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+/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.