Tuning crystal structure of potassium dihydrogen phosphate at ambient conditions

At ambient conditions, potassium dihydrogen phosphate (KDP) usually exists in tetragonal structure, and only transforms to monoclinic structure at a temperature around 503 K. In this study, stable monoclinic KDP is developed at room temperature using a mixture of graphene oxides and unsaturated KDP solution. Furthermore, a reversible phase transition between monoclinic and tetragonal structure is observed by tuning the distance between graphene oxide layers. The first-principles calculation suggests that the tetragonal structure is more energetically favorable when KDP is sandwiched by two graphitic surfaces with a large interlayer distance, whereas the monoclinic structure is more stable with a small interlayer distance. This is attributed to the interaction between ${\mathrm{K}}^{+}$ cations and \ensuremath{\pi} electrons in the graphitic surface, which distorts the local structural configurations. The increase of interaction strength with decreasing interlayer distance not only reduces the crystal-liquid interfacial free energy, which promotes the formation of the monoclinic structure in unsaturated solution, but also stimulates transformation of existing tetragonal KDP to monoclinic structure. The observations shed light on the nucleation mechanism for crystal structure that is usually infeasible and unstable at ambient conditions, and provide an avenue to tune crystal structures for a wide range of metal salts.