The crystal structure of parkerite (Ni₃Bi₂S₂) Available to Purchase

Parkerite (Ni₃Bi₂S₂) is orthorhombic with a = 5.545(4) Å, b = 5.731(3) Å, c = 4.052(3) Å, space group Pmam (D_2h⁵), Z = 1. The crystal structure, determined on synthetic material, consists of a C centered arrangement of Bi atoms with the Ni in two non-equivalent positions octahedrally coordinated to four Bi and two S and juxtaposed across shared octahedral faces with Ni-Ni distances less than 2.80 Å the Ni(2) site has an occupancy of 0.5. The S is coordinated to three Ni at the apex of a trigonal pyramid giving Ni-S distances of 2.02 Å and 2.05 Å, which are significantly shorter than in other nickel sulfides. The parkerite structure is shown to be a derivative of the shandite (Ni₃Pb₂S₂) structure, the essential differences being the distribution of the Ni atoms about the heavy atoms and the apparent direct coordination of S to the Pb(2) atom in shandite. The short Ni-S distances in parkerite are related to a π bonding contribution to these bonds and further crystal chemical analysis of the compound predicts that only four electrons populate the non-bonding d orbitals on each Ni. The distribution of Ni-Bi and Ni-S bonds readily accounts for the excellent {010} cleavage of parkerite.