Analysis of the Raman Frequency as an Order Parameter Close to the Melting Point in Biphenyl

Abstract

We analyze the observed Raman frequencies of the three lattice modes (modes A, B and 70cm-1) near the melting point in biphenyl by calculating the temperature dependence of the order parameter from the mean field theory. This is based on the correlation between the frequency and the order parameter close to the melting temperature (Tm=343K) in biphenyl. The Raman frequencies of the modes A and B decrease rapidly as the melting temperature is approached, whereas the Raman frequency of the 70 cm-1 is smoothly decreasing, as observed experimentally. It is indicated that the structural phase change toward the melting point in biphenyl is associated with the decrease in the Raman frequencies of the lattice modes A and B, in particular mode A, as observed experimentally in this molecular crystal. This method of calculating the Raman frequency in relation to the order parameter as calculated from the mean field theory is significant in comparison with some previous methods such as using the quasi-harmonic approximation and also from the crystal volume by means of the mode Grüneisen parameter.