14N PNQR investigation of the effect of pressure on phase transition in malononitrile

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Abstract
The 14N pure nuclear quadrupole resonance frequencies in the molecular crystal malononitrile were measured as a function of hydrostatic pressure up to 5.6 kbar. Above T=21.55°C, the second order phase transition previously studied by Zussman and Alexander was observed at high pressures. Above the transition pressure pc, each resonance line splits into two. The splittings Δν along an isotherm at 23°C were fitted to the equation Δν=B (p−pc)β and the critical exponent β was found to be close to 0.5. The variation of Tc with pressure was measured to 2 kbar and the line of transition could be described by the linear equation Tc(p) = 21.51+16.5734p. The present results for the variation of Δν along an isotherm were compared with previous results obtained along an isobar.