Structural and electronic properties of $\alpha$-(BEDT-TTF)${}_2{\mathrm{I}}_3$, $\beta$-(BEDT-TTF)${}_2{\mathrm{I}}_3$, and $\kappa$-(BEDT-TTF)${}_2{X}_3$ ($X$ = I, F, Br, Cl) organic charge transfer salts

${\left(\mathrm{BEDT}\text{−}\mathrm{TFF}\right)}_2{\mathrm{I}}_3$ charge transfer salts are reported to show superconductivity and pressure-induced quasi-two-dimensional Dirac cones at the Fermi level. By performing state of the art ab initio calculations in the framework of density functional theory, we investigate the structural and electronic properties of the three structural phases $\alpha$, $\beta$, and $\kappa$. We furthermore report about the irreducible representations of the corresponding electronic band structures, symmetry of their crystal structure, and the origin of band crossings. Additionally, we discuss the chemically induced strain in $\kappa \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2{\mathrm{I}}_3$ achieved by replacing the iodine layer with other halogens: fluorine, bromine, and chlorine. In the case of $\kappa \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2{\mathrm{F}}_3$, we identify topologically protected crossings within the band structure. These crossings are forced to occur due to the nonsymmorphic nature of the crystal. The calculated electronic structures presented here are added to the organic materials database (OMDB).

Figure 1

Crystal structure for different structural phases of ${\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2{\mathrm{I}}_3$. (a) Molecular ordering for the $\alpha$, $\beta$, and $\kappa$ phases. (b) Layered structure of $\alpha \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2{\mathrm{I}}_3$ [49].

Figure 2

Band structure, irreducible representations, and density of states of $\alpha$-, $\beta$- and $\kappa \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2{\mathrm{I}}_3$. The Brillouin zones are plotted in units of the reciprocal lattice vectors and show the paths for the band structure calculations. The orange plane in panel (d) indicates the region for the band structure plot in Fig. 3. (a) $\alpha$-phase band structure and DOS. (b) $\beta$-phase band structure and DOS. (c) $\kappa$-phase band structure and DOS. (d) Triclinic Brillouin zone. (e) Monoclinic Brillouin zone.

Figure 3

Tilted linear crossing within the band structure of $\alpha \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2{\mathrm{I}}_3$ under a pressure of 1.76 GPa. The plot region within the Brillouin zone is shown in Fig. 2.

Figure 4

Band structure, irreducible representations, and density of states for $\kappa \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2{\mathrm{F}}_3$. Path $\overline{\mathrm{\Gamma }Y}$ is highlighted in pink.