The structural phase change in s-triazine: Reconciliation of x-ray diffraction and NQR measurements

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Abstract
Previous x‐ray diffraction studies of the low‐temperature phase of s‐triazine identified the order parameter with a shear strain of the unit cell and interpreted the data in terms of a weakly first‐order transition. Independent measurements of the nuclear quadrupole resonance spectrum showed that the 14N lines split in the low temperature phase; this splitting was taken to be proportional to the order parameter and its variation with temperature was described in terms of that associated with a second‐order phase change, with a fluctuation contribution in the region of the transition. The present work shows that the first order nature of the transition is a necessary consequence of the symmetry of the system and the temperature dependence of the NQR frequencies can be explained if they are expressed as a sum of a term proportional to the order parameter together with a relatively large quadratic term whose magnitude can be qualitatively understood by a consideration of the environment of the nitrogen atoms.

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