Crystal-Structure Prediction and Experimental Investigation of the Polymorphic Lanthanum Fluoride Selenides LaFSe and La<sub>2</sub>F<sub>4</sub>Se
Constantin Buyer, Hagen Grossholz, Sarah Wolf, Dejan Zagorac, Jelena Zagorac, J. Christian Schön, Thomas Schleid
Abstract
Lanthanum fluoride selenides (A-LaFSe, B-LaFSe and La2F4Se) have been synthesized through high-temperature experiments from an appropriate La/LaF3/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 P4/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 P63/mmc with a = 421.602(2) pm, c = 818.163(7) pm, and Z = 2 with the CeHSe-type structure; and La2F4Se in the trigonal space group R3̅m with a = 417.86(2) pm, c = 2326.78(9) pm, and Z = 3 in the Ce2F4Se-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.