Light scattering and optical diffusion from willemite spherulites

• Willemite spherulites embedded in glass act as optical diffusers of visible light.
• The scale of the microstructure of willemite spherulites causes the effect.
• The phenomenon is shown to be in essence a form of Mie scattering.
• Optical diffusers based on willemite have potential for high temperature use.

Willemite is a zinc silicate mineral used in modern day pottery as a decorative feature within glazes. It is produced by controlled heat treatment of zinc oxide-containing ceramic glazes. The heat-treated glazes devitrify, producing thin nanoscale needle-like willemite crystals growing in spherulitic morphologies through branching of the needles. We show here that this resulting morphology of willemite crystals in an inorganic glass matrix has a previously unreported strong interaction with light, displaying remarkable optical diffraction patterns. Thin sections of such spherulites act as optical diffusers, enabling light beams to be spread up to 160° in width. Analysis of the interaction between the willemite spherulites and light suggests that the high density of willemite crystals in the spherulites and the length scales associated with both the thickness of the needles and the spacings between branches are together responsible for this optical diffusion behaviour.