Modeling the X-ray diffraction pattern of opal-CT

Abstract X-ray diffraction patterns of opal-CT were modeled using a recursive calculation technique that allows for ordered and disordered intergrowths of planar units. The opal-CT structure was modeled as interstratifications of cristobalite and tridymite layers that were based on an idealized trigonal silica sheet, as found in the high-temperature structures. Modeled patterns matched observed patterns well, including the overall shapes of the broad peaks at ~19.5-24.5° 2θ and 35.9° 2θ (CuKα). The calculations allow several important interpretations of opal-CT: XRD patterns of opal-CT can be modeled by an interstratification of cristobalite and tridymite; both disordered and ordered interstratifications occur; particle size can be inferred from the width of the 35.9° 2θ peak; significant information regarding the structural state of opal is contained in weak, often-ignored bands (e.g., one at ~41-46° 2θ); and H2O is not present in an ordered fashion within the cristobalite and tridymite sheets.