Brent D. Cutforth

Electrical conductivity of linear chain mercury compounds

Abstract The electrical conductivity of the compounds Hg2,86AsF6 and Hg2.91SbF6 containing linear cationic chains of mercury atoms with a mercury-mercury distance that is incommensurate with the lattice has been measured from 4.2 to 300K. It is shown that they have anisotropic metallic conductivity that increases markedly with decreasing temperature.

THE LINEAR CHAIN MERCURY COMPOUNDS: Hg2.86AsF6 AND Hg2.91SbF6. ELECTRICAL CONDUCTIVITY, OPTICAL REFLECTANCE, AND THERMOELECTRIC STUDIES

The compounds Hg,,8,ASF6 and Hg,,,, SbF, were discovered a t McMaster University in 1971 during an investigation of the formation of polymercury cations Hg/+.l The reactions of mercury with SbFS and AsFS in solution in SO2 were found t o lead to the formation of the linear Hg:+ and Hg?’ cations in the form of their soluble hexafluoroarsenate and hexafluoroantimonate salts, together with an insoluble crystalline material with a golden metallic appearan~e.’ ,~ Determination of the structures of these insoluble products showed that they have the compositions H~J,,~,ASF, and Hg2.9, SbF,.4v5 A better method for making large crystals of these materials has subsequently been found t o be the reaction of metallic mercury with Hg,(AsF,), or Hg,(SbF,), a t 10 to -20°C in solution in so,., The reaction is slow but good crystals are usually obtained after approximately one week. The two compounds have essentially the same structure (FIGURE l ) . 4 3 5 The AsF,or SbF6octahedra are situated in a tetragonal lattice with a = 7.54 8, a n d c = 12.32 A for Hg,,,,AsF, and a = 7.67 A and c = 12.61 A for Hg,,,,,SbF,. There are four ASF6or SbF,ions per unit cell. These anions form a framework with linear, nonintersecting, mutually perpendicular channels in which infinite linear mercury chains are situated. A schematic view of the relationship of the mercury chains in four planes perpendicular to the c axis is shown in FIGURE 2. The shortest distance between two chains in consecutive planes occurs where the chainscross and is 3.09 A in Hg2,86A~F6 and 3.15 A in Hg2,,,SbF6. The mercury atoms in the chains have a nominal formal charge of +0.35 and are separated, on average, by 2.64 A. Since the intrachain mercury-mercury distance is incommensurate with the tetragonal lattice, each mercury atom has a different chemical environment.

Preparation and crystal structure of the Hg42+ ion

The compound Hg4(AsF6)2 containing mercury in the oxidation state of +½ has been obtained from the reaction of mercury with arsenic pentafluoride in liquid SO2; the Hg42+ ion is shown by X-ray methods to be very nearly linear with Hg–Hg distances of 2·57 and 2·70 A.