Rigid Unit Modes in disordered nepheline: a study of a displacive incommensurate phase transition

Calculations of the Rigid Unit Modes (RUMs) allowed in the nepheline struct ure are used to explain the diffuse scattering previously seen in electron diffraction experiments. The RUM calculations also show that the modulation wavelength for incommensurate nephelines is essentially determined by the framework topology. X-ray diffraction is used to measure the intensity of t he diffuse scattering asa function of temperature. The diffuse intensity in creases sharply at 308 K. This effect is interpreted as being due to the so ftening of a phonon mode, indicating a phase transition. Measurements of th is phase transition below the transition temperature are made using hard mo de infrared spectroscopy.