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.