Heat Capacity and Thermodynamic Functions of Neodymium Orthoniobate

The heat capacity of polycrystalline neodymium orthoniobate NdNbO4 in the range 2–1370 K is studied by relaxation, adiabatic, and differential scanning calorimetry. The existence of a magnetic transformation with a Neel temperature below 2 K in the low-temperature region is verified. The reversible phase transition at 987 K is shown to be a second-order transition. The contribution of the Schottky anomaly (Schottky contribution) to neodymium orthoniobate heat capacity is estimated. The gained data are used to calculate temperature-dependent standard thermodynamic functions in the range from 0 to 1400 K. Their values at 298.15 K are $$C_{p}^{^\circ }$$ (298.15 K) = 121.4 ± 0.4 J/(K mol), S°(298.15 K) = 134.7 ± 0.5 J/(K mol), H°(298.15 K) – H°(0 K) = 21.24 ± 0.17 kJ/mol, Ф°(298.15 K) = 63.43 ± 0.21 J/(K mol).