Hidden degrees of freedom in aperiodic materials

Area of Science:

• Crystallography
• Materials Science
• Solid-State Chemistry

Background:

• Many crystalline materials feature incommensurate sublattices described within a superspace framework.
• Aperiodic crystals, including bioorganic materials, exhibit complex arrangements beyond three dimensions.

Purpose of the Study:

• To directly observe superspace symmetry breaking during a phase transition in an incommensurate host-guest system.
• To investigate the role of higher dimensionality in driving phase transitions in aperiodic crystals.

Main Methods:

• Neutron diffraction was employed to study the solid-solid phase transition.
• The channel inclusion compound of nonadecane/urea was used as the model system.

Main Results:

• Direct observation of superspace symmetry breaking was achieved.
• A phase transition resulted in unit cell doubling, affecting the modulation of one substructure by another.
• A second phase transition was identified, also governed by superspace dimensionality.

Conclusions:

• The study demonstrates nature's ability to access a greater number of phases in aperiodic crystals.
• Superspace symmetry breaking provides an unanticipated pathway for the rearrangement of degrees of freedom.
• Higher dimensionality in superspace plays a crucial role in controlling phase transitions.