Glasses and amorphous materials
Which statement about the atomic arrangement of an oxide glass such as soda-lime glass is correct?
- ANearest-neighbor coordination and bond lengths, as in SiO₄ tetrahedra, are nearly uniform (short-range order), but there is no long-range periodicity
- BThe atoms are arranged completely at random, like a gas, and both the coordination number and the bond lengths vary from atom to atom
- CIt has the same periodic arrangement as a crystal, but its grains are only a few nm across, so no peaks appear in X-ray diffraction
Show answer and explanation
Answer: A. Nearest-neighbor coordination and bond lengths, as in SiO₄ tetrahedra, are nearly uniform (short-range order), but there is no long-range periodicity
Even in an oxide glass, the SiO₄ unit, in which four oxygens are tetrahedrally coordinated around Si, is almost the same as in the crystal, and the Si-O distance is uniform. However, the Si-O-Si angles and the way tetrahedra link vary, so regularity is lost beyond about 1 nm and there is no long-range periodicity. Zachariasen described this as a continuous random network. In X-ray diffraction a broad halo appears instead of sharp peaks, and the nearest-neighbor distance and coordination number can be obtained from the radial distribution function. The statement that atoms are completely disordered overlooks short-range order. The crystallite hypothesis, which treats glass as an aggregate of tiny crystals, is not the main view, because the crystal size estimated from the halo width is no larger than a unit cell, and the small-angle scattering that grain boundaries would produce is not observed (Warren).