Molecular Origin of Auxetic Behavior in Tetrahedral Framework Silicates

Recent analytical models for the Poisson's ratios (νᵢⱼ) of tetrahedral frameworks are applied to {α}-cristobalite and {α}-quartz for the first time. Rotation and dilation of the SiO₄ tetrahedral subunits are considered. Each mechanism leads to negative ν₃₁ values, whereas negative and positive values are possible when they act concurrently. The concurrent model is in excellent agreement with experiment and explains the dichotomy between negative and positive ν₃₁ values in α-cristobalite and α-quartz, respectively. The predicted strain-dependent trends confirm those from molecular modeling.