Abstract
We report on the experimental observation of the polymorphic transformation from the monoclinic α- to the orthorhombic β-phase of thin films of the molecular organic radical p-nitrophenyl nitronyl nitroxide (p-NPNN). This solid-solid phase transition to the thermodynamically most stable β-phase encompasses clearly differentiated changes in structure and morphology and may be inhibited if the thickness of the films of the initial α-phase lies below a certain critical value. In our case, the α-phase of p-NPNN (α-p-NPNN) has been stabilized as thin film obtained by thermal evaporation in high vacuum (thickness ⩽ 1 μm).