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An isobaric survey of the system praseodymium oxide + oxygen has been made at oxygen pressures between 0 and 1 atm and in the temperature range 200 to 1150 °C. The derived isobaric sections enable a three-dimensional phase diagram (pressure, temperature, composition) to be constructed with considerable certainty and detail for the composition range PrOj.gg to PrOj.g*. Reasonable extensions to cover the complete range between the sesquioxide and dioxide are proposed, and a projection of the diagram on to the temperature-composition plane is presented. At lower temperatures several discrete, ordered phases of narrow homogeneity range exist. These constitute an homologous series P r„02n_2, with n = 4, 7, 9 ,1 0 ,1 1 ,1 2 , oo. At higher temperatures two wide-range solid solutions obtain: <r, a body-centred cubic phase with a maximum composition range ca. PrOj.gQ to PrOj.yo; and a, a face-centred cubic phase of composition P r O ^ to P r 0 2. The fields of stability of the various phases are defined and the ambient conditions at many invariant axes (peritectoid and eutectoid) enumerated. Miscibility gaps with upper consolute points are exceptional; order-disorder peritectoid transformations are common. Hysteresis in phase transformations is confirmed, and the results further demonstrate the existence of metastable states in phase reactions involving an increase in structural order. The appearance of these pseudo-phases and the nature of non-stoichiometry is explained in terms of a plausible model invoking microdomain texture in defect solids. This model is believed to be appropriate for other non-stoichiometric systems also. Earlier experimental data on the system are examined, and found to be consistent with the present results.

The structures of the intermediate phases ZrsSc2013 and Zr3Sc4Ol2 in the system ZrO2-Sc203 have been determined from intensity data obtained with a H/~gg-Guinier focusing X-ray powder camera and Cu Ket radiation. The space group in both cases is R3, and the hexagonal unit-cell dimensions are respectively a=9-53(2), c=17.44(2),~ and a=9.37(8), c=8.71(0)A. Both these structures are derived from the fluorite-type parent MO2 by ordered omission of oxygen atoms; the observed rhombohedral distortion is the result of lattice relaxation. It is possible to recognize structural sub-units from which these and other fluorite-related structures can be built up. Such sub-units are likely to play an important role in any adequate description of grossly non-stoichiometric phases.