Skutterudite vs. ReO3 structures for MX3 solids:: electronic requirements

Electronic band-structure calculations on ReO3 and CoP3 have been performed to analyse the different structural preferences found for these two compounds. The electronic origin of these differences is associated with the formation of the characteristic non-metal four-membered rings of the skutterudite structure, which is energetically unfavorable for compounds with small, strongly electronegative anions. The combination of two factors has been identified to be responsible for the structural differences between these compounds. The first is the electronegativity difference between the two types of atoms that form the solid: for strongly electronegative non-metal atoms like O or F the ReO3-type structure is expected to be the most stable, while for the combination of less electronegative atoms like the pnictides with late transition metals the skutterudite structure is preferred. The second factor is the relative size of the constituting atoms: small atoms like F and O favor the ReO, structure, while larger atoms like P and As stabilize the skutterudite one.