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) Å, b = 7.0557(2) Å, c = 4.4005(2) Å), LiF (sp. gr. $Fm\bar {3}m$, a = 4.0234(3) Å), 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) Å, c = 7.136(1) Å). 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 T_tr = 763 ± 15 K, which increases the conductivity σ_dc 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 T_β−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 σ_dc activation for α-SmF₃ is ΔH_σ = 0.37 ± 0.05 eV.