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