During my two-week STSM at Dr. Mirta Rubčić’s lab at the University of Zagreb, I worked with Dr. Edi Topić on variable-temperature X-ray diffraction (VT-XRD) experiments of two benchmark compounds of the COST action—4’-hydroxyacetophenone (HAP) and 4-hydroxybenzaldehyde (HBA)—as well as several structurally related compounds. Each compound was heated from room temperature to melting, while collecting XRD diffractograms every 5 °C. In the cases were the previously performed DSC scans revealed different thermal behavior when crystallizing from the melt, the samples were melted and recrystallized in situ, using a capillary setup, collecting diffractograms both on heating and cooling.

The main goal of the STSM was to obtain experimental data on the temperature dependence of the structural parameters of the compounds, which can be used for the determination of the thermal expansion coefficients of these materials and are needed for the development of crystal structure prediction methods. The experiments could also provide a structural view on the thermal behavior, phase transitions and melt crystallization of the compounds. Moreover, by studying a series of structurally related molecules, insights into how the presence of different substituent groups impacts thermal behavior, and the formation and stability of polymorphs, could also be obtained.

Indeed, not only were the cell parameters determined as a function of temperature for all compounds, but the experiments also offer a greater understanding of the polymorphic behavior of HOP and HBP. VT-XRD confirmed the presence of a solid-solid phase transition in HOP, as the compound was heated to its melting point, which resulted in the formation of a new polymorph. In the case of HBP, previous differential scanning calorimetry (DSC) and thermomicroscopy experiments had already detected a solid-solid phase transition during cooling crystallization from the melt, accompanied by a thermosalient effect (where the crystals physically moved or jumped as they cooled). VT-XRD verified this transition and revealed that HBP crystallizes from the melt into a previously unknown polymorph, which then undergoes the thermosalient phase transition upon cooling.

Overall, the STSM grant provided me with a valuable opportunity to gain hands-on experience with a new technique (VT-XRD), helping me understand its strengths and limitations. It also allowed me to build connections with other researchers, sparked fresh ideas for new experiments, and paved the way for future collaborations.