Optical and microradian x-ray diffraction from opal-like films: Transition from 2D to 3D regimes

In this work diffraction of light and x-rays (XRD) on opal-like structures (OLS) is considered. New methodological approaches to collection, processing and interpretation of experimental data are presented. The microradian XRD reveals the type and degree of imperfection of opal-like structures. It was shown that the OLS ordered in the twinned face centered cubic (FCC) structures with the lattice constant of 835 ± 10 nm but also with clear tendency to the random hexagonal close-packed (RHCP) structure along the [111] axis. A novel representation of the light diffraction data in the “incident angle - scattered angle” coordinates (θ, Θ) allows one to easily distinguish the reflections originating from 2D diffraction from the ones governed by 3D Bragg diffraction. Structural disorder becomes apparent in the (θ, Θ) representation, as well. In addition, it was demonstrated that the immersion spectroscopy method can be used to selectively switch diffraction reflections. This phenomenon is caused by optical inhomogeneity of the SiO₂ particles that form opal-like crystals.