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

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 p_c each resonance line splits into two. The line splittings were fitted to the equation Δν=B (p−p_c)^β, and the critical exponent β was found to be close to 0.5. The pressure dependence of the transition temperature T_c is described by the (linear) equation T_c(p) =198.8+18.2p, [T_c(°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.