The rare earth ruthenium pyrochlore Ho2Ru2O7(s): thermodynamic properties by electrochemical cell and differential scanning calorimetric measurements

Abstract

The Gibbs energy of formation of Ho₂Ru₂O₇(s) has been determined by employing a solid-state electrochemical cell incorporating an oxide ion conducting electrolyte. The reversible electromotive force (e.m.f.) of the following cell has been measured: cell: (−)Pt/{Ho₂O₃(s) + Ho₂Ru₂O₇(s) + Ru(s)}//CSZ//O₂(p(O₂) = 21.21 kPa)/Pt(+). The Gibbs energy of formation of Ho₂Ru₂O₇(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: {Δ_fG°(Ho₂Ru₂O₇,s)/(kJ mol⁻¹) ± 2.6} = −2518.6 + 0.6225(T/K); (937 ≤ T/K ≤ 1265). The standard molar heat capacity C^°_p,m(T) of Ho₂Ru₂O₇(s), was measured using a heat flux type differential scanning calorimeter (DSC) in the temperature range from 307 K to 765 K. The heat capacity was fitted into a mathematical expression and can be represented by: C^°_p,m((Ho₂Ru₂O₇,s,T) (J K⁻¹ mol⁻¹)) = 289.8 + 3.06 × 10⁻²T (K) − 33.74 × 10⁵/T² (K); (307 ≤ T (K) ≤ 765). The heat capacity data of Ho₂Ru₂O₇(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.