Anomalous thermal expansion of cordierite, Mg2Al4Si5O18, understood through lattice simulations

Abstract

Cordierite, Mg₂Al₄Si₅O₁₈, finds widespread use as a high-performance material because of its very low thermal expansion, but its thermal properties are not understood at a fundamental level. Here we examine the thermal expansion in cordierite using lattice dynamics and molecular dynamics simulations with transferable force fields. The model reproduces the low positive thermal expansion along the orthorhombic [1,0,0] and [0,1,0] directions and the negative thermal expansion (NTE) along the [0,0,1] direction. Calculations of the mode strain Grüneisen parameters show that many phonon branches up to 10 THz contribute to the NTE. The overall strain Grüneisen parameters give similar results for the three principal strains: all are negative at lower temperatures and positive at higher temperatures. While they show the presence of a significant tension effect, the overall behavior is controlled by the form of the elastic compliance tensor. The results will usefully inform future studies of anisotropic systems.