Dipartimento Ingegneria Meccanica, Università di Roma Tor Vergata, Rome, Italy.
J Phys Chem B. 2010 Jun 24;114(24):8134-9. doi: 10.1021/jp102609y.
The character of the R(II)-R(I) and the R(I)-R(V) transitions was studied in alkane samples by analyzing the hysteresis the specific heat shows between heating and cooling measurements, using photopyroelectric calorimetry. The hysteresis systematically shows up in all the investigated samples over the R(II)-R(I) transition, similarly to what also observed over the liquid-R(II) and the R(V)-crystal phase transitions, thus confirming the first order character of all such phase transitions. Over the R(I)-R(V) transition a hysteresis is also present but with varying characteristics depending on the width of the temperature range of the R(I) phase, the first order character being smaller for decreasing R(I) range. The observed results were explained in terms of possible varying strength of the coupling term in the free energy between the distortion and tilt angle order parameters in the different samples, a variation which can induce a change in the order character of the transition. The results are compared to those obtained in an alkane sample under increasing ambient pressure, which turned the transition from first to second order. In both cases it was suggested that the variations of the coupling strength between the two order parameters could be associated with changes in the effectiveness of the molecular interaction between the layers relative to that occurring within each layer.
利用光热释电技术，通过分析加热和冷却测量之间比热表现出的滞后现象，研究了烷烃样品中 R(II)-R(I) 和 R(I)-R(V) 转变的特性。在所有研究的样品中，滞后现象系统地出现在 R(II)-R(I) 转变中，类似于在液体-R(II) 和 R(V)-晶体相变中观察到的情况，从而证实了所有这些相变都是一级相变。在 R(I)-R(V) 转变中也存在滞后现象，但特征随 R(I) 相温度范围的宽度而变化，随着 R(I) 范围的减小，一级相变的特征变小。观察到的结果用自由能中变形和倾斜角序参量之间的耦合项的可能变化强度来解释，这种变化可以导致转变的有序特征发生变化。结果与在烷烃样品中在升高环境压力下获得的结果进行了比较，这使转变从一级变为二级。在这两种情况下，都表明两个序参量之间的耦合强度的变化可能与层间分子相互作用的有效性相对于每层内发生的相互作用的变化有关。
