Negative thermal expansion of quartz glass at low temperatures: An ab initio simulation study

Using a mixed classical Molecular dynamics (MD)/ab initio simulation scheme combined with a quasi-harmonic approximation, we calculate the linear thermal expansion coefficient αL(T) in vitreous silica glasses. The systems are first cooled down by classical MD simulations. Then they are structurally relaxed by ab initio DFT calculations. The vibrational properties are calculated employing the frozen phonon method, and these results are finally used to calculate the Helmholtz free energy as a function of volume. In agreement with experiments, our simulations predict that αL(T) is negative at low temperatures up to T ≈ 150 K. In this low-temperature regime, the simulation results are in quantitative agreement with experiments. To elucidate the origin of the negative thermal expansion, we analyze in detail the microscopic mode Grüneisen parameters in the system and show that the anomalous behavior of αL(T) can be related to the fact that the Grüneisen parameters for the lowest modes become negative at low temperatures - i.e., the lowest eigenfrequencies become stiffer with increasing volume.