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The C -t A transformation of Pr2O3 has been examined. The transformation is sluggish and takes place at and above 750°C. C-Pr203 is oxidized at a lower temperature than the A-form and oxidation proceeds in two stages in both cases. A hydrate Pr2O3 . H20, formed by the interaction of C-Pr203 and water at relatively high temperature, is described and its behaviour compared with that of Pr(OH)3. The C-form undergoes hydration at 40°C while the A-form does not. All the observations have been explained in terms of the defect structure of C-Pr203.Rare earth sesquioxides exist in the cubic (C), hexagonal (A) and monoclinic ( B ) forms and the phase transformations of some of the sesquioxides have been reported.1-6 These transformations are sluggish and affected by the origin, purity and thermal history of the oxide.5s6 The transformations were hitherto considered to be irreversible when " dry heating " was employed,3 but hydrothermal techniques 5 have revealed that the transformations are reversible. In this communication, studies on the C+A transformation of Pr2O3 are reported, since the earlier reports were conflicting. The C-form is a defect form of the fluorite structure of Pro2 with one-fourth of the anion positions vacant, while the hexagonal form is a close-packed structure and is related to the C structure.7 A comparative study of the reactivity, oxidation and hydration behaviour of the C and A forms of Pr2O3 has been made in the present work.EXPERIMENTAL Lindsay 99-99 % praseodymium oxide Pr6Q11, ignited to constant weight, was used. The lattice constant was a = 5.468 &002 A, in agreement with the literature value.7 Cubic praseodymium sesquioxide (yellow in colour) was prepared by reducing Pr6011 for 24 h at 600°C in a stream of dry oxygen-free hydrogen. Hydrogen was passed over platinized asbestos (at 3O0-35O0C), anhydrous magnesium perchlorate, phosphorus pentoxide and cubic Pr2O3 maintained at 4O0-45O0C, before passing it over Pr6011 to effect reduction. Cubic Pr2O3 was a good deoxidizer.8 The composition of the sesquioxide was determined as Pro, (x = 1.500-1-504) by ignition in air to Pr6011. The lattice constant was 11.160 k 0.006 A, which agrees with the literature value 1 within experimental error. Hexagonal Pr2O3 (light green in colour) was prepared by reducing Pr6011 at 900°C in dry hydrogen for 8 h.The lattice constants were a = 3.84f0.03 A and c = 5-98 f0.03 A which agree with the literature values.1Transformation of C-Pr203 to A-Pr2O3 was studied by heating C-Pr203 at different temperatures for different times in a stream of dry hydrogen. The samples thus heated were quenched and subjected to X-ray analysis.

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