On the icosahedral metal–phosphorus coordination in melliniite: a gift from the sky for materials chemistry

Abstract

Recently a new mineral, melliniite, was reported from a meteorite sample. This mineral has an ideal chemical composition of (Ni,Fe)₄P and a crystal structure where the phosphorus atoms are coordinated by twelve nearest neighboring metal atoms. No other phosphide has been reported to have such high metal coordination. Therefore melliniite provides new and important information about the chemical interaction in transition metal chalcogenides and possibly pnictides. We demonstrate here, using first principles theory, that the stability and icosahedral metal–phosphorous coordination of melliniite are due to a balance between covalent Fe–P binding, configurational entropy and a weaker nickel–phosphorus binding, that has only a weak directional dependence.