Abstract
Measurements of the linear coefficient of thermal expansion of UO 2 from 1.5 to 35 K show a first-order magnetic transition atT N=30.36 K (single crystal) and 30.40 K (sintered compact) with an associated strain Δl/l ≅ 25×10−6. Comparison of the magnetic contributions to thermal expansion and heat capacity leads to a magnetic Grüneisen parameter γ m ≅ 4 (T<35 K). Addition of ThO 2 reducesT N to 24.0 K (10% ThO 2 ), 17.5 K (20%), 14 K (25%). For more than 25% ThO 2 the transition is broadened, but a very significant magnetic contribution to the thermal expansion remains up to 80% ThO 2 . Broadened transitions are also found in specimens of UO 2 containing 8% and 20% ZrO 2 . The variation ofT N with concentration is not in accord with theoretical models based on Heisenberg or Ising exchange interactions. A dilute sample containing 10% UO 2 /90% ThO 2 has a negative expansion coefficient below 8 K, with a minimum at ∼3 K. We suggest this is associated with a Jahn-Teller splitting of the U4+ ionic states and qualitatively supports the theory of the first-order transition proposed by Allen.
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Visiting Research Scientist 1972–1973. Permanent address: Department of Physics, University of Nottingham, Nottingham, England.
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White, G.K., Sheard, F.W. The thermal expansion at low temperatures of UO2 and UO2/ThO2 . J Low Temp Phys 14, 445–457 (1974). https://doi.org/10.1007/BF00658873
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DOI: https://doi.org/10.1007/BF00658873