Researchers using the power of the ChemMatCARS 15-ID-B,C,D beamline at the APS are investigating polymorphism — the ability of a compound to form different crystal structures — to better understand the deterministic role of crystal nucleation and growth.
One stone two birds: the β → α and β → γ transformations in glycine can occur concurrently in some single crystals in a non-topotactic manner. The nucleation and growth of the product phases are enhanced by moisture.
The metastable β polymorph of glycine exhibits a single-crystal-to-single-crystal transformation (SCSCT) to either the α or γ phase with retention of the crystal habit of the parent β phase. X-ray diffraction and optical microscopy reveal that of 51 single crystals of the β parent phase, 24 form single domain crystals of α or γ with a single orientation matrix, confirming an SCSCT. The remaining 27 β parent crystals transform to crystals with a few domains of either α and/or γ, each domain arising from a single nucleation event. In three cases the β → α and β → γ 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 β → α and β → γ 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 β 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% α-glycine was observed as the only product.
Qi Jiang, Alexander G. Shtukenberg, Michael D. Ward and Chunhua Hu, “Non-Topotactic Phase Transformations in Single Crystals of β-Glycine,” Crystal Growth and Design, Article ASAP, DOI: 10.1021/acs.cgd.5b00187, Published May 4, 2015.