We report evidence of grown-in inhomogeneous strain affecting hexa methylene tetramine ( HMT ) crystals grown from ethanol-water solution. Irrespective of the excellent optical quality displayed by the samples during growth, as evaluated in-situ by optical transmission methods, azimuth-retardation maps of the specimens, tracked by an ad-hoc developed polarimetric technique, show that the crystals are affected by strain-induced inhomogeneous birefringence. This feature may provide a rationale to the erratic estimates of the electro-optical coefficient of HMT reported in literature. We conjecture that this birefringence may originate jointly from the thermo-elastic effect and the softness of the material, and then that it may be intrinsic to hydrogen- and Van der Waals- bonded crystals, even if grown at low temperatures. Were this true, it could justify the inhomogeneous strain indirectly observed in a variety of organic compounds.

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