NEUTRON-DIFFRACTION INVESTIGATION OF CRYSTAL AND MOLECULAR-STRUCTURE OF ANISOTROPIC SUPERCONDUCTOR HG3ASF6

The crystal and molecular structure of Hg3AsF6 has been investigated by single-crystal neutron diffraction. This metallic compound crystallizes in the body-centered tetragonal space group I41/amd with cell dimensions of a=7.549 (5) Å and c=12.390 (9) Å. The crystal structure consists of two orthogonal and nonintersecting linear chains of Hg0.33+ cations passing through a lattice of octahedral AsF6- anions. The intrachain Hg-Hg distance of 2.64 (2) Å is derived from planes of diffuse scattering normal to a* and b*. Since the a and b axis lattice constants are not simple multiples of the Hg-Hg intrachain distance, the mercury chains are incommensurate with the tetragonal lattice; hence we have the apparent formula Hg2.86AsF6. These results are in essential agreement with a previously reported x-ray diffraction study. However, from the neutron diffraction data, we have established that the Hg chains are not strictly one-dimensional. The maximum room-temperature deviation from the chain axis is 0.07 (1) Å with neighboring chains distorted away from each other. The closest interchain Hg-Hg contact is 3.24 (2) Å. Furthermore, analytical data consistently indicate a stoichiometric empirical formula of Hg3AsF6. These results together with precise density measurements imply that the incommensurate structure is stabilized by anion vacancies, such that there are four formula weights of Hg2.86(AsF6)0.953 per unit cell. © 1978, American Chemical Society. All rights reserved.