Phase Stability of Monoclinic KDP Single Crystalline Fibers Obtained in Room-Temperature Solution

For a long time, it was believed that the monoclinic potassium dihydrogen phosphate (KDP) crystal could not grow directly in solution, unlike its deuterated isomer DKDP. This perception was overturned when the crystal was observed to crystallize in highly supersaturated aqueous solutions. Till now, the phase stability of the monoclinic KDP remains unknown. In this article, a comprehensive investigation is conducted on the phase stability and phase transition behavior of the monoclinic KDP crystal. The crystal transition in solution and the phase transformation to the tetragonal phase after grinding confirm the metastable nature of the monoclinic phase. Nevertheless, despite its metastability, the monoclinic crystal in the solid state maintains a structural stability in air for an extended duration of over 30 days and has no phase transition across a wide temperature range from −180 to 230 °C. These research findings enrich the KDP phase diagram and enable a deeper understanding of the nucleation, crystal growth, and thermodynamic properties of both the monoclinic and tetragonal KDP crystals.