Energetics of Substituted Pollucites Along the CsAlSi₂O₆-CsTiSi₂O_6.5 Join: A High-Temperature Calorimetric Study

Enthalpies of drop solution for a suite of substituted pollucites with the compositions CsTi_xAl_1-xSi₂O_6+0.5x,; 0 ≤ x ≤, which are synthesized using the sol-gel method, have been measured in molten lead borate (2PbO·B₂O₃) at 701°C. As Ti⁴⁺ substitutes for Al³⁺, the enthalpies of drop solution become less endothermic and show exothermic heats of mixing within the composition range from x = 0.3 to 1. This nonideal mixing behavior is consistent with the trend seen in variation of lattice parameters, and we interpret it to be a result of the short-range order associated with the framework cations Al³⁺, Si⁴⁺, and Ti⁴⁺ in the structure. Using enthalpies of drop solution of SiO², Al₂O₃, TiO₂, and Cs₂O, standard molar enthalpies of formation of these phases from their constituent oxides and from the elements are derived for the first time.