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Orientational Transition in a Liquid Crystal Triggered by the Thermodynamic Growth of Interfacial Wetting Sheets

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Nuclear dynamics and phase polymorphism in solid formic acid.

Maciej Krzystyniak¹, Kacper Drużbicki², Giovanni Romanelli³

¹ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0QX, UK. matthew.krzystyniak@stfc.ac.uk and School of Science and Technology, Nottingham Trent University, Clifton Campus, Nottingham, NG11 8NS, UK.

Physical Chemistry Chemical Physics : PCCP

|March 18, 2017

Summary

This summary is machine-generated.

This study investigates crystalline formic acid polymorphism using neutron scattering and electronic structure calculations. It reveals temperature-induced shearing, not proton transfer, drives phase changes above 220 K, solving a long-standing puzzle.

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Area of Science:

Condensed matter physics
Materials science
Chemical physics

Background:

Polymorphism in hydrogen-bonded systems is crucial for material properties.
Crystalline formic acid is an archetypal system for studying phase transitions.
Previous hypotheses suggested proton-transfer-driven phase coexistence.

Purpose of the Study:

To investigate the degree of polymorphism in crystalline formic acid.
To test Zelsmann's hypothesis on the coexistence of β1 and β2 phases.
To elucidate the mechanism of structural changes above 220 K.

Main Methods:

Combined structural and dynamical neutron-scattering techniques.
Density-functional electronic-structure calculations.
Neutron-Compton-scattering for exploring phase polymorphism.

Main Results:

Established the degree of polymorphism in crystalline formic acid.
Contradicted the proton-transfer-driven phase coexistence hypothesis.
Observed temperature-induced shearing of hydrogen-bonded chains due to loosened crystal packing.

Conclusions:

The structural change above 220 K involves crystal packing loosening and chain shearing.
Solves a fifty-year-old puzzle regarding formic acid polymorphism.
Demonstrates the utility of neutron-Compton-scattering for phase polymorphism studies.