Thermal conductivity of Glycerol’s liquid, glass, and crystal states, glass-liquid-glass transition, and crystallization at high pressures

To investigate the effects of local density fluctuations on phonon propagation in a hydrogen bonded structure, we studied the thermal conductivity κ of the crystal, liquid, and glassy states of pure glycerol as a function of the temperature, T, and the pressure, p. We find that the following: (i) κ crystal is 3.6-times the κ liquid value at 140 K at 0.1 MPa and 2.2-times at 290 K, and it varies with T according to 138 × T −0.95; (ii) the ratio κ liquid (p)/κ liquid (0.1 MPa) is 1.45 GPa−1 at 280 K, which, unexpectedly, is about the same as κ crystal (p)/κ crystal (0.1 MPa) of 1.42 GPa−1 at 298 K; (iii) κ glass is relatively insensitive to T but sensitive to the applied p (1.38 GPa−1 at 150 K); (iv) κ glass-T plots show an enhanced, pressure-dependent peak-like feature, which is due to the glass to liquid transition on heating; (v) continuous heating cold-crystallizes ultraviscous glycerol under pressure, at a higher T when p is high; and (vi) glycerol formed by cooling at a high p and then measured at a low p has a significantly higher κ than the glass formed by cooling at a low p. On heating at a fixed low p, its κ decreases before its glass-liquid transition range at that p is reached. We attribute this effect to thermally assisted loss of the configurational and vibrational instabilities of a glass formed at high p and recovered at low p, which is different from the usual glass-aging effect. While the heat capacity, entropy, and volume of glycerol crystal are less than those for its glass and liquid, κ crystal of glycerol, like its elastic modulus and refractive index, is higher. We discuss these findings in terms of the role of fluctuations in local density and structure, and the relations between κ and the thermodynamic quantities.