Molecular dynamics study of the high‐temperature elasticity of SiO2 polymorphs: Structural phase transition and elastic anomaly

Abstract

AbstractThe elastic properties of cristobalite and quartz, the typical polymorphs of SiO2, are studied with particular emphasis on the structural phase transition and the high‐temperature phase. Using the equilibrium molecular dynamics (MD) method with a stress‐fluctuation formula, we have successfully evaluated the adiabatic elastic constants (Cij) of both cristobalite and quartz in a wide temperature range, including the α–β phase‐transition region. In the results for cristobalite, the anomalous property of a negative Poisson's ratio appears over the entire temperature range of 300–1800 K, where the bulk modulus is extremely low compared with the shear modulus. However, the mechanisms differ between the α‐ and β‐phases, according to the structural data obtained from the MD simulations. On the other hand, quartz exhibits a negative Poisson's ratio in the narrow temperature region between ca. 750 and 825 K before the phase transition occurs. After the transition to the β‐phase, the bulk modulus is shown to increase sharply, and a positive value of Poisson's ratio is recovered. For cristobalite as well as quartz, we have confirmed that the recovery of bulk Cij's in the β‐phase can be attributed to internal relaxations, which arise from the cooperative motions of corner‐linked SiO4 tetrahedra. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)