Electronic structure and transport properties of TlInSe2 and Tl0·5Li0·5InSe2

Abstract

We report calculations of the electronic structure of thermoelectric ternary chalcogenide TlInSe 2 in the pressure range 0-30 GPa and the Li-substituted compound Tl 0.5 Li 0.5 InSe 2 using density functional theory. Moreover, with Boltzmann transport theory the electronic transport properties of these compounds are investigated at the optimal p-doping level for a maximized power factor. We follow two possible band engineering routes by applying pressure and elemental substitution with Li to investigate a possible enhancement of the electronic properties for thermoelectric applications.Our study employs several exchange-correlation functionals including the spin-orbit interaction as well as the B3LYP hybrid functional. The band gap in TlInSe 2 obtained by using the Tran-Blaha modified Becke-Johnson functional is in good agreement with experimental data. We find a direct band gap for TlInSe 2 at the M-point and a slightly larger energy gap at the Z-point. The spin-orbit (SO) splitting is extracted from the calculated electronic band structure. When applying pressure to TlInSe 2 the Seebeck coefficient strongly decreases and band crossing results in metallic properties above 20 GPa. In contrast to TlInSe 2 , an indirect band gap is found for Tl 0.5 Li 0.5 InSe 2 with the valence band maximum located at an off-symmetry point along the M-X direction and the conduction band minimum located at an off-symmetry point along the X-P direction. In contrast to TlInSe 2 at ambient pressure, taking the SO coupling into account for Tl 0.5 Li 0.5 InSe 2 and TlInSe 2 at 20 GPa is necessary as it markedly changes the transport properties. Optimally doped p-type TlInSe 2 at ambient pressure has the most favorable electronic band structure for thermoelectric applications superior to both, optimally doped p-type TlInSe 2 under pressure and optimally doped p-type Tl 0.5 Li 0.5 InSe 2 .

Structural, Vibrational, and Electronic Properties of 1D-TlInTe2 under High Pressure: A Combined Experimental and Theoretical Study

“…SOC plays a significant role during metallization for TlInSe 2 under pressure. 9 A possibility of the topological nontrivial phase has already been predicted for TlSe family. For instance, the first-principles calculations on monolayer TlSe reveals an occurrence of topological crystalline insulator state at ambient pressure, which transforms to a topological insulating state under tensile strain.…”

“…However, TlInSe 2 becomes (semi)metal with and without SOC around 20 and up to 30 GPa, respectively. SOC plays a significant role during metallization for TlInSe 2 under pressure …”

“…Soon after the discovery of non-trivial topological quantum states in chalcogen-based semiconductors with narrow band gaps and strong spin–orbit coupling (SOC), the two-dimensional (2D) layered semiconductors opened up a new avenue to scrutinize novel pressure-induced phenomena, such as quantum phase transitions, topological superconductors, charge density waves, structural phase transitions, Lifshitz transitions, and so forth. − In contrast to layered 2D materials, one-dimensional (1D) chain materials from the TlSe family such as TlInTe 2 , TlInSe 2 , TlGaTe 2 , and so forth are relatively under-explored from the perspective of high pressure research, , although they exhibit astonishing properties at ambient pressure. − For instance, TlInSe 2 exhibits exceptionally high thermoelectric properties, which is correlated to the formation of an incommensurate superlattice . TlInTe 2 exhibits a high figure of merit (1.78 for p-doped and 1.84 for n-doped at 300 K) with an intrinsic ultra-low lattice thermal conductivity (<0.5 W/mK in the temperature range 300–673 K) owing to the spatial fluctuation of the Tl 1+ cation inside the polyhedral framework of a sublattice formed by Tl 1+ and Te 2– ions. ,,− …”

Strategies and challenges of high-pressure methods applied to thermoelectric materials

“…146 In contrast to these results, a negative effect of applied hydrostatic pressure on the power factors of TlInSe 2 and Tl 0.5 Li 0.5 InSe 2 was predicted. 147…”

Investigations on the electronic, optical, thermoelectric and thermodynamic properties of quaternary coinage metal HgSeBr: a DFT study

“…Specifically, the conduction band minimum aligns with the high-symmetric point Γ for CuHgSeBr, whereas the valence band maximum is situated at a non-symmetry point, either closer to the Γ point or positioned between Γ and X points. The presence of non-symmetrical points in the band structure has been documented in prior studies concerning high-performance thermoelectric materials [50][51][52][53]. Since non-symmetrical points appear in our materials and structure, we aim to assess its thermoelectric efficiency.…”