Photoelastic trends from halides to pnictides by a bond-orbital method.

The photoelastic effect, or strain dependence of refractive index n(o), essentially controls the details of light scattering in liquids, glasses and crystals. Some knowledge of its behavior as a function of structure and chemical composition is therefore extremely valuable in a search for devices which seek to maximize or minimize such light scattering mechanisms. This paper derives a bond-orbital theory for the hydrostatic photoelastic effect (or the dependence of n(o) on density rho) for three-dimensionally coordinate crystalline structure (for which density fluctuations are directly related to primary bond length fluctuations) including four-, six-, and eight-fold coordinated cations. The theory sets out the manner in which (delta)n(o)/(delta) (rho) depends on such fundamentals as valence, ionic radii, and degree covalence, and highlights a term not previously derived in any prior theories of linear dielectric response.