Mixed matlokite hosts of composition BaFBrxI1–x(0x1) (pure and doped with Sm2+, Eu2+) were studied with x-ray crystallography, luminescence, Raman, and electron paramagnetic resonance (EPR) spectroscopy. Results are presented for BaFBr0.5I0.5 which demonstrate that a ferrielectric domain structure is formed due to the fact that the heavy halogen ions form separate sublattices with randomly distributed domain walls. The space group of a domain is P4 mm (No. 99). The EPR data from Eu2+ allowed to determine the volume fraction of domains.