Large thermal anisotropy in monoclinic niobium trisulfide: a thermal expansion tensor study.

We present a method based on the Grüneisen formalism to calculate the thermal expansion coefficient (TEC) tensor that is applicable to any crystal system, where the number of phonon calculations associated with different deformations scales linearly with the number of lattice parameters. Compared to simple high-symmetry systems such as cubic or hexagonal systems, a proper consideration of low-symmetry systems such as monoclinic or triclinic crystals demands a clear distinction between the TEC tensor and the lattice-parameter TECs along the crystallographic direction. The latter is more complicated and it involves integrating the equations of motion for the primitive lattice vectors, with input from the TEC tensor.

First-principles study of the lattice dynamics of Sb2S3.

We present a lattice dynamics study of orthorhombic antimony sulphide (Sb2S3) obtained using density-functional calculations in conjunction with the supercell force-constant method. The effect of Born effective charges is taken into account using a mixed-space approach, resulting in the splitting of longitudinal and transverse optical (LO-TO) phonon branches near the zone center. Zone-center frequencies agree well with Raman scattering experiments.