Structural phase transitions: refining distortion-mode amplitudes instead of atomic coordinates

Abstract

Structure Determination from Powder Data (SDPD) is rapidly becoming a feasible alternative when suitable single crystals for structure determination cannot be grown, even for flexible organic molecules. We have been working on SDPD of flexible organic molecules from laboratory X-ray data using simulated annealing as implemented in DASH for several years now. Recently, while working on three molecular Ca 2+ salts, we suspected, based on the unit-cell volumes, that the crystal structures were hydrates. Although far from unexpected for molecular Ca 2+ salts, and although the presence of water molecules could readily be deduced, the exact number of water molecules (two or three) could not a priori be established due to the small size of a water molecule with respect to the organic fragments. Moreover, because the molecules were flexible and due to the additional degrees of freedom of the Ca 2+ counter ion, structure solution attempts with the maximum number of waters (three) were hampered by the increase in the number of degrees of freedom due to the water molecules. Therefore, the crystal structures were solved in an iterative approach using a combination of simulated annealing in DASH [1] to determine the positions of the molecular ions followed by multiple partial Rietveld refinements in TOPAS [2] to locate the missing water molecules one by one; we note that this is common practice in structure determinations from singlecrystal data. The partial Rietveld refinements progressed smoothly, yielding chemically sensible Ca 2+ coordinations; the final Rietveld refinements showed excellent fits.