DECASIL (SIO2) - MULTIPLE TWINNING AND 2-DIMENSIONAL DISORDER

The synthetic porous SiO2 polymorph decasil crystallizes in the monocl inic space group C2/m. Its structure is characterized by interconnecte d rods of small cages built up from SiO4 tetrahedra. These rods are al igned parallel to the c axis. Decasil often displays structural disord er producing diffuse X-ray diffraction effects, which consist of strea ks between Bragg reflections. These streaks are constrained to layers of the reciprocal lattice perpendicular to the c axis. Combining knowl edge of the crystal structure and the observation of diffuse diffracti on, a model of disorder and a method to calculate the diffuse diffract ion intensity are developed. The model of disorder involving different ly oriented rods allows the application of a recursive method of inten sity calculation, previously developed for stacking faults.