Crystal Chemistry of Transition Metal Arsenides and the High Pressure Behavior of CoAs

Transition metal arsenide compounds provide a number of intriguing structural and bonding problems, and provide challenges in the description of their crystal chemistry that are quite different from those of traditional oxide and silicate minerals. The most striking of these is perhaps the the range of crystal structures adopted by monoarsenides of composition MAs as the metal is varied across the first transition series. NiAs and CoAs/FeAs, for example, exhibit different crystal structures despite the fact that the ionic radii of the metals are essentially identical, but differ by one and two electrons at the metal for Co and Fe, respectively. These compounds may be profitably understood as alloy compounds with a nominal oxidation state of 2+ at the metal, in contrast to the traditionally assigned 3+, with a Jahn-Teller distortion driving the distortion in CoAs/FeAs from the ideal NiAs arrangement. At approximately 6-8 GPa, single crystals of CoAs (modderite) undergo a transformation to a lower-symmetry phase which at this point is not determined as a result of the twinning that takes place at the onset of the transition. Using a combination of first-principles computational methods and semi-empirical molecular orbital techniques in concert with high pressure diffraction data, we describe the evolution of the electronic structure of CoAs with pressure and discuss possible structural alternatives for the high pressure phase and the phase transformation mechanism. This study illustrates the surprising power of electronic effects in controlling crystal structure preference and shows the utility of employing a range of computational methods as a useful complement to diffraction experiments and a valuable aid in understanding the structural and bonding properties of high pressure mineral phases.