Phase Transitions of NH₄Cl under Low Temperature or High Pressure Observed by Sequential Powder X-ray Diffraction

The compression behavior and phase transitions of NH₄Cl were investigated by using X-ray diffraction. A clear volume anomaly at 0.7-1.0 GPa confirmed the first-order nature of the phase II-IV transition at 298 K. Low-temperature X-ray diffraction measurements at ambient pressure revealed a slight hysteresis in this transition, suggesting differences in the degree of order of the orientation of the NH₄⁺ ion in the cooling and heating processes. We first clarified that the crystal structure of phase V has antiferro-ordered orientation with a tetragonal system, and the structural analysis of phase V indicated that hydrogen bonding plays a key role in the alternately varied N-Cl distances.