14N PNQR study of the effect of pressure on the phase transition and molecular reorientation in s-triazine

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Abstract
14N pure nuclear quadrupole resonance frequencies and inverse linewidth in s-triazine were measured as functions of hydrostatic pressure to 6.2 kbar along two isotherms at 23.3 and −0.4 °C. The second order phase transition, previously studied at atmospheric pressure, was observed at high pressures. Above the transition pressure pc each resonance line splits into two. The line splittings were fitted to the equation Δν=B (p−pc)β, and the critical exponent β was found to be close to 0.5. The pressure dependence of the transition temperature Tc is described by the (linear) equation Tc(p) =198.8+18.2p, [Tc(°K);p (kbar)]. The present results along isotherms were compared with previous results along an isobar. In the low pressure region the inverse linewidth T2* is dominated by hindered rotations of the s-triazine molecule. The activation volume for this motion is ΔV*=26 cm3/mole.