Electrostatic potential for O-H-O in tetragonal ammonium dihydrogenphosphate

Area of Science:

• Crystallography
• Solid-state chemistry
• Hydrogen bonding

Background:

• Ammonium dihydrogen phosphate (NH4H2PO4) is a widely studied crystalline material.
• Understanding its hydrogen bonding network is crucial for predicting its physical properties.

Purpose of the Study:

• To elucidate the precise crystal structure of NH4H2PO4.
• To characterize the hydrogen bonding environment and proton distribution within the crystal lattice.

Main Methods:

• Single-crystal X-ray diffraction at room temperature (298 K).
• Analysis of crystallographic data including lattice parameters, atomic positions, and thermal parameters.
• Refinement of the crystal structure using 99 independent reflections.

Main Results:

• Determined the tetragonal crystal structure (space group I42d) with lattice parameters a = 7.500(1) Å and c = 7.550(2) Å.
• Identified an O-H-O hydrogen bond system with an O...O distance of 2.493(3) Å.
• Described the hydrogen atom occupying a single site with anisotropic thermal vibrations, O...H distance of 1.31(9) Å, and O-H-O angle of 145(8)°.

Conclusions:

• The proton in NH4H2PO4 appears to reside in a single potential well site with significant thermal motion.
• The possibility of a double-peak proton distribution, similar to isomorphous compounds, cannot be excluded based on current data.
• Further high-resolution neutron diffraction studies may be necessary to definitively resolve the proton's probability distribution.