Thermal effects of phase transitions of Cerium and Neodymium

Phase transitions of high-purity cerium and neodymium are investigated by using high temperature scanning calorimeter (HT-1500 Seteram) in the temperature range 1500 K. The calorimeter was calibrated by standard compounds (benzoic acid, Sn, Pb, Zn, Al, Ag, Cu). The accuracy of enthalpy of transitions (ΔH_tr) was about ± 2 %. For cerium two types of transformation are detected: at 350-372 K - hexagonal close packing (hcp) - face-centered cubic lattice (fcc) transition, and in 880-960 K the face-centered cubic lattice (fcc) transformation into body-centered cubic lattice (bcc). For neodymium the changing of hexagonal close packing (hcp) into body-centered cubic lattice (bcc) is detected at 1093-1113 K. This anomalies are in good agreement with the literature data of electrical resistance, thermal conductivity and temperature conductivity. The thermal characteristics of transitions - enthalpy, entropy, temperature domains - are reported, which are as follows: for cerium hcp-fcc transition ΔH_tr = 2436.3 J/mole; ΔS_tr = 6.69 J/K.mole; fcc-bcc transition ΔH_tr = 4742.7 J/mole; ΔS_tr = 5.0 J/K.mole; For neodium hcp-bcc transition ΔH_tr = 3234.2 J/mole; ΔS_tr = 2.9 J/K.mole. (author)