| dc.description.abstract | First principles based phase diagram calculations were performed for the hexagonal closest packed
octahedral-interstitial solid solution system aTiOX, or aTi½ 1 XOX ([ ]¼Vacancy; 0rXr1=2), by the
cluster expansion (CE) method. The predicted diagram has ordered ground-state (GS) phases at: X¼0
(hcp aTi, P63mmc); X¼1/6 (Ti6O: P31c, R3, R3); X 1=3 (Ti3O: R3, P31c, R3
0
); and X¼1/2 (Ti2O: Pbcn;
contrary to previous reports that it is anti-CdI2). Both at X¼ 1/6 and X¼ 1/3 density functional theory
(DFT) calculations predict at least triply degenerate GS (three structure energies at each composition
that are within 0.001 eV; i.e. within DFT error). In the CE, this leads to exactly triply degenerate GS at
both compositions. The calculated phase diagram includes a broad field between Ti6O and Ti3O in
which the stable phases have layer-structures that are related to the anti-CdI2-structure. It appears that
disordering of the Ti6O- and Ti3O-GS involves cascades of first-order transitions that are suggestive of
Devil’s staircases. | en_US |
