Evidence of residual entropy in the cubic spinel Zn₂TiO₄

Abstract

The standard free energies of formation of Zn₂Ti0₄ and ZnTi0₃ have been determined in the temperature range 930° to i ioo'x from electromotive force measurements on reversible solid oxide galvanic cells;Ag-5at%znll I Pt, + CaO-Zr0₂ ZnO I II Ag-5at%Zn Y₂0r Th0₂ CaO-Zr0₂ + ,Pt Zn₂Ti0₄+ ZnTi0₃ and II Ag-5at%Zn CaO-Zr0₂ + ,Pt ZnTi0₃+ Ti0₂ The values may be expressed by the equations,₂ZnO (wurtz) + Ti0₂(rut) -> Zn₂Ti0₄(sp), f:!:.Go = -750-2-46T (±75)cal;ZnO(wurtz) +Ti0₂(rut) -> ZnTi0₃(ilmen) ,f:!:.Co = -]600-0·]99T(±50)cal.Combination of the free energy values with the calorimetric heat of formation, and low-temperature and high-temperature heat capacity of Zn2Ti04 reported in literature, suggests a residual entropy of ],9 (±0·6) cal K-1 mol ? for the cubic spinel. Ideal mixing of Zn₂+ and Ti₄+ ions on the octahedral sites would result in a configurational contribution to the entropy of 2· 75 cal K-1 rnol ".The difference is indicative of short-range ordering of cations on octahedral sites.