High-pressure-induced structural changes in molecular crystals preserving the space group symmetry: anisotropic distortion/isosymmetric polymorphism

Pressure-induced structural changes in molecular crystals that are not accompanied by a change in space group symmetry are considered. Anisotropic structural strain of the same polymorph and isosymmetric polymorphic transition are two possibilities which are illustrated in the examples of Co(III)nitropentaammine chloride, the monoclinic and ortho rhombic polymorphs of paracetamol, the α- and γ-polymorphs of glycine, benzoquinone (anisotropic structural strain) and sodium oxalate (isosymmetric polymorphic transition). The experiments described were carried out in diamond anvil cells, and the structural changes were monitored by X-ray powder and single crystal X-ray diffraction, and IR and Raman spectroscopy.