Reply to ``Comment on `Light scattering study of the phase transition in sym-triazine' ''
Rae’s1 comments on the author’s paper concerning the phase transition in sym‐triazine2 are answered. (AIP)
The structural phase change in s-triazine. I. The crystal structure of the low-temperature phase
The crystal structure of s-triazine at 150K has been determined by X-ray diffraction. The unit cell is monoclinic with a=0.6884 nm, b=0.9569 nm, c=0.7093 nm, beta =126.61 degrees . The low-temperature structure can be generated from the high-temperature hexagonal form by shearing the crystal in the a*c plane and rotating the molecules about the b axis. The shear and rotation angles have been measured at a number of temperatures between 110K and the transition temperature of 198.07K. They are found to be proportional to each other and their variation with temperature is in good agreement with that expected from the order parameter in the case of a quasi-continuous transition.
14N NQR study of molecular reorientation and phase transition in s‐triazine
14N NQR frequencies, linewidth, and spin–lattice relaxation times were measured in s‐triazine as a function of temperature between 77 and 280 °K. The second order phase transition from hexagonal to monoclinic which takes place at the critical temperature Tc=198.8 °K was studied in detail. The splittings of the NQR lines in the monoclinic phase, which are proportional to the order parameter, were found to vanish with the critical exponent β=1/2 on approaching Tc. Above 210 °K the temperature dependence of the linewidth and of the spin–lattice relaxation time indicates that the s‐triazine molecule undergoes molecular reorientation around its threefold axis. The activation enthalpy for this motion was found to be ΔH=14.0 kcal/mole. These relaxation phenomena were analyzed using a density matrix formalism. Explicit expressions for the decay exponents and signal function were derived.