Structural Crystallization Processes Research Articles

Local Structural Features Elucidate Crystallization of Complex Structures.

Complex crystal structures are composed of multiple local environments, and how this type of order emerges spontaneously during crystal growth has yet to be fully understood. We study crystal growth across various structures and along different crystallization pathways, using self-assembly simulations of identical particles that interact via multiwell isotropic pair potentials. We apply an unsupervised machine learning method to features from bond-orientational order metrics to identify different local motifs present during a given structure's crystallization process. In this manner, we distinguish different crystallographic sites in highly complex structures. Tailoring this order parameter to structures of varying complexity and coordination number, we study the emergence of local order along a multistep crystal growth pathway─from a low-density fluid to a high-density, supercooled amorphous liquid droplet and to a bulk crystal. We find a consistent under-coordination of the liquid relative to the average coordination number in the bulk crystal. We use our order parameter to analyze the geometrically frustrated growth of a Frank-Kasper phase and discover how structural defects compete with the formation of crystallographic sites that are more high-coordinated than the liquid environments. The method presented here for classifying order on a particle-by-particle level has broad applicability to future studies of structural self-assembly and crystal growth, and they can aid in the design of building blocks and for targeting pathways of formation of soft-matter structures.

First principles calculations on the origin of violet-blue and green light photoluminescence emission in SrZrO3 and SrTiO3 perovskites

The photoluminescence (PL) emission from SrZrO3 (SZ) and SrTiO3 (ST) crystalline, quasi-crystalline, and quasi-amorphous samples, prepared by the polymeric precursor method, was examined by ab initio quantum mechanical calculations. It was used in the modeling the structural model consisting of one pyramidal TiO5 or ZrO5 unit piled upon the TiO6 or ZrO6, which are representative of disordered structures of quasi-crystalline structures such as ST and SZ. The structural crystallization process was monitored by X-ray diffraction. In quasi-crystalline powders, the photoluminescence (PL) in the visible region showed different peak positions and intensities in SZ and ST. The PL emission was linked to distinct distortions in perovskite lattices and the emission of two colors—violet-blue in SZ and green in ST—was also examined in the light of favorable structural and electronic conditions.