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Related Experiment Videos

Highly compressed ammonia forms an ionic crystal.

Chris J Pickard1, R J Needs

  • 1Scottish Universities Physics Alliance, School of Physics and Astronomy, University of St Andrews, St Andrews, KY16 9SS, UK. cjp10@st-andrews.ac.uk

Nature Materials
|August 30, 2008
PubMed
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This summary is machine-generated.

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Under high pressure, ammonia (NH3) transforms into novel ionic crystalline phases composed of ammonium (NH4+) and amide (NH2-) ions. At extreme pressures, it reverts to hydrogen-bonded structures.

Area of Science:

  • Materials Science
  • Computational Chemistry
  • Planetary Science

Background:

  • Ammonia (NH3) is a vital industrial compound and a fundamental hydrogen-bonded system.
  • Understanding ammonia's properties under pressure is crucial for planetary physics.
  • Existing knowledge focuses on hydrogen-bonded solid phases of ammonia.

Purpose of the Study:

  • To computationally predict new high-pressure crystalline phases of ammonia.
  • To investigate the structural transitions of ammonia under extreme compression.
  • To explore the fundamental properties of compressed ammonia.

Main Methods:

  • First-principles density-functional-theory calculations were employed.
  • A computational search was conducted to identify potential crystalline phases.

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  • Thermodynamic stability of predicted phases was assessed over a range of pressures.

    • Main Results:

    • Prediction of new high-pressure ionic crystalline phases of ammonia.
    • These phases consist of alternating layers of ammonium (NH4+) and amide (NH2-) ions.
    • Ionic bonding is predicted to be stable within experimentally accessible pressure ranges.
    • At extreme pressures, ammonia is predicted to return to hydrogen-bonded neutral NH3 structures.

    Conclusions:

    • Ammonia undergoes a transition to ionic phases (NH4+ and NH2-) under high pressure.
    • This transition is driven by the formation of stable ions and volume reduction.
    • Under extreme compression, ammonia reverts to neutral hydrogen-bonded structures.