These intermetallic compounds, having the same close-packed C15 structure but very different heats-of-formation, were used to explore possible differences in the point-defect structures predicted by first-principles calculations. Although there was an appreciable mismatch in atomic size between the constituent atoms, no evidence was found for constitutional vacancies in either compounds; on both sides of stoichiometry. In ZrCo, with an open B2 structure and a heat-of-formation almost as high as that of ZrCo2, the point defect structure was characterized by the existence of triple defects (2 vacancies at Co sites, accompanied by a Co antisite defect in the Zr sub-lattice) that were typical of strongly ordered systems. The analysis showed that, regardless of the degree of ordering, maintenance of a high coordination number was a preferred bonding mode for atoms in a topologically close-packed structure. The defect structure in the open B2 structure appeared to be governed by the atomic size factor; once the bonding states were fully occupied.

First-Principles Study of Point-Defect Structures in C15 ZrCo2 and ZrCr2 and B2 ZrCo. M.Krčmar, C.L.Fu: Physical Review B, 2003, 68[13], 134110 (8pp)