Phase chemistry of the system Nb–Rh–O

•First complete isothermal section for the system Nb–Rh–O has been established.•An electrochemical cell is used to measure the thermodynamic properties of NbRhO4.•Oxygen potential diagram at 1223 K is computed from thermodynamic data.•Temperature–composition diagrams at constant oxygen partial pressures.•A complete thermodynamic characterization of the system Nb–Rh–O is presented.

Phase relations in the system Nb–Rh–O at 1223 K were investigated by isothermal equilibration of eleven compositions and analysis of quenched samples using OM, XRD, SEM and EDS. The oxide phase in equilibrium with the alloy changes progressively from NbO to NbO2, NbO2.422 and Nb2O5−x with increasing Rh. Only one ternary oxide NbRhO4 with tetragonal structure (a=0.4708 nm and c=0.3017 nm) was detected. It coexists with Rh and Nb2O5. The standard Gibbs energy of formation of NbRhO4 from its component binary oxides measured using a solid-state electrochemical cell can be represented by the equation;ΔGf,oxo(J/mol)=−38,350+5.818×T(±96)Constructed on the basis of thermodynamic information of the various alloy and oxide phases are oxygen potential diagram for the system Nb–Rh–O at 1223 K and temperature–composition diagrams at constant partial pressures of oxygen.

Graphical abstractIsothermal section of ternary phase diagram for the system Nb–Rh–O at 1223 K. Figure optionsDownload full-size imageDownload as PowerPoint slide