Chemical perturbation and lattice instability in molecular crystals

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Abstract
A method of successive perturbation is used to investigate the nature of lattice instability associated with a chemical substitution. The p‐dihalobenzene series is studied by this method using the techniques of x‐ray diffraction and laser Raman spectroscopy. The Raman phonon spectra in the series p‐C6H4XY (X=Cl, Br and Y=Cl, Br, I) correlate well but a lattice instability exhibited by the breakdown in phonon correlation is encountered by substituting just one more iodine for X in p‐C6H4XI. A temperature dependence study is conducted to investigate the role of dynamic (motional) instability. The lattice structures obtained both by thermal perturbations (three different phases of p‐C6H4Cl2) and by chemical perturbations (various p‐dihalobenzenes) show five characteristic close contacts. A novel application is made of the principal component analysis using these close contacts and corresponding angles as variables to understand the nature of structural perturbations. The result shows that the principal component scores for thermal perturbations and chemical perturbations define two nearly orthogonal axes; this suggests that at least for this series the nature of these two types of perturbations is widely different.