Phase stability determination of negative thermal expansion silicates by theory and experiment

Abstract

Materials that exhibit zero thermal expansion have numerous applications, ranging from everyday ceramic hobs to telescope mirrors to devices in optics and micromechanics. These materials include glass ceramics containing crystal phases with negative thermal expansion in at least one crystallographic direction, such as Ba_1−xSr_xZn_2−2yMg_2ySi₂O₇ solid solutions. However, the volume increase associated with the martensitic phase transformation in these crystals often hinders their use as zero thermal expansion materials at operating temperatures near the transition temperature T_t. Here, an approach to rapidly predict T_t of such materials as a function of chemical composition based on a combination of density functional theory simulations and experiments has been developed and applied to Ba_1−xSr_xZn_2−2yMg_2ySi₂O₇. Its central element is the modeling of free energy as a function of temperature and chemical composition using a composition-dependent Debye model augmented by an empirical correction, which incorporates the effects of anharmonic lattice vibrations. This approach provides T_t predictions with an estimated uncertainty of about ±100 K, which is similar to the accuracy of computationally much more demanding simulations of polymorphous phase transitions. In addition, this approach allows computationally efficient determination of the chemical compositions at which the Ba_1−xSr_xZn_2−2yMg_2ySi₂O₇ phase with the desired thermal properties will be formed during synthesis, facilitating the targeted design of zero thermal expansion materials.