Potentiometric determination of stabilities of NiSb2O4 and NiSb2O6

An isothermal section of the phase diagram of the system Ni-Sb-O at 873 K w as established by isothermal equilibration and XRD analyses of quenched sam ples. Two ternary oxides, namely NiSb2O4 with Sb in trivalent and NiSb2O6 w ith Sb in pentavalent state, were identified. Making use of coexistence of NiSb2O4 with NiO and Sb, as well as NiSb2O6 with NiO and NiSb2O4, the follo wing galvanic cells were designed to measure the Gibbs energies of formatio n of the ternary oxides: I Chromel, Mo, Sb, NiO, NiSb2O4/15 CSZ/NiO, Ni, Mo, Chromel II Pt, NiO, NiSb2O4, NiSb2O6/15 CSZ/air (P-O2 = 0.21 atm), Pt where 15 CSZ stands for ZrO2 stabilized by 15 mol% CaO The reversible emfs of the cells were measured over the temperature ranges 769-931 K and 894-1138 K respectively. The results could be represented by the following expressions: E +/- 0.3 (mV) = 88.3-0.1043 T (K) E +/- 0.3 (mV) = 1053.5-0.5544 T (K) The standard Gibbs energies of formation of NiSb2O4 and NiSb2O6 were comput ed from the emfs: Delta G(f)degrees (NiSb2O4) +/- 4.2 (kJ mol(-1)) = - 988.3 + 0.3472 T (K) ( 769-931 K) Delta G(f)degrees (NiSb2O6) +/- 4.3 (kJ mol(-1)) = - 1394.9 + 0.5472 T (K) (894-1138 K) The reasonability of the above data were assessed by computing the entropy change for the solid-solid reactions leading to the formation of ternary ox ides from the respective pairs of constituent binary oxides. It was found t hat NiO forms a much more stable compound with Sb2O5 than with Sb2O3. (C) 1 999 Elsevier Science S.A. All rights reserved.