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Observation of banded spherulite in a pure compound by rhythmic growth

Banded spherulitic growth of crystal is observed in some materials with spherically symmetric growth front and periodic radial variation of birefringence. This variation of birefringence in quasi two dimensional geometry produces concentric interference colour bands when viewed through crossed polarisers. In most materials, the banded spherulites are found to be formed by radially oriented periodically twisted fibrillar crystallites. Here, we report the formation of banded spherulites due to the rhythmic growth of concentric crystallite-rich and crystallite-poor bands for a pure compound consisting of strongly polar rod like molecules. The compound exhibits coexistence of untwisted fibrillar crystallites and an amorphous phase in its most stable solid state. On sufficient supercooling of the sample from its melting point, the banded spherulites are formed with a periodic variation of composition of untwisted radially aligned fibrillar crystallites and an amorphous solid phase. We have developed a time dependent Ginzburg-Landau model to account for the observed banded spherulitic growth.