Solid solutions based on BaZn2Si2O7 with thermal expansions from negative to highly positive – a review

Abstract

The compound BaZn₂Si₂O₇ shows a transition from a monoclinic low temperature phase to an orthorhombic high temperature phase at 280 °C which is accompanied by an increase in the volume of 3.3%. The temperature of the phase transition can be shifted to higher temperature by a replacement of Zn²⁺ by Mg²⁺ or the divalent transition metal cations, Mn²⁺, Co²⁺, Ni²⁺ or Cu²⁺ as well as by the replacement of Si⁴⁺ by Ge⁴⁺. By contrast, the replacement of Ba²⁺ by Sr²⁺ leads to the shift of the phase transition to lower temperatures. The coefficient of thermal expansion of the low temperature phase is exceptionally high and in the range from 10 to 15 10⁻⁶ K⁻¹, while that of the high temperature phase is small or even negative. The reason of the latter effect is a rotation of bridged ZnO₄ tetrahedra in opposite directions. This results in a strongly negative thermal expansion of the b-axis, while the expansion of the a- and c-axes are positive. Although melts of the stoichiometric compounds do not build a continuous network, glass formation can be achieved by the addition nucleation inhibitors, such as ZrO₂, Al₂O₃ or La₂O₃ or an excess of SiO₂. The quenched glasses crystallize at the surface while bulk crystallization is not observed. This changes if noble metals or higher concentrations of ZrO₂, WO₃ or P₂O₅ are added to the glass, then also bulk crystallization is possible. Homogeneously crystallized glass ceramics can also be obtained, by sinter crystallization of powdered glass or using sol–gel derived crystalline powders. Adjusting the composition of the respective solid solutions enables to tailor the coefficient of thermal expansion from negative to highly positive values.