It was noted that, while nearly all compounds of the form, AnBm, obeyed Dalton’s rule of integer stoichiometry (n, m integers), there existed a class of systems, exemplified by the rocksalt structured, Sc1–xxS, that exhibited large deviations from stoichiometry via vacancies; even at low temperatures. By combining first-principles total energy calculations with lattice statistical mechanics, an astronomical number of possible structures was scanned; identifying the stable ground states. Surprisingly, all of them had the same motifs: (111) planes with (112) vacancy rows arranged in (110) columns. Electronic structure calculations of the ground states (identified out of some 3 x 106 structures) revealed the remarkable origins of non-stoichiometry.

Origins of Nonstoichiometry and Vacancy Ordering in Sc1–xxS. G.L.W.Hart, A.Zunger: Physical Review Letters, 2001, 87[27], 275508 (4pp)

Figure 9

Diffusion of B in TiB2