text: Four crystalline forms (polymorphs) of  are known: α, β, γ, and δ. Each case features tetrahedral  centers.
  Here a, b, and c are lattice constants, Z is the number of structure units per unit cell, and ρ is the density calculated from the structure parameters.
  The orthorhombic form (δ) rapidly changes to one of the other forms on exposure to the atmosphere. A possible explanation is that the  ions originating from the absorbed water facilitate the rearrangement. Rapid cooling of molten  gives a glass.
  Molten  has a high viscosity at its melting point and a comparatively low electrical conductivity, which increases markedly with temperature. As indicated by a Raman scattering study, the viscosity is explained by the presence of polymers,. Neutron scattering study indicated the presence of tetrahedral  centers, which requires aggregation of  monomers as well..
  In the gas phase,  molecules are linear with a bond length of 205 pm.