The ferroelastic phase transition in hydrogen cyanide studied by density functional theory

Area of Science:

• Condensed Matter Physics
• Materials Science
• Computational Chemistry

Background:

• Hydrogen cyanide (HCN) exhibits complex phase behavior with temperature.
• Understanding its crystal structures and lattice dynamics is crucial for predicting material properties.

Purpose of the Study:

• To calculate the crystal structures and lattice dynamics of tetragonal and orthorhombic HCN phases.
• To investigate the mechanism of the ferroelastic phase transition.
• To identify the role of phonon modes in thermal expansion and phase transition driving forces.

Main Methods:

• Density functional theory (DFT) calculations were employed.
• Negative pressure was used as a proxy for elevated temperatures.
• Phonon spectrum analysis was performed.

Main Results:

• The ferroelastic phase transition in HCN involves the softening of a transverse acoustic mode.
• The phase transition was confirmed to be discontinuous.
• Acoustic modes were identified as responsible for significant thermal expansion and the thermodynamic driving force for the transition.

Conclusions:

• Lattice dynamics calculations provide insights into the phase transition mechanisms in molecular crystals like HCN.
• Acoustic mode behavior is key to understanding thermal expansion and phase stability.