The topologies of the charge densities in Zr and Ru

We report on the atomic scale phenomena responsible for the variation of oxygen solubility in Zr and Ru. First-principles calculations reveal that the topologies of the charge densities in these hexagonal close-packed metals are distinct. Neither element was found to possess the topology of the prototype, Mg. There are 12 bond paths terminating at each Ru atom. These are the bonds between nearest neighbors. Only five bond paths terminate at each Zr atom and the Zr atoms are not bound to one another. Instead, they are bonded through non-nuclear maxima. As a result, channels of low charge density that can accommodate oxygen anions are present in Zr.