NEGATIVE POISSON RATIOS IN CRYSTALLINE SIO2 FROM 1ST-PRINCIPLES CALCULATIONS

THE Poisson ratio of a solid characterizes its response to uniaxial stress. It is defined as the negative ratio of the transverse strain to the corresponding axial strain. Normally, this ratio is positive, as most solids expand in the transverse direction when subjected to a uniaxial compression. Although a negative Poisson ratio is not forbidden by thermodynamics, it is rare in crystalline solids: the results of recent experiments1 which observed a negative Poisson ratio in alpha-cristobalite were therefore unexpected. We have investigated the elastic behaviour of alpha-cristobalite and other forms of silica with first-principles calculations and classical interatomic potentials. Our calculations reproduce the negative Poisson ratio in alpha-cristobalite, and predict that alpha-quartz, the most common form of crystalline silica, will also exhibit a negative Poisson ratio under large uniaxial tension. We attribute the occurrence of a negative Poisson ratio in low-density silica polymorphs to the high rigidity of the SiO4 tetrahedra.