Polymorphism in phenol under pressure: dielectric properties and molar polarizations of polycrystalline phenol I and II

The dielectric properties of phenol I have been measured as isothermal functions of pressure between 140 and 1520 bar and between +35 and −10 °C. No dielectric relaxation was observed. The static (50 kHz) permittivity at 10 °C is given with a precision of 0.3% by[Formula: see text]where P is in bar. The extrapolated value at 1 bar is 2.882 ± 0.009 which compares with literature values between 2.74 and 2.84. The 50 kHz molar polarization, [Formula: see text], at 10 °C is given by[Formula: see text]where V and Vo are the molar volumes at pressures P and 1 bar, respectively. At 10 °C and 1 bar, the electronic and atomic polarizations are estimated to be 26.9 ± 0.3 cm3 and 5.1 ± 0.4 cm3, respectively. Expressions for the isobaric temperature dependence of εo′ are reported for several pressures. Below 1500 bar, (∂εo′/∂T)P is negative, as expected from the density change but in contrast with previous results and with results obtained while varying the temperature under a constant applied pressure of 1 kbar. The accuracy of the temperature dependence at 1 bar is not high, judging from the thermal expansivity calculated from it.εo′ of phenol II at 10 °C and an estimated pressure of 2000 bar is 3.10 ± 0.06, with the corresponding molar polarization 30.75 ± 0.5 cm3. No intrinsic dielectric relaxation was observed in phenol II and the molar polarization shows no marked discontinuity at the transition. Phenol II is, therefore, like phenol I, a hydrogen-bonded solid in which the hydroxyl hydrogen atoms are ordered, either fully or in chains. The dielectric parameters and Arrhenius activation energies of two transient dispersions which appeared whenever phenol II formed are discussed.