Study on attribution of laser Raman spectroscopy for hopeite crystal films

Abstract

Laser Raman spectroscopy was applied to clarify the chemical structure of hopeite crystal films. Orthophosphate [PO ³⁻₄ ], which is a regular tetrahedron, has four basic vibration modes, + ν₁, ν₂, ν₃ + of which ν₁, and ν₃ are observed at 800 to 1300 cm⁻¹. Here, the main peak corresponds to ν₁ and the other peaks correspond to ν₃ untied and split. A reference sample of 85% H₃PO₄ showed two peaks in the same region, the main peak corresponding to ν₁ and the sub-peak corresponding to ν₃ degenerated. It was found that a basic vibration mode of ν₃ appears at 1150 cm⁻¹. Raman spectra were observed for hopeite dissolved in HCL solution. Three peaks were found in the region, but the spectral pattern was quite different from that of crystalline hopeite, and was similar to that of H₃PO₄ aqueous solution. The peak intensity ratio of I ₁₀₇₅/I ₆₉₀ differed between liquid-state hopeite and H₃PO₄ aqueous solution, but the band frequencies of the three peaks were consistent with each other. It was confirmed that the three peaks correspond to the P(OH)₃ and PO stretching vibrations of [H₃PO₄] and the PO₂ stretching vibration of [H₂PO ⁻₄ ] formed by the dissociation of H₃PO₄. The PO₂ stretching vibration observed at 1075 cm⁻¹ depends on the dissociation state of H₃PO₄.