First Principles Phase Diagram Calculations for the Octahedral-Interstitial System ZrOX, 0 ≤ X ≤ 1/2

First principles based phase diagram calculations were performed for the octahedral-interstitial solid solution system alpha ZrOX (alpha Zr[](1-X)O-X; [] equivalent to vacancy; 0 <= X <= 1/2). The cluster expansion method was used for ground state analysis, and to calculate the phase diagram. The predicted diagram has four ordered ground-states in the range 0 <= X <= 1/2, but one of these, at X = 5/12, is predicted to disproportionate at T approximate to 20 K, well below the experimentally investigated range T approximate to 420 K. Thus, at T greater than or similar to 420 K, the first-principles based calculation predicts three ordered phases rather than the four that have been reported by experimentalists.