Lattice dynamics of bismuth tellurohalides

Lattice dynamics of two bismuth tellurohalides, BiTeI and BiTeCl, and the influence of spin-orbit coupling on their vibrational properties are investigated using first-principles calculations in the density functional perturbation formalism. We also report on the Raman study of these bismuth tellurohalides as well as BiTeBr. It is shown that the inclusion of spin-orbit interaction results in a sizable softening of the phonon spectra of BiTeI and BiTeCl. For both compounds a strong anisotropy in the dielectric tensor and in the Born effective charges is found. Predicted Raman active mode frequencies in BiTeI and BiTeCl are in good agreement with the experimental Raman data.

Figure 1

A triple layer of Te-Bi-I and two triple layers of Cl-Bi-Te, forming the unit cell of BiTeI and BiTeCl, respectively. In the middle part, the structure of Te/Br-Bi-Te/Br is shown. Projections of the crystal structures on the (11$\overline{2}$0) plane are displayed on the right panel.

Figure 2

Phonon dispersions of BiTeI and BiTeCl calculated with spin-orbit coupling at the optimized lattice parameters. The right panels show the phonon density of states (DOS). The coloured areas give the partial DOS of halogen atoms.

Figure 3

Phonon dispersions of BiTeI (along some high-symmetry directions of the BZ) and BiTeCl (in the $\Gamma \mathrm{A}$ symmetry line) calculated with (solid lines) and without (dashed lines) spin-orbit coupling included.

Figure 4

Experimental Raman spectra of BiTeI, BiTeBr, and BiTeCl. The spectra were measured in the geometry when the polarization of scattered light was parallel (VV, black solid line) and perpendicular (UV, red dotted line) to the polarization of the incident light.

Figure 5

Atomic displacements for two $A_1$ modes in BiTeI: at (a) 88 cm${}^{-1}$ and (b) 141 cm${}^{-1}$.

Figure 6

Atomic displacements for two $E$ modes in BiTeCl: at (a) 118 cm${}^{-1}$ and (b) 119 cm${}^{-1}$.