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The cooling curves of small samples of nominally stoichiometric UO(2.00), initially laser heated to temperatures just below the melting point, reveal a significant heat capacity peak near 2670 K. Samples exposed to a reducing environment exhibit undercooling, characteristic of a first order phase transition, while under oxidising conditions the premelting transition readily disappears. These findings confirm Bredig's original prediction of a premelting transition in this material, in common with that found in other (nonactinide) fluorites near 0.85T(m). A simple model is presented to explain the observed behaviour. The model is based on Frenkel disordering of the oxygen sublattice - a process which is rendered cooperative by attractive interactions between complementary Frenkel defects (oxygen interstitials and vacancies). The quantitative degree of maximum disorder (realised just above the transition) is, on the other hand, controlled by repulsive interactions between like defects. Crucial to reproducing the observed behaviour away from stoichiometry is the introduction of a (linear) dependence of the nonconfigurational partial entropy of formation on the prevailing concentration of intrinsic Frenkel defects in UO(2 +/- x).

Additional information

Authors: HIERNAUT J P, JRC Karlsruhe (DE);RONCHI C, JRC Karlsruhe (DE);HYLAND G J, University of Warwick, Department of Physics, Coventry (GB)
Bibliographic Reference: Article: International Journal of Thermophysics, Vol. 14 (1993) No. 2, pp. 259-283
Record Number: 199311068 / Last updated on: 1994-11-29
Original language: en
Available languages: en
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