Calorimetric Investigation of a New Solid Phase in Triphenylphosphite

Adiabatic heat capacity measurements were made on triphenylphosphite, molecular formula C₁₈H₁₅O₃P, between 140 and 335 K. The results confirm the existence of a phase which is intermediate in stability between the undercooled liquid and the crystal phase. This phase, called the glacial phase, was recently reported for the first time. At 227 K this phase disappears as the crystallization starts. The melting temperature was found to be 297.7 ± 0.2 K, the melting enthalpy 25090 ± 100 J mol^-1. The glass transition temperature was 201.8 K, the ratio between the heat capacity of the undercooled liquid and the glass at this temperature being 1.68. T₀, the temperature at which the entropy of the undercooled liquid equals that of the crystalline phase was calculated to be 169 K. A discontinuity in the specific heat of the liquid phase was found close to the melting point. This is related to the existence of a narrowly avoided thermodynamic transition point.