System Dy-Fe-O: thermodynamic properties of ternary oxides using Calvet calorimetry and solid-state electrochemical cell

摘要

The enthalpy increments and the standard molar Gibbs energies of formation of DyFeO_3(s) and Dy_3Fe_5O_(12)(s) have been measured using a Calvet micro-calorimeter and a solid oxide galvanic cell, respectively. A co-operative phase transition, related to anti-ferromagnetic to paramagnetic transformation, is apparent from the heat capacity data for DyFeO_3 at approx 648 K. A similar type of phase transition has been observed for Dy_3Fe_5O_(12) at approx 560 K which is related to ferrimagnetic to paramagnetic transformation. Enthalpy increment data for DyFeO_3(s) and Dy_3Fe_5O_(12)(s), except in the vicinity of the second-order transition, can be represented by the following polynomial expressions:{H~0_m(T) - H~0_m(298.15 K)} (J mol~(-1)) (+-1.1 percent)=-52754+142.9 X (T (K))+2.48 X 10~(-3) X (T (K))~2+2.951 X 10~6 X (T (K))~(-1); (298.15<= T (K) <= 1000) for DyFeO_3(s), and{H~0_m(T) - H~0_m(298.15 K)} (J mol~(-1)) (+-1.2 percent)=-191048+545.0 X (T (K))+2.0X10~(-5) X (T (K))~2+8.513 X 10~6 X (T (K)) ~(-1); (298.15 <= T (K) <= 1000)for Dy_3Fe_5O_(12)(s). The reversible emfs of the solid-state electrochemical cells: (-)Pt/{DyFeO_3(s) + Dy_2O_3(s) + Fe(s)}//YDT/CSZ//{Fe(s) + Fe_(0.95)O(s)}/Pt(+) and (-)Pt/{Fe(s) + Fe_(0.95)O(s)}//CSZ//{DyFeO_3(s) + Dy_3Fe_5O_(12)(s) + Fe_3O_4(s)}/Pt(+), were measured in the temperature range from 1021 to 1250 K and 1035 to 1250 K, respectively. The standard Gibbs energies of formation of solid DyFeO_3 and Dy_3Fe_5O_(12) calculated by the least squares regression analysis of the data obtained in the present study, and data for Fe_(0.95)O and Dy_2O_3 from the literature, are given by:DELTA_F G~0_m(DyFeO_3,s) (kJ mol~(-1)) (+-3.2) = -1339.9+0.2473 X (T (K)); (1021 <= T (K) <= 1548) andDELTA_F G~0_m(Dy_3Fe_5O_(12),s) (kJ mol~(-1)) (+-3.5) = -4850.4 + 0.9846 X (T (K)); (1035 <= T (K) <= 1250).The uncertainty estimates for DELTA_F G~0_m include the standard deviation in the emf and uncertainty in the data taken from the literature. Based on the thermodynamic information, oxygen potential diagram and chemical potential diagrams for the system Dy-Fe-O were developed at 1250 K.