X-Ray Diffraction of Vitreous Silica

X-ray diffraction patterns of vitreous silica and pyrex glass each show a strong tut broad diffraction ring at a scattering angle corresponding to a value"rf" in the Bragg Law of 4,32 A for silica and 4.26 Ä tor pyrex. Two methods of interpretation suggest themselves; a) the diffuse ring is due to extremely small crystals of cristobalite, b) due to scattering from a non-crystalline random network. On the first hypothesis the observed ring is the strong (111) reflection from cristobalite crystals about 15 A in size. A number of objections, particularly those having to do with the width of the ring in connection with heat treatment, seem to rule out the crystallite hypothesis. On the random network hypothesis, it is postulated that the atoms are bound together in the same way as in the crystalline forms of SiO^, but forming a continuous non-crystalline network. Each Silicon is tetrahedrally surrounded by 4 oxygens, and each oxygen shared between two tetrahedral groups. Each tetrahedral group has 4 neareat neighbours at 3.1 A, and 12 next nearest neighbours at 5.0 Ä. Beyond this the distances are indefinite. An expression for the intenaity of scattering from such an array leads to an intensity curve in satisfactory agreement with the experimental curve. The random network picture of the vitreous State, deduced from X-ray diffraction study, is in complete agreement with the conclusions reached by Zachariasen from consideration of the chemical and physical properties of glass.