Abstract
A calorimetric study of La 2NiO 4 and Pr 2NiO 4 has been carried out with adiabatic and ac calorimetry. Thermodynamic functions of both compounds have been calculated at rounded temperatures. Heat capacity anomalies corresponding to structural transitions from the low-temperature tetragonal P4 2/ncm phase to the higher-temperature orthorhombic Bmab phase have been found at 56K for La 2NiO 4 and at 121.2K for Pr 2NiO 4. The three-dimensional magnetic ordering of the Ni ions produces a small peak in the heat capacity at 328K annd 324K, respectively, for both compounds in agreement with the expected low-dimensional behavior. No long-range order of Pr 3+ has been detected down to 2K. A broad excess heat capacity in Pr 2NiO 4 over the estimated lattice contribution was found to be due to the electronic energy levels of Pr 3+. An energy gap of around 50K between the ground and the first-excited level with equal degeneracies has been deduced.
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