New Family of Materials with Negative Coefficients of Thermal Expansion: The Effect of MgO, CoO, MnO, NiO, or CuO on the Phase Stability and Thermal Expansion of Solid Solution Phases Derived from BaZn2Si2O7.

Recently, a silicate with the composition SrxBa1-xZn2Si2O7 was reported, which exhibits a negative coefficient of thermal expansion. The compound BaZn2Si2O7 shows a highly positive coefficient of thermal expansion up to a temperature of 280 °C and then transfers to a high temperature phase, which exhibits a coefficient of thermal expansion near zero or negative over a limited temperature range up to around 500 °C. This high temperature modification can be stabilized to room temperature if Ba(2+) is replaced by Sr(2+). In the solid solution SrxBa1-xZn2Si2O7, also Zn(2+) can be replaced in a wide concentration range by other cations with the respective valency. In the present study, Zn was partially or completely replaced by Mg, Co, Mn, Ni, or Cu. If the high temperature phase is stable at room temperature, the thermal expansion is negative, and if the partial substitution exceeds a certain concentration threshold, the low temperature phase with the crystal structure of BaZn2Si2O7 and highly positive thermal expansion is formed. The lowest mean coefficients of thermal expansion were measured for the composition Ba0.5Sr0.5Zn1.4Co0.6Si2O7 with a value of -2.9 × 10(-6) K(-1). In general, a lower Zn-concentration leads to a higher anisotropy and a lower mean coefficient of thermal expansion.