75LiF + 25SmF3 Eutectic Composite and Ionic Conductivity of SmF3 near the Polymorphic α–β Transition

Abstract

75 mol % LiF + 25 mol % SmF3 eutectic composite is obtained by fusing components in a CF4 atmosphere. The composite is multiphase and consists of β-SmF3 (sp. gr. Pnma, a = 6.6703(2) A, b = 7.0557(2) A, c = 4.4005(2) A), LiF (sp. gr. $$Fm\bar {3}m$$, a = 4.0234(3) A), and a small amount (up to 4.5% of the total trifluoride content) of α-SmF3 (sp. gr. $$P\bar {3}c1$$, a = 6.946(1) A, c = 7.136(1) A). The conductometric measurements are carried out in a heating–cooling cycle in the range of 463–827 K. A first-order phase transition is detected upon heating at Ttr = 763 ± 15 K, which increases the conductivity σdc of the composite by a factor of ~10. After the experiment, the α-SmF3 phase content in the composite increases by a factor of almost 3. According to the differential scanning calorimetry data, the polymorphic transition in pure SmF3 component occurs at Tβ−α = 764.6 ± 0.2 K. The first-order transition in the composite is assigned to the polymorphic β-SmF3 → α-SmF3 transition. Upon cooling, a reverse transition from the high-temperature α-SmF3 (LaF3 type) to the low-temperature form β-SmF3 (β-YF3 type) is kinetically retarded. The enthalpy of σdc activation for α-SmF3 is ΔHσ = 0.37 ± 0.05 eV.