Off-campus UNL users: To download campus access dissertations, please use the following link to log into our proxy server with your NU ID and password. When you are done browsing please remember to return to this page and log out.
Non-UNL users: Please talk to your librarian about requesting this dissertation through interlibrary loan.
Analysis of phase transitions in molecular crystals
Abstract
The $\beta$ and $\gamma$ phases of single crystal malononitrile, CH$\sb2$(CN)$\sb2$, are investigated using Raman spectroscopy below 200 cm$\sp{-1}$. Nine out of twelve Raman-active optical lattice modes are assigned in each phase. The cyano-C-cyano molecular bending mode is also observed below 200 cm$\sp{-1}$. Three of the lattice modes shift in frequency on passing through the phase transition at 294.7 K. These results are compared to previous Raman spectroscopic studies of malononitrile. Thirteen stiffness constants of $\beta$-phase malononitrile are determined from acoustic phonon velocities utilizing Brillouin spectroscopy. The stiffness constants constitute the first report concerning the elastic behavior of malononitrile. Molecular dipole and cyano functional group interactions are used to explain the $\beta$-phase compressional stiffness constants of malononitrile. Changes in molecular dipole interactions are used to describe the $\beta$-$\gamma$ phase transition in malononitrile. An analogy between the smectic A $\leftrightarrow$ nematic phase transition observed in cyano-based liquid crystals and the $\beta$-$\gamma$ phase transition in malononitrile is made. Based on the description of the $\beta$-$\gamma$ phase transition, an assumption is made about the $\gamma$- and $\alpha$-phase stiffness constants of malononitrile.
Subject Area
Chemistry
Recommended Citation
Bonafede, Salvatore J, "Analysis of phase transitions in molecular crystals" (1993). ETD collection for University of Nebraska-Lincoln. AAI9415951.
https://digitalcommons.unl.edu/dissertations/AAI9415951