Nuclear dynamics and phase polymorphism in solid formic acid

• Published in: Physical chemistry chemical physics : PCCP Vol. 19; no. 13; pp. 964 - 974
• Main Authors: Krzystyniak, Maciej, Dru bicki, Kacper, Romanelli, Giovanni, Gutmann, Matthias J, Rudi, Svemir, Imberti, Silvia, Fernandez-Alonso, Felix
• Format: Journal Article
• Language: English
• Published: England 29.03.2017
• Subjects: Condensed matter; Crystal structure; Dynamical systems; Formic acid; Hydrogen bonding; Loosening; Mathematical analysis; Polymorphism
• ISSN: 1463-9076; 1463-9084
• DOI: 10.1039/c7cp00997f

We apply a unique sequence of structural and dynamical neutron-scattering techniques, augmented with density-functional electronic-structure calculations, to establish the degree of polymorphism in an archetypal hydrogen-bonded system - crystalline formic acid. Using this combination of experimental and theoretical techniques, the hypothesis by Zelsmann on the coexistence of the β 1 and β 2 phases above 220 K is tested. Contrary to the postulated scenario of proton-transfer-driven phase coexistence, the emerging picture is one of a quantitatively different structural change over this temperature range, whereby the loosening of crystal packing promotes temperature-induced shearing of the hydrogen-bonded chains. The presented work, therefore, solves a fifty-year-old puzzle and provides a suitable framework for the use neutron-Compton-scattering techniques in the exploration of phase polymorphism in condensed matter. Neutron experiments and simulations solve the formic acid puzzle.