Two intermetallic compounds ${\mathrm{ZrCo}}_2$ and ${\mathrm{ZrCr}}_2$ with the same close-packed $C15\mathrm{}$ structure but significantly different heat-of-formation values are chosen to examine any possible differences in the point defect structures from the first-principles calculations. Although there is a substantial mismatch in the atomic size between constituent atoms, we find no evidence for the existence of constitutional vacancies for both compounds on either side of stoichiometry. By contrast, in ZrCo compound with the open $B2\mathrm{}$ structure and a heat-of-formation nearly as high as that of ${\mathrm{ZrCo}}_2,$ the point defect structure is characterized by the existence of triple defects (two vacancies at the Co sites accompanied by the Co antisite defect at the Zr sublattice), typical for the strongly ordered systems. Our analysis shows that, regardless of the degree of ordering, maintaining high coordination number is a preferred bonding mode for atoms in a topologically close-packed structure. On the other hand, the defect structure in the open $B2\mathrm{}$ structure appears to be governed by the atomic size factor once the bonding states are fully occupied.

• Received 30 April 2003

DOI:https://doi.org/10.1103/PhysRevB.68.134110