Compositional, structural and physical studies of some graphite hexafluoroarsenates and their relatives

Abstract

The F/As molar ratio for the vacuum-stable products obtained by treating graphite, at 20°, with (1) AsF 5, (2) AsF 5 plus F 2, and (3) O 2AsF 6, has been established to be 6. Route (1) gives mixtures of first and second-stage salts, whereas (2) and (3) readily yield first-stage material. This is attributed to the greater oxidizing potential of the reagents in (2) and (3). The volatiles from the route (1) synthesis are AsF 5 and AsF 3, and from (2) AsF 5. The interlayer spacing (I c) of the occupied graphite galleries in the vacuum-stable hexafluoroarsenates is ca. 7.6 Ā and c ⋍ 7.6 + 3.35 (n−1) Ā (where n is the stage). For the products of routes (1) and (2), prior to removal of volatiles, I c ⋍ 8.0 Ā and c ⋍ 8.0 + 3.35 (n−1) Ā. X-ray powder diffraction data for the first-stage salt C 14AsF 6 have established that the AsF − 6 species are nestled in contiguous three-fold sets of C-atom hexagons of the graphite. This requires a staggering of the enclosing C-atom layers, as in graphite. Each neighboring pair of C-atom sheets contains an ordered closest-packed assembly, appropriate for C 14AsF 6. Aside from the restriction imposed by AsF − 6-nestling, the layers of composition C 14AsF 6 are otherwise randomly stacked. Materials prepared by direct interaction of graphite and AsF 5, prior to removal of volatiles, have a larger interlayer spacing, I c ⋍ 8.0 Ā than C 14AsF 6 (I c ⋍ 7.6 Ā). X-ray data for such materials are accounted for by a random distribution of un-nestled AsF x species, between eclipsed carbon-atom sheets. The C xAsF 6 salts are all good conductors (approximately the conductivity of Al metal). Addition of fluorine to the salts decreases their conductivities substantially (approximately that of graphite).