Composition and cation-ordering effects in quasi-one-dimensional BiSI–SbSI chalcohalides

We investigate the structural, electronic, elastic, and vibrational properties of BiSI, SbSI, and four ordered Bi 0.5 Sb 0.5 SI configurations using density functional theory. Structural optimizations with PBE+D3 and vdW-DF3-opt2 show that the nonlocal functional provides better overall agreement with the experimental lattice parameters of the end members. All six systems are indirect-gap semiconductors, while the mixed configurations do not simply interpolate between the BiSI and SbSI gaps. Spin–orbit coupling produces a strongly composition-dependent gap reduction, from 0.673 eV in Bi 4 S 4 I 4 to 0.205 eV in Sb 4 S 4 I 4 . The elastic tensors reveal a pronounced anisotropic response and show that nonlocal interactions redistribute the stiffness among deformation modes rather than producing uniform stiffening. In contrast, the four mixed configurations have very similar polycrystalline elastic moduli. Their lattice dynamics are more sensitive to cation ordering. Bi content therefore controls the relativistic electronic response more clearly, whereas Bi/Sb ordering leaves its strongest fingerprint in the low-frequency lattice dynamics.