System Er–Ru–O: High temperature study of the heavy rare earth pyrochlore Er2Ru2O7(s) by electrochemical cell and differential scanning calorimeter

•The phase equilibrium in the Er–Ru–O system was experimentally determined.•Electrochemical measurements on solid oxide galvanic cells were carried out.•Heat capacity was determined using a differential scanning calorimeter.•Complete thermochemical data was generated for the pyrochlore Er2Ru2O7(s).

The Gibbs free energy of formation of Er2Ru2O7(s) has been determined using solid-state electrochemical technique employing oxide ion conducting electrolyte. The reversible electromotive force (e.m.f.) of the following solid-state electrochemical cell has been measured:(−)Pt/{Er2O3(s) + Er2Ru2O7(s) + Ru(s)}//CSZ//O2(p(O2) = 21.21 kPa)/Pt(+).The Gibbs free energy of formation of Er2Ru2O7(s) from elements in their standard state, calculated by the least squares regression analysis of the data obtained in the present study, can be given by:{ΔfG°(Er2Ru2O7,s) / (kJ∙mol−1) ± 2.2} = − 2517.3 + 0.6099 ∙ (T/K); (934.6 ≤ T/K ≤ 1236.3).Standard molar heat capacity C°p,m(T) of Er2Ru2O7(s) was measured using a heat flux type differential scanning calorimeter (DSC) in two different temperature ranges, from 129 K to 296 K and 307 K to 845 K. The heat capacity in the higher temperature range was fitted into a polynomial expression and can be represented by:C°p,m(Er2Ru2O7,s,T)(J∙K−1∙mol−1) = 293.88 + 2.397 10−2 T(K) − 54.74717 105/T2(K); (307 ≤ T(K) ≤ 845).The heat capacity of Er2Ru2O7(s), was used along with the data obtained from the oxide electrochemical cell to calculate the standard enthalpy and entropy of formation of the compound at 298.15 K.