A study of the mean - square displacement amplitudes of Si , AI , and 0 atoms in framework structures : Evidence for rigid bonds , order , twinning , and stacking faults

A study of the mean-square displacement amplitudes (MSDA) of the T(Al,Si) and 0 atoms in ordered framework silicates and aluminosilicates indicates that the sizes, shapes, and orientations of the displacement ellipsoids in these crystals are consistent with a rigid bond model. In particular, the MSDAs of the T and 0 atoms in the direction of the TO bonds tend to be equal and to increase with temperature. When this relationship does not hold, it appears to be related to the presence of static disorder. This observation leads to criteria that may be used to detect disorder in crystals. A simple rigid bond model is presented that, to a reasonable degree, describes the ellipticity and the orientation of the displacement ellipsoids of the bridging 0 atoms.