The thermal expansion at low temperatures of UO2 and UO2/ThO2

Abstract

Measurements of the linear coefficient of thermal expansion of UO2 from 1.5 to 35 K show a first-order magnetic transition atTN=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 Gruneisen parameter γm ≅ 4 (T<35 K). Addition of ThO2 reducesTN to 24.0 K (10% ThO2), 17.5 K (20%), 14 K (25%). For more than 25% ThO2 the transition is broadened, but a very significant magnetic contribution to the thermal expansion remains up to 80% ThO2. Broadened transitions are also found in specimens of UO2 containing 8% and 20% ZrO2. The variation ofTN with concentration is not in accord with theoretical models based on Heisenberg or Ising exchange interactions. A dilute sample containing 10% UO2/90% ThO2 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.