Thermodynamic Properties of YbRhO 3 and Phase Relations in the System Yb-Rh-O K.T. Jacob, Preeti Gupta, Donglin Han, and Tetsuya Uda (Submitted March 1, 2016; in revised form May 12, 2016; published online June 21, 2016) Thermodynamic properties of the ternary oxide YbRhO 3 were determined by using a solid-state electrochemical cell incorporating calcia-stabilized zirconia as the solid electrolyte in the tem- perature range from 900 to 1300 K. The standard Gibbs energy of formation of YbRhO 3 from component binary oxides Yb 2 O 3 with C-rare earth type structure and Rh 2 O 3 with orthorhombic structure can be represented by the equation, D f ðoxÞ G o ð130Þ=J/mol ¼43164 þ 3:436 ðT/KÞ: Standard enthalpy of formation of YbRhO 3 from elements in their normal standard states is 21153.18(±3) kJ/mol and its standard entropy is 100.93(±0.6) J/K/mol at 298.15 K. The decomposition temperature of YbRhO 3 is 1671(±3) K in pure oxygen, 1566(±3) K in air and 1047(±3) K at an oxygen partial pressure of P O 2 =P o ð Þ¼ 10 6 , where P o = 0.1 MPa is the standard pressure. Decomposition temperature was confirmed by DTA/TGA. Phase diagrams for the system Yb-Rh-O are computed using the thermodynamic data. Keywords differential scanning calorimetry (DSC), differential thermal analysis (DTA), electromotive force (EMF), enthalpy of formation, entropy, equilibrium diagram, Gibbs energy 1. Introduction As part of systematic thermodynamic and phase diagram studies on ternary systems containing rhodium and oxygen (M-Rh-O), [1–17] properties of ytterbium orthorhodite (YbR- hO 3 ) were measured. Unlike 3d transition metal rhodites which have the spinel structure, lanthanide rhodites crystal- lize in the perovskites structure. Because of the large spin and orbital angular momentum of lanthanide ions, lan- thanide rhodites (LnRhO 3 ) exhibit interesting magnetic properties. [18,19] The Rh 3+ ion in perovskites structure is in low spin state without any significant magnetic moment. Therefore, LnRhO 3 compounds are good for the study of magnetic interaction between Ln 3+ ions. Paramagnetic behaviour of LnRhO 3 (Ln = lanthanide element except Ce and Pm) above 5 K have been investigated. [18] Ohnishi et al. [19] observed antiferromagnetic transition of YbRhO 3 at 2.3 K using magnetic and specific heat measurements. Among the two types of possible magnetic interactions (superexchange and dipole-dipole), the superexchange interaction was found to be dominant. [19] Shaplygin and Lazarev [20] have reported YbRhO 3 as a semiconductor with electrical resistivity of 1.66 kX cm at room temperature. YbRhO 3 has orthorhombic perovskite (GdFeO 3 -type) struc- ture with space group Pbnm. [19,20] Important applications of lanthanide rhodites are in the fields of catalysis and electrochemistry. [21–23] 3d transition metals (M) can partially substitute for Rh generating solid solutions LnM x Rh (1x) O 3d , which can be tailored for different applications. Phase relations in the system (Yb 2 O 3 -Rh 2 O 3 ) in air have been studied by Skrobot and Grebenshchikov, [24] who identified only one stable ternary oxide YbRhO 3 , which decomposes on heating to metal Rh and Yb 2 O 3 . There is no information on thermodynamic properties of YbRhO 3 in the literature. Hence, a solid-state electrochemical cell was used to measure the standard Gibbs energy of formation of YbRhO 3 in the temperature range from 900 to 1300 K. Thermal studies were undertaken to support electrochemical measurements. The standard en- thalpy of formation and entropy of YbRhO 3 were assessed from the data, Temperature-composition phase diagrams are computed by using thermodynamic data at different partial pressures of oxygen. An isothermal section of the ternary system Yb-Rh-O at 1273 K is also constructed from the results of this study and available information on the binary systems in the literature. 2. Experimental 2.1 Materials Powders of Rh metal (99.95 mass% pure, trace metal basis) and Yb 2 O 3 (99.99 mass% pure, trace metal basis) were obtained from Alfa Aesar. Rh 2 O 3 (99.8 mass% pure, K.T. Jacob and Preeti Gupta, Department of Materials Engineering, Indian Institute of Science, Bangalore 560012, India; Donglin Han and Tetsuya Uda, Department of Materials Science and Engineering, Kyoto University, Sakyo-ku, Kyoto 6068501, Japan. Contact e-mails: katob@materials.iisc.ernet.in and ktjacob@hotmail.com. JPEDAV (2016) 37:503–509 DOI: 10.1007/s11669-016-0482-y 1547-7037 ÓASM International Journal of Phase Equilibria and Diffusion Vol. 37 No. 4 2016 503