THE MECHANISM OF COUPLING IN THE MODULATED STRUCTURE OF NEPHELINE

The crystal structures of a nepheline, K(0.54)Na(3.24)Ca(0.03)square(0.19)Al(3.84)Si(4.16)O(16), and that of the same sample annealed at high temperature to induce K-vacancy disorder, have been determined at several temperatures down to 15 K by single-crystal X-ray diffraction. The largest structural change in both crystals with decreasing temperature is the decrease of the T1-O1-T2 angle, corresponding to an increase in the tilt of the T1 and T2 tetrahedra within the framework. The tetrahedra in the annealed sample have a smaller tilt than in the natural sample at any given temperature. The correlation of the tilts of the tetrahedra with changes in the intensities of satellite reflections confirms that the satellites arise from a displacive modulation of the framework of tetrahedra. Distance-least-squares simulations suggest that the modulation creates larger and smaller cavities within the extra-framework channels that contain the K atoms. Analysis of the K-O bond lengths with both the state of K-square order and temperature indicate that the coupling between K-square order and the framework modulation occurs through the K-O2 bond. An increase in the average K-O2 bond length with decreasing temperature or increasing K-square order supports the modulation of the framework. Shortening of the K-O2 bond leads to rotations of the tetrahedra that are opposite to those associated with the modulation, and thus suppresses it.