THEORETICAL-STUDIES OF THE STABILITY OF ORDERED A8B COMPOUNDS

Ten out of 27 possible A8B (prototype Pt8Ti) compounds, where A = Ni,Pd,Pt, and B are IVA, VA, and VIA elements, have been experimentally observed. For these compounds to be thermodynamically stable, their T = 0 formation energy DELTAH must be negative, and DELTAH must lie below the tie line connecting neighboring ground states. Using the first-principles total-energy approach as implemented in the linearized augmented-plane-wave LAPW method, we have calculated the T = 0 DELTAH of (Ni,Pd,Pt)B-8 (B = V, Cr, Mo, and W) and of their neighboring known (or suspected) ground-state structures. We find that (i) DELTAH < 0 for these compounds except for Pd8Cr, for which DELTAH > 0; (ii) DELTAH(AB-8) lies below the tie line connecting the neighboring known (or suspected) ground states and the end point (pure element). These A8B structures appear to be true, stable ground states for these systems. We further elucidate the stability or instability of these systems in terms of their underlying electronic structures, and also calculate their electron-phonon interactions. We conclude that these materials are unlikely to be superconductors.