Revisiting NH4H2PO4: 1H MAS, CRAMPS, and Spin-Lattice Relaxation

Numerous nuclear magnetic resonance (NMR) investigations of the KH2PO4 family of hydrogen-bonded compounds that display ferroelectric behavior have reported results related to structure, molecular motion, and phase transitions. The current study looks at how high magnetic field and high-resolution solid-state techniques affect the measured NMR parameters for ammonium dihydrogen phosphate (NH4H2PO4). Under magic-angle spinning (MAS), the ammonium and acid protons do not achieve a common spin temperature as previously stated in the literature [i.e, the spin-lattice relaxation times (T-1) are different]. While the relaxation behavior in the static sample can be well-described with a single exponential at ambient temperature and below, the behavior becomes increasingly nonexponential as the sample is taken above ambient temperature. This behavior may arise from the motion of the ammonium moiety or from ionic conduction. The three principal values of the H-1 shielding tensors for the different species of protons in both NH4H2PO4 and KH2PO4 were measured with slow spinning H-1 combined rotational and multiple pulse spectroscopy experiments. The behavior of spinning sidebands under the multipulse sequence is briefly discussed.