate(VI)/(VII). X-ray diffraction structure and electron-transfer rate by dielectric relaxometry and DC conductimetry

Abstract

A structure study by XRD of single crystals of the key stoichiometric mixed-valent salt MnO₄· K₂MnO₄ allows detailed consideration of the electron-transfer mechanisms in DC conduction and dielectric relaxation. A simple primitive monoclinic cell was established, with discrete MnO^–₄ and MnO^2–₄ 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 Å(cf. 4.25 Å for Mn^VII—Mn^VII and 4.875 Å 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)× 10⁵ 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 × 10^–7 S cm^–1, cf. measured (1.0 ± 0.3)× 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.