Reversible phase transitions are found between the cubic C1 and orthorhombic C23 polymorphs of by differential thermal analysis (DTA) under hydrostatic pressures. The transition temperature varies approximately linearly with pressure at the rate of ∼25 (±10%) deg kb−1 from the highest pressure attained (∼6 kb) down to 1.8–2.0 kb, where there is pronounced curvature or a possible kink; at lower pressures, the slope is ∼45 (±10%) deg kb−1 and the reverse transition is sluggish and thus not observed. The “diffuse” transition in C1 could not be observed unambiguously but it is plausible that it intersects the C1–C23 transition in the vicinity of the region of strong curvature. That C23 is the stable low temperature polymorph is strongly reinforced by the present experiments.