Phase Relations between Temperature-Induced Phase Transition, Hysteresis and Bending Strain in an Elastically Flexible Molecular Crystal

Crystal Growth & Design, vol. 26, pp. 7364–7375

Abstract

Abstract Relations between structural properties and their variations due to phase transitions under variable thermodynamic conditions are of central interest in material science. More than a decade ago, the discovery of elastic bending in single crystals of caffeine cocrystal solvates opened up a new area of research in the mechanical properties of molecular crystals. Here, we report on a structural transition at T > 320 K that is accompanied by negative area expansion of the plane normal to the bending axis, and a nonlinear increase of intermolecular distances within π···π stacks and the unit cell volume. Upon applying heating–cooling cycles, the transition at Tc is found to be reversible; albeit, from 360 K ≤ T ≤ 380 K to T = 100 K, structurally irreversible states are observed that are characterized by hysteresis in the area of the planes and intermolecular distances. The phase transformation, as well as irreversibilities, are associated with significant crystal degradation, as suggested by increasing mosaicity as a function of T, which qualitatively indicates the evolution of defects in the crystals. During these various thermal processes, the acid–base dimers rotate as well as move. The ratio (η) between variation in rotations and distances reveals a similarity in its order in the high-temperature phase, as well as those in the irreversible structural states, to that calculated from reported deformations during mechanical bending at T = 100 K.

Authors 14

1. Somnath Dey Aachen

   RWTH Aachen University

   Affiliation as printed

   RWTH Aachen University , , ,

2. Hans Gildenast Aachen

   RWTH Aachen University

   Affiliation as printed

   RWTH Aachen University , , ,

3. S. Nandi

   Forschungszentrum Jülich

   Affiliation as printed

   Forschungszentrum Jülich GmbH , , ,

4. Branislav Viliam Hakala

   Forschungszentrum Jülich

   Affiliation as printed

   Forschungszentrum Jülich GmbH , , ,

5. Hend Shahed

   Forschungszentrum Jülich

   Affiliation as printed

   Forschungszentrum Jülich GmbH , , ,

6. Andrzej Grzechnik Aachen

   Forschungszentrum Jülich · RWTH Aachen University

   Affiliation as printed

   Forschungszentrum Jülich GmbH , , ,

   RWTH Aachen University , , ,

7. S. Ramakrishnan

   University of Bayreuth

   Affiliation as printed

   University of Bayreuth , , ,

8. Sander van Smaalen

   University of Bayreuth

   Affiliation as printed

   University of Bayreuth , , ,

9. Carsten Paulmann

   Universität Hamburg · Deutsches Elektronen-Synchrotron DESY

   Affiliation as printed

   DESY , , ,

   Universität Hamburg , , ,

10. Martin Tolkiehn

    Deutsches Elektronen-Synchrotron DESY

    Affiliation as printed

    DESY , , ,

11. Anagha Ghosh

    Central Glass and Ceramic Research Institute · Council of Scientific and Industrial Research

    Affiliation as printed

    CSIR-Central Glass and Ceramic Research Institute , 196, Raja S. C. Mullick Road, Jadavpur , ,

12. Swastik Mondal

    Central Glass and Ceramic Research Institute · Council of Scientific and Industrial Research · Academy of Scientific and Innovative Research

    Affiliation as printed

    Academy of Scientific and Innovative Research (AcSIR) , ,

    CSIR-Central Glass and Ceramic Research Institute , 196, Raja S. C. Mullick Road, Jadavpur , ,

13. Georg Roth Aachen

    RWTH Aachen University

    Affiliation as printed

    RWTH Aachen University , , ,

14. Lars Peters Aachen

    RWTH Aachen University

    Affiliation as printed

    RWTH Aachen University , , ,