Structural modulation induced by MIIIA metals in Ba3MQ4X (M = Al, Ga, In; Q = S, Se; X = Cl, Br): an experimental and computational analysis.

Four new chalcohalides Ba3AlQ4X (Q = S, Se; X = Cl, Br) have been discovered by a conventional high-temperature method. All of them crystallize in the Pnma space group. The structure is composed of zigzag XBa layers and isolated AlQ4 tetrahedra. Detailed structural comparisons show that the cation size of the MIIIA metals and variable coordination modes of the Ba2+ cations cooperatively influence the framework geometries of the title compounds. The first principles method was also used to investigate the electronic structures and optical properties. Their calculated birefringences range from 0.044 to 0.050 @1064 nm. Real space atom cutting analysis shows that the AlS4, GaS4, and InS4 tetrahedra make the main contribution to the birefringence.