Rigid Unit Modes in disordered nepheline: a study of a displacive incommensurate phase transition |
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Authors: | S A Hayward A K A Pryde RF de Dombal M A Carpenter M T Dove |
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Institution: | (1) Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, UK, GB |
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Abstract: | Calculations of the Rigid Unit Modes (RUMs) allowed in the nepheline structure 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 the
diffuse scattering as a function of temperature. The diffuse intensity increases sharply at 308 K. This effect is interpreted
as being due to the softening of a phonon mode, indicating a phase transition. Measurements of this phase transition below
the transition temperature are made using hard mode infrared spectroscopy.
Received: 17 February 1999 / Revised, accepted. 15 October 1999 |
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Keywords: | Nepheline Incommensurate Rigid unit mode |
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