Decasil (SiO2):: Multiple twinning and two-dimensional disorder

The synthetic porous SiO2 polymorph decasil crystallizes in the monoclinic space group C2/m. Its structure is characterized by interconnected rods of small cages built up from SiO4 tetrahedra. These rods are aligned parallel to the c axis. Decasil often displays structural disorder producing diffuse X-ray diffraction effects, which consist of streaks between Bragg reflections. These streaks are constrained to layers of the reciprocal lattice perpendicular to the c axis. Combining knowledge of the crystal structure and the observation of diffuse diffraction, a model of disorder and a method to calculate the diffuse diffraction intensity are developed. The model of disorder involving differently oriented rods allows the application of a recursive method of intensity calculation, previously developed for stacking faults.