PHASE-TRANSITIONS AND CRYSTAL-FIELD LEVELS IN ND2NIO4

The heat capacity of Nd2NiO4 has been measured between 2 and 350 K wit h adiabatic and ac calorimetry, Thermodynamic functions have been calc ulated at rounded temperature intervals. The three-dimensional magneti c ordering of the Ni ions produces a small peak in the heat capacity a t 325 K, indicating strong two-dimensional magnetic behavior. A sharp first-order peak has been found at 138 K corresponding to the structur al transition from the low-temperature tetragonal P4(2)/ncm phase to t he higher-temperature orthorhombic Bmab phase. No three-dimensional or dering of the Nd sublattice has been detected. The excess heat capacit y, after subtracting the estimated lattice contribution, is in agreeme nt with the crystal-field energy levels of the Nd3+ ions reported from inelastic neutron diffraction experiments. A broad shoulder below 10 K can be adscribed to an additional splitting of 17 K in the lowest cr ystal-field doublet of Nd3+. It is due to the interaction with the int ernal magnetic field created by a canting of the Ni2+ magnetic moments .