To better understand the events visually observed on the thermal microscope, variable-temperature powder X-ray diffraction (VT-PXRD) and DSC of the three polymorphs were performed. Heating the sample bench of XRD showed transformation to a new polymorph IV at 200°C for all the three polymorphs such as I, II and III. This transient Form IV was characterized by its unique powder XRD lines and in all cases the product on cooling to room temperature was Form III, which is the nearest stable phase (Fig. S9). Thus, a heat–cool cycle exhibited transformation to polymorph III via a new phase IV. In order to observe possible phase changes below ambient temperature (−25°C to 30°C), DSC was carried out on the sample in the range −150°C to 200 °C. Form I showed an endotherm at 147°C and saw tooth-like endotherms (zigzag profile) at 170–185°C due to Form I → IV conversion (confirmed by VT-PXRD experiments, Fig. S9). PXRD of high-temperature Form IV obtained from different polymorphs I–III are compared in Fig. S10. On cooling the same material, Form IV converted to Form III at 0–10°C and on reheating Form III, a small endotherm at 150–160°C was observed, after which it again converted to high-temperature phase Form IV (Form I → IV ↔ III is a solid-to-solid phase transformation); melting occurred at 249°C (Figs. S11 and S12). In the case of Form II, an endotherm was observed at 170°C and upon further heating a second endotherm occurred at 183–185°C, which indicates Form II → IV conversion. Reheating of the same material showed a small endotherm at 150–160°C (Form III → IV) followed by melting at 249°C (Figs. S13 and S14). During heating of Form III, a sawtooth wave DSC profile was observed at 181–190°C, indicating the conversion of Form III to IV. Cooling of the same material showed transformation to Form III, and on reheating small endotherms were observed at 150–162°C (Form III → IV), followed by melting at 251°C (Figs. S15 and S16). All the above experiments were performed multiple times with identical results. We also performed competitive slurry experiments to establish that Form II is more stable (thermodynamically stable state) compared with the remaining two Forms I and III. DSC and VT-PXRD confirmed that Form IV is stable at high temperature (after 180°C) and Forms III and IV are reversible during cooling and heating (III ↔ IV). Thus, the solid-to-solid phase transitions on temperature-modulated powder XRD and DSC exhibit similar transformations under heat–cool conditions. Compounds B and C melt at 274°C and 254 °C, respectively, without any phase transformation (Fig. S17).