X-ray diffraction study of a Nd3Ga5SiO14 crystal at 295 and 90 k and the structural basis for chirality

An accurate X-ray diffraction study of a Nd₃Ga₅SiO₁₄ crystal (sp. gr. P321, Z = 1) has been performed using repeated X-ray diffraction data sets collected on a diffractometer equipped with a CCD area detector at 295 and 90 K. The asymmetry of the disorder in the atomic positions in Nd₃Ga₅SiO₁₄ is described in two alternative ways: in terms of anharmonic atomic displacements (at 295 K R/wR = 0.80/0.92%; at 90 K R/wR = 0.82/0.76%) and using a split model (at 295 K R/wR = 0.85/0.97%; at 90 K R/wR = 0.80/0.74%). An analysis of the probability density function of finding an atom at a particular point in space shows that most of the atoms in the unit cell are asymmetrically or highly anisotropically disordered in the vicinity of their sites. Electron density regions for cations and anions that form pseudohelices were found. Such regions mimic a screw axis of symmetry and are responsible for the chirality of the structure.