High thermal expansion in the solid solution series BaM_2−x Ni _x Si₂O₇ (M = Zn, Mg, Co)-the effect of Ni-concentration on phase transition and expansion

Abstract

In crystalline phases with the compositions BaMg₂Si₂O₇, BaCo₂Si₂O₇, and BaZn₂Si₂O₇, the components MgO, CoO, and ZnO can be partially substituted by NiO. The formed solid solutions exhibit a phase transition from a low to a high temperature phase which runs parallel to an increase in volume, which is most pronounced in the case of BaZn₂Si₂O₇. The phase transition temperatures and the thermal expansion behavior of these solid solutions are investigated as a function of the NiO concentration. The phase transition temperature of pure BaZn₂Si₂O₇ increases from 280 up to 915 °C, if ZnO is partially replaced by NiO. Furthermore, the phase transition temperatures of the MgO- and the CoO-containing compounds also increase with the increasing NiO concentration. The thermal expansion behavior was investigated by dilatometry and for selected samples by high-temperature X-ray diffraction in a temperature range between room temperature and a temperature below the respective phase transition. The measured coefficients of thermal expansion lie in the range from 6.2 to 21.4×10⁻⁶ K⁻¹. The thermal expansion of all studied compounds depends on the crystallographic direction. The largest variation is observed in the compound BaZn₂Si₂O₇. The smallest differences in the expansion in different directions are measured for the sample BaCo_1.5Ni_0.5Si₂O₇, where the maximum difference between the coefficients of thermal expansion of the different lattice parameters was just 3.9×10⁻⁶ K⁻¹.