Stability of the pyrochlore Tm2Ru2O7(s) & comparison of its stability with other heavy rare earth ruthenium pyrochlores

Abstract

The Gibbs energy of formation of Tm2Ru2O7(s) has been determined employing a solid-state electrochemical cell incorporating an oxide ion conducting electrolyte. The reversible electromotive force (e.m.f.) of the following cell was measured:Cell : (−)Pt/{Tm2O3(s) + Tm2Ru2O7(s) + Ru(s)}//CSZ//O2(p(O2) = 21.21kPa)/Pt(+)The standard molar Gibbs energy of formation of Tm2Ru2O7(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°(Tm2Ru2O7,s)/(kJ · mol−1) ± 1.9} = − 2511.9 + 0.625 ⋅ (T/K); (977 ≤ T/K ≤ 1265).Standard molar heat capacity Cop,m(T) of Tm2Ru2O7(s), was measured using a heat flux type differential scanning calorimeter (DSC) in two different temperature ranges, from 128 K to 276 K and 307 K to 686 K. The heat capacity in the higher temperature range was fitted into a mathematical expression and can be represented by:C°p,mTm2Ru2O7sT)(J⋅K−1·mol−1=297.8+2.0⋅10−3TK−62.1⋅105/T2K.307≤TK≤686The standard molar heat capacity data of Tm2Ru2O7(s), along with the data obtained from the oxide electrochemical cell were used to calculate the standard enthalpy of formation and absolute molar entropy of the compound. The standard Gibbs energy of formation obtained earlier for other heavy rare earth ruthenium pyrochlores were compared with the present results.