Growth and morphological control of metastable KDP crystals using defect-free seeds

Many functional crystal materials with significant practical value belong to metastable phases. However, the controllable growth of metastable crystals, compared with their thermodynamically stable crystalline phases, faces substantial technical bottlenecks, primarily attributed to their tendency to transform into their stable phases. Herein, we demonstrate that defect-free microscale seeds of a metastable phase, which possess smooth surfaces and high intrinsic two-dimensional (2D) nucleation barriers, can effectively suppress undesirable phase transformations, thereby realizing controlled growth and morphological regulation of the metastable phase. We employ a solution-seeding method to precisely determine the solubility of the metastable monoclinic phase of potassium dihydrogen phosphate (KH₂PO₄, KDP) and the critical supersaturation for initiating growth on its distinct crystal faces. This allows for the clear delineation of the growth regions for the stable tetragonal and metastable monoclinic phases of KDP. By controlling supersaturation and growth kinetics within the stable zone of the tetragonal phase, we further achieve the growth of the monoclinic phase with precise morphological control, producing one-dimensional (1D) single crystal fibers (SCFs) and three-dimensional (3D) rod-shaped crystals. This study provides a methodology for investigating the growth and morphological control of metastable crystalline phases and is expected to strongly support the exploration and development of new metastable-phase functional crystalline materials for advanced device applications.