A new study of the structure of vitreous silica has been made under greatly improved conditions. Using Rh Kα radiation with the method of fluorescence excitation, reliable intensity values were measured to 4π sinθ/λ = 20. The interpretation was in terms of pair functions, thereby eliminating the approximations in earlier work. Each silicon is tetrahedrally surrounded by 4 oxygen atoms, with a Si–O distance which is closely 1.62 Å. Each oxygen atom is bonded to 2 silicon atoms. The Si–O–Si bond angle α shows a distribution V(α) extending all the way from 120° to 180°, with a maximum at α = 144°. This wide variation in α is an important distinction between the vitreous and the crystalline forms of silica, and it furnishes an important criterion for any proposed model of vitreous silica. Good agreement with the measured pair function distribution curve was obtained by assuming a random orientation about the Si–O bond directions. The interpretation leads to the familiar random network model, but with the new results the model is more precise.

A new study of the structure of vitreous boron oxide has been made under greatly improved conditions. Using Rh K~ radiation with the method of fluorescence excitation, reliable intensity values were measured to 4n sin 0/2 = 20 ,~-1, and a rigorous interpretation was obtained in terms of pair functions. Each boron is triangularly surrounded by 3 oxygen atoms with a B-O distance of 1"37 A. Each oxygen atom is bonded to 2 boron atoms. The major part of the glass is made up of boroxol groups B306 (a hexagonal ring of 3 boron atoms and 3 oxygen atoms with 3 corner oxygen atoms outside the ring). These groups are linked together by sharing the corner oxygen atoms, with randomness in orientation about the B-O bond directions at the shared oxygen atoms. Within the ring the B-O-B angle is 120 ° and at the shared oxygen it is approximately 130 °. There is satisfactory agreement between the calculated and measured pair function distribution curves. In vitreous B203 the scheme of bonding does not fill space, and any boroxol group has neighboring groups which are not directly connected to it by B-O and O-B bonds. The measured pair function areas indicate neutral atoms, so that the bonding is predominantly covalent.

An exact expression in the form of a double integral is obtained for the ratio of the intensity of double scattering to that of single scattering 1(2)/1(1) for a diffractometer sample of an amorphous material.By expressing the double scattering in terms of the independent scattering of the atoms, and approximating this quantity by an expression involving two parameters, the double integral is evaluated and tabulated as a function of 20 and the two parameters. With these tabulated values, the ratio 1(2)/1(1) for unmodified scattering only, or for both unmodified and modified scattering, can be obtained for a sample of any composition. For the unmodified scattering from vitreous SiO2 using Rh Ke radiation,

T~ec/~ ratios of the hexagonal AIN an~ BeO structures show that they do not have a regularly tetrahedral dlstnbutIOn of bonds. The structural details have been studied by x-ray analysis and are discussed in relation ~o the concepts. of t~e electronegati~ity and ~Iectroneutrality of atoms. This gives a basis for an understand-Ing of the dlstnbutIOn of the wurtzlte and ZInC blende lattices in the binary compounds of the group III/V and II/VI elements.

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