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

Kazuki Komatsu1, Takanori Murakami1, Satoshi Nakano2

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Ammonium chloride (NH4Cl) phase transitions were studied under compression. X-ray diffraction revealed a first-order phase transition and clarified the antiferro-ordered structure of phase V, highlighting the role of hydrogen bonding.

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Area of Science:

  • Materials Science
  • Solid State Physics
  • Crystallography

Background:

  • Ammonium chloride (NH4Cl) exhibits complex phase transitions under varying pressure and temperature.
  • Understanding these transitions is crucial for predicting material behavior in different environments.

Purpose of the Study:

  • To investigate the compression behavior and phase transitions of NH4Cl.
  • To elucidate the structural characteristics of NH4Cl phases, particularly phase V.
  • To determine the role of hydrogen bonding in NH4Cl phase transitions.

Main Methods:

  • X-ray diffraction was employed to study NH4Cl under compression.
  • Low-temperature X-ray diffraction measurements were conducted at ambient pressure.

Main Results:

  • A volume anomaly between 0.7-1.0 GPa confirmed a first-order phase II-IV transition at 298 K.
  • Hysteresis observed in low-temperature measurements suggests order-disorder differences in NH4+ ion orientation.
  • The crystal structure of phase V was identified as antiferro-ordered within a tetragonal system.

Conclusions:

  • The phase II-IV transition in NH4Cl is confirmed as first-order.
  • Hydrogen bonding is critical for the varied N-Cl distances observed in the antiferro-ordered phase V structure.