Bi_2+2nO_2+2nCu_2−δSe_2+n–δX_δ (X = Cl, Br): A Three-Anion Homologous Series

Abstract

Both layered multiple-anion compounds and homologous series are of interest for their electronic properties, including the ability to tune the properties by changing the nature or number of the layers. Here we expand, using both computational and experimental techniques, a recently reported three-anion material, Bi4O4Cu1.7Se2.7Cl0.3, to the homologous series Bi2+2n O2+2n Cu2−δSe2+n–δXδ (X = Cl, Br), composed of parent blocks that are well-studied thermoelectric materials. All of the materials show exceptionally low thermal conductivity (0.2 W/mK and lower) parallel to the axis of pressing of the pellets, as well as narrow band gaps (as low as 0.28 eV). Changing the number of layers affects the band gap, thermal conductivity, carrier type, and presence of a phase transition. Furthermore, the way in which the different numbers of layers are accessed, by tuning the compensating Cu vacancy concentration and halide substitution, represents a novel route to homologous series. This homologous series shows tunable properties, and the route explored here could be used to build new homologous series out of known structural blocks.

Prediction of higher thermoelectric performance in BiOCuSe by weakening electron-polar optical phonon scattering

“…9,23 However, our band structure compares well with that calculated with the more expensive HSE + SOC functional after a rigid band shift of 0.44 eV. 23,29 The top of the valence band of BiOCuSe is composed of Se and Cu orbitals and the bottom of the conduction band is constituted of Bi orbitals. The valence band maximum (VBM) is located along the M-Γ direction.…”

“…A Hubbard U value of 4 eV was added to the Cu 3d orbitals, because the Cu 3d electrons play an important role in Cu based multinary semicondutors. 29,41 The phonon band structure and phonon density of states (DOS) were obtained by combining VASP with phonopy. 42 A 4 × 4 × 4 supercell (512 atoms) was built to calculate the phonon band structure.…”

“…The strained cells were optimized by using optB86b functional and the band edge energies were calculated by using PBE + U + SOC functional as the electronic structure is better described by the latter. 29 The deformation potential constants E 1 were calibrated by the Cu 3p core level energy at the Γ point.…”

Electrical and Thermal Transport Properties of n‐type Bi6Cu2Se4O6 (2BiCuSeO + 2Bi2O2Se)

“…As shown in Figure 1, the unit cell of Bi , and all peaks can be well indexed. [28] The impurity phases appeared when the doping content of Br and Cl increases to x = 1.0 and x = 0.8, respectively, indicating the solid solution limit of Cl and Br in Bi 6 Cu 2 Se 4 O 6 . structure plays an important role in the charge and phonon anisotropic transport in Bi 6 Cu 2 Se 4 O 6 in different directions, which will be discussed below.…”

“…[25] Interestingly, due to Se vacancies, [26] Bi 2 O 2 Se could exhibit n-type conductive transports. [27] To fully functionalize the features of BiCuSeO and Bi 2 O 2 Se, a new series of Bi 6 Cu 2 Se 4 O 6 oxyselenides are synthesized with the 1:1 ratio of BiCuSeO and Bi 2 O 2 Se through a simple solidstate reaction method that followed the suggestions by Rosseinsky et al [28,29] We expect to realize the low thermal conductivities and n-type conducting transports in Bi 6 Cu 2 Se 4 O 6 via succeeding from the advances of BiCuSeO and Bi 2 O 2 Se, respectively. Indeed, we found the low thermal conductivity in Bi 6 Cu 2 Se 4 O 6 due to the strong interlayer phonon scattering and the complex layered structures.…”