Mechanisms of ultralow and anisotropic thermal expansion in cordierite Mg2Al4Si5O18: Insight from phonon behaviors

Materials with negative or ultralow thermal expansion are of crucial importance for technological applications since they make it possible to tailor the coefficient of thermal expansion (CTE) of composite to a specific positive, negative or even zero value. In this work, first-principle calculations were performed to investigate the thermal expansion behavior in cordierite Mg2Al4Si5O18, which is a representative silicate widely used in the ceramic industry and of promising application due to its ultralow CTE and good thermal shock resistance. According to the quasi-harmonic approximation and the Gruneisen theory, temperature dependences of linear CTEs along a, b, and c directions were predicted. The transverse acoustic modes and low-energy optic modes are identified to take the most of the responsibility for the negative CTE, especially at low temperatures while the high-energy optic modes contribute positively to the thermal expansion, leading to increasing CTE at higher temperatures. The ultralow linear CTEs result from the weighted average of all the modal contributions with negative or positive Gruneisen parameters. In addition, the anisotropy of thermal expansion originates from its layered crystal structure containing rigid tetrahedron rings in a-b plane staking along c direction. This work provides an insight into the mechanism of ultralow and anisotropic thermal expansion in Mg2Al4Si5O18 and further enriches the scope of material design for use in applications needing to control thermal expansion.