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 C°_{p, 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: EN:(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 Fe_0.95O and Nd₂O₃ from the literature, is given by Δ_fG°;_m(NdFeO₃, s)/kJ·mol^-1(±2.0)=-1345.9+0.2542(T/K); (1000=T/K =1650). The error in Δ_fG°_m(NdFeO₃, s, T) includes the standard deviation in emf and the uncertainty in the data taken from the literature. Values of Δ_fH°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.