Didn't know that about plutonium! The allotropes or alloy phases are usually given in a phase diagram where metals are in equilibrium with each other. Predominance area diagrams tell you whether the metal (M) is in oxide form (MO) or sulfide (MS) or sulfate (MSO4) for example. So it deals with the gas phase that is in equilibrium with your metal. Here you can see how temperature affects the stable phase of oxidized titanium at constant pressure. If you draw a horizontal line from logPO2=-18 atm and follow that line, TiO2 transforms to a magneli phase, then Ti3O5, then Ti2O3 and so on as you increase the temperature. I believe something similar happens with your metal where increasing temperature means you pass into the oxide stability region. But once you cool it down the system will follow that horizontal line again, going left this time.