Non-Topotactic Phase Transformations in Single Crystals of β-Glycine

The metastable beta polymorph of glycine exhibits a single-crystal-to-single-crystal transformation (SCSCT) to either the alpha or gamma phase with retention of the crystal habit of the parent beta phase. X-ray diffraction and optical microscopy reveal that of 51 single crystals of the beta parent phase, 24 form single domain crystals of alpha or gamma with a single orientation matrix, confirming an SCSCT. The remaining 27 beta parent crystals transform to crystals with a few domains of either alpha and/or gamma, each domain arising from a single nucleation event. In three cases the beta -> alpha and beta -> gamma transformations occurred within the same glycine single crystal. In all cases, regardless of the number of domains, the transformation occurred with retention of the original habit of the parent phase. Both beta -> alpha and beta -> gamma transformations proceed in a non-topotactic manner, as evident from the random orientations of the daughter phases in the crystallographic reference frame of the parent beta phase. The transformation rates, as measured by advancement of the growth front with polarized optical microscopy, vary from crystal to crystal as well as for different regions within the same crystal. Moreover, the transformation rate increases substantially with relative humidity, and at a relative humidity of 90% alpha-glycine was observed as the only product.