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<p>Abstract</p> <p>Background</p> <p>The crystal structure of 2,2-trimethylenedioxy-4,4,6,6-tetrachlorocyclo triphosphazene has been determined at 120, 274 and 293 K. The result at 293 K confirms the room temperature <it>Cmc2</it>₁structure, but at the lower temperatures the space group is <it>Pna2</it>₁. Nevertheless the basic structure remains the same, with only small displacements of the atoms, amounting to an average of 25 pm between 120 and 293 K.</p> <p>Results</p> <p>X-ray diffraction and DSC results indicate that the phase transition takes place in two steps between 274 – 293 K and provides an understanding of previous NQR results. In the intermediate temperature range the molecules are displaced from their room temperature positions in such a way as to give an average structure with <it>Cmc2</it>₁symmetry.</p> <p>Conclusion</p> <p>The overall phase transition is consistent with the occurrence of a soft lattice mode at room temperature in which a large displacement of the molecule in the <it>x</it>-direction is coupled with a flexing motion about an axis defined by the nitrogen atoms in the N1 and N3 positions.</p>