Raman spectral characterization of NH4H2PO4 single crystals: Effect of pH on microstructure

Single crystals of ammonium dihydrogen phosphate (NH₄H₂PO₄, ADP) were grown from aqueous solutions using “point seeds” method. The Raman spectra of the NH₄H₂PO₄ single crystals were recorded as a function of pH. The band intensities and frequencies were monitored to determine the diagnostic bands for the crystals grown from the solutions of different pH values of aqueous solution. In the Raman spectra of ADP crystal samples at different pH, the position and intensity of the peaks in the range 300–900 cm⁻¹ did not change; the peaks in the range 1400–1700 cm⁻¹ shifted their position, although they had the same intensity. This indicates that the structure of ${{(\text{PO}}_4)}^{3-}$ did not change with pH, whereas the structure of ${\mathrm{NH}}_4^{+}$ changed with pH. In the Raman study in the range 1700–4000 cm⁻¹, the primary discrimination occurred for the intensity of the peaks at 3050 cm⁻¹ and 3147 cm⁻¹, which is attributed to the vibration of O–H⋯O hydrogen bond, it can be inferred that pH slightly affected the O–H⋯O hydrogen bonding. This Raman spectral characterization may help to better understand the microstructural change under different conditions and optimize the growth and property of this type of material.