CRYSTAL CHEMISTRY OF SULFIDES AND ARSENIDES: SOME SURPRISES IN THE FIRST TRANSITION SERIES

Transition metal sulfides and arsenides provide a wealth of intriguing structure and bonding problems for the crystal chemist, particularly in the variety of structure types observed for transition metal monosulfides and monoarsenides. While the monoxides of the first transition series adopt almost exclusively the rock salt structure, and the corresponding sulfides adopt the nickel arsenide structure at high temperature, at low temperature the monosulfides exhibit a variety of vacancy-ordered structures, and the monoarsenides show a surprising variation in structure type through the series. Qualitative considerations of ionic radius, electronegativity and orbital interactions can help create a unified picture of the structure and bonding in simple structures, which can then be used to understand more complex structures, particularly those formed at high pressure.

This contribution will review the crystal chemistry of the monosulfides and monoarsenides of the first transition series, focusing on NiS and NiAs as illustrative cases. Considerations of orbital overlap, ionic radius, electronegativity and electron counting are supplemented by ab-initio electronic band structure calculations that provide a quantitative understanding of crystal chemical fundamentals. Finally, these concepts are applied to two problems: first, understanding the transformation of Ni₃S₂ at 23 GPa to a high-pressure phase with distinct alloy-like and valence-like layers. This transformation takes place through increased participation of d-orbital states in Ni-S bonding at high pressure, and points to the surprising occurrence of square pyramidal coordination of nickel in nickel sulfides. The second problem concerns the variation in structure type among FeAs and CoAs on the one hand, and NiAs on the other, and illustrates the powerful effect of electron count at the metal on crystal structure preference.