Temperature Driven Phase Transformation in Zeolitic Imidazolate Framework

Metal organic frameworks (MOFs) are known to exhibit a variety of crystal structures under pressure, temperature, and gas adsorption. Desolvated zeolitic imidazole frameworks show temperature dependent phase transition. On cooling to 140 K, its volume contracts by 23\%, while its symmetry remains the same. Through first-principles calculations, we attempt to understand the dynamical properties of high and low temperature phases. We identified the soft acoustic phonon modes of high temperature phase that are responsible for the phase transitions. The transition is driven by the vibrational entropy. Our calculated transition temperature is 170 K which is quite close to the experimental value of 140 K.