Physical Chemistry of Crystalline (K,NH₄)H₂PO₄ in Aqueous Solution: An in Situ Molecule Vibration Spectral Observation of the Early Formation Stage

The competing occupancy of cation position by NH₄⁺ and K⁺ during crystallization produces local distortions in structure, resulting in the difficulties in growing high-quality (K,NH₄)­H₂PO₄ single crystals with particular mixing concentration. Crystal properties such as structure, defect density, and purity always depend on the early formation stage of crystalline (K,NH₄)­H₂PO₄ molecule. Identification of the effect of K⁺/NH₄⁺ mole ratios in KH₂PO₄–NH₄H₂PO₄ aqueous solution on (K,NH₄)­H₂PO₄ molecule motion at the early stage of crystallization can provide the strategy to growing high-quality (K,NH₄)­H₂PO₄ crystals with particular mixing concentration. In situ molecule vibration spectroscopy was used to identify the early formation stage of crystalline (K,NH₄)­H₂PO₄ in KH₂PO₄–NH₄H₂PO₄ aqueous solution with various K⁺/NH₄⁺ mole ratios. (K,NH₄)­H₂PO₄ molecule motion was imaged via integrating the structural information on IR/Raman-active NH₄⁺, H₂PO₄^–, and hydrogen bonding infrastructures. K⁺<i>/</i>NH₄⁺ mole ratio in KH₂PO₄–NH₄H₂PO₄ aqueous solution determines the supersaturation in crystallization system as well as the competing incorporation of K⁺ and NH₄⁺ into the lattice. Both lower supersaturation and stronger competition between cations hinder the crystallization of (K,NH₄)­H₂PO₄, resulting in the remarkable spectral difference before and after the formation of crystalline (K,NH₄)­H₂PO₄. Our results demonstrate the concept of competed incorporation between different cations into the anionic framework from the molecular viewpoint