High thermal expansion in the solid solution series BaM2−x Ni x Si2O7 (M = Zn, Mg, Co)-the effect of Ni-concentration on phase transition and expansion

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 BaCo1.5Ni0.5Si2O7, where the maximum difference between the coefficients of thermal expansion of the different lattice parameters was just 3.9×10−6 K−1.