The structure of opal-CT revisited

• Opal-CT structure is best explained by tridymite-dominant paracrystalline model.
• Paracrystalline model is supported by various spectroscopic methods.
• XRD and electron microscope evidence are reconciled with paracrystalline model.
• Paracrystalline model has implications for nomenclature of silica polymorphs.

There is a conflict of evidence with regard to the structure of the silica polymorph known as opal-CT. The current widely accepted interpretation of the structure views opal-CT as made up essentially of a disordered interlayering of crystalline cristobalitic and tridymitic stacking units, in about equal proportions where the d spacing of the diffraction maximum is close to 4.11 Å, but becoming progressively more ordered and cristobalitic as this spacing decreases in value and approaches the d(101) reflection at 4.04 Å of α-cristobalite. The most compelling evidence for this interpretation comes from matching calculated and experimental XRD profiles, HRTEM lattice images and SAED patterns. This interpretation is here described as the “crystalline” model. However, the crystalline model is not supported by a variety of spectroscopic techniques that can be used to probe both long-range and short-range order. In general, these techniques which include FTIR, Raman, NMR and XANES spectroscopies are more consistent with a structure, especially where the diffraction maximum approaches 4.11 Å, as being dominated by disordered material similar to amorphous opal (opal-A) but containing a small volume of crystalline stacking units which are mainly (sometimes exclusively) of a tridymitic nature. This interpretation is referred to as the “paracrystalline” model. From a critical review of the literature, suggestions are made as to how these two conflicting strands of evidence can be reconciled, as well as matters concerning appropriate nomenclature of opaline materials.