Crystal-Structure Prediction and Experimental Investigation of the Polymorphic Lanthanum Fluoride Selenides LaFSe and La₂F₄Se

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Lanthanum fluoride selenides (A-LaFSe, B-LaFSe and La 2 F 4 Se) have been synthesized through high-temperature experiments from an appropriate La/LaF 3 /Se mixture and characterized using single-crystal as well as powder X-ray diffractometry and UV/Vis diffuse reflectance spectroscopy. A-type LaFSe crystallizes in the tetragonal space group P 4/ nmm with a = 413.79(3) pm, c = 715.24(5) pm, and Z = 2 with the PbFCl-type structure; B-type LaFSe in the hexagonal space group P 6 3 / mmc with a = 421.602(2) pm, c = 818.163(7) pm, and Z = 2 with the CeHSe-type structure; and La 2 F 4 Se in the trigonal space group R 3̅ m with a = 417.86(2) pm, c = 2326.78(9) pm, and Z = 3 in the Ce 2 F 4 Se-type structure, respectively, in agreement with the earlier work. The experiments are complemented by crystal-structure predictions for LaFSe, which were performed using global optimization with empirical potentials and ab initio energy local minimizations. The results of the calculations concur with the experimentally observed structures and predict additional, so far unknown LaFSe polymorphs. The electronic properties were investigated both experimentally and theoretically, demonstrating the possibilities for band gap engineering in LaFSe.