Thermodynamic Studies on NdFeO3(s)

Abstract

The enthalpy increments and the standard molar Gibbs energy of formation of NdFeO3(s) have been measured using a high-temperature Calvet microcalorimeter and a solid oxide galvanic cell, respectively. A λ-type transition, related to magnetic order–disorder transformation (antiferromagnetic to paramagnetic), is apparent from the heat capacity data at ∼687 K. Enthalpy increments, except in the vicinity of transition, can be represented by a polynomial expression: {H°m(T)−H°m(298.15 K)}/J·mol−1 (±0.7%)=−53625.6+146.0(T/K) +1.150×10−4(T/K)2 +3.007×106(T/K)−1; (298.15≤T/K ≤1000). The heat capacity, the first differential of {H°m(T)−H°m(298.15 K)} with respect to temperature, is given by Cop, m/J·K−1·mol−1=146.0+2.30×10−4(T/K)−3.007×106(T/K)−2. The reversible emf's of the cell, (−) Pt/{NdFeO3(s) +Nd2O3(s)+Fe(s)}//YDT/CSZ//{Fe(s)‘FeO’(s)}/Pt(+), were measured in the temperature range from 1004 to 1208 K. It can be represented within experimental error by a linear equation: E/V:(0.1418±0.0003)−(3.890±0.023)×10−5(T/K). The Gibbs energy of formation of solid NdFeO3 calculated by the least-squares regression analysis of the data obtained in the present study, and data for Fe0.95O and Nd2O3 from the literature, is given by ΔfG°m(NdFeO3, s)/kJ·mol−1(±2.0)=−1345.9+0.2542(T/K); (1000≤T/K ≤1650). The error in ΔfG°m(NdFeO3, s, T) includes the standard deviation in emf and the uncertainty in the data taken from the literature. Values of ΔfH°m(NdFeO3, s, 298.15 K) and S°m(NdFeO3, s, 298.15 K) calculated by the second law method are −1362.5 (±6) kJ·mol−1 and 123.9 (±2.5) J·K−1·mol−1, respectively. Based on the thermodynamic information, an oxygen potential diagram for the system Nd–Fe–O was developed at 1350 K.