Heat capacity study of La2NiO4 and Pr2NiO4

A calorimetric study of La2NiO4 and Pr2NiO4 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 56 K for La2NiO4 and at 121.2 K for Pr2NiO4. The three-dimensional magnetic ordering of the Ni ions produces a small peak in the heat capacity at 328 K annd 324 K, respectively, for both compounds in agreement with the expected low-dimensional behavior. No long-range order of Pr3+ has been detected down to 2 K. A broad excess heat capacity in Pr2NiO4 over the estimated lattice contribution was found to be due to the electronic energy levels of Pr3+. An energy gap of around 50 K between the ground and the first-excited level with equal degeneracies has been deduced.