High Thermopower and Low Thermal Conductivity in Semiconducting Ternary K−Bi−Se Compounds. Synthesis and Properties of β-K₂Bi₈Se₁₃ and K_2.5Bi_8.5Se₁₄ and Their Sb Analogues

Abstract

were synthesized by a molten flux method. The black needles of compound I were formed at 600°C and crystallized in the monoclinic P2 1 /m space group (No. 11) with a ) 17.492(3) Å, b ) 4.205(1) Å, c ) 18.461(4) Å, ) 90.49(2)°. The final R/R w ) 6.7/5.7%. Compound II is isostructural to I. Both I and II are isostructural with K 2 Bi 8 S 13 which is composed of NaCl-, Bi 2 Te 3 -, and CdI 2 -type units connecting to form K + -filled channels. The thin black needles of III and IV obtained at 530°C crystallize in the same space group P2 1 /m with a ) 17.534 (4) Å, b ) 4.206(1) Å, c ) 21.387(5) Å, ) 109.65(2)°and a ) 17.265(3) Å, b ) 4.0801(9) Å, c ) 21.280(3) Å, ) 109.31 (1)°, respectively. The final R/R w ) 6.3/8.3% and 5.1/3.6%. Compounds III and IV are isostructural and potassium and bismuth/antimony atoms are disordered over two crystallographic sites. The structure type is very closely related to that of I. Electrical conductivity and thermopower measurements show semiconductor behavior with ∼250 S/cm and ∼-200 µV/K for a single crystal of I and ∼150 S/cm and ∼-100 µV/K for a polycrystalline ingot of III at room temperature. The effect of vaccum annealing on these compounds is explored. The optical bandgaps of all compounds were determined to be 0.59, 0.78, 0.56, and 0.82 eV, respectively. The thermal conductivities of melt-grown polycrystalline ingots of I and III are reported.

Spin–orbit coupling and weak antilocalization in the thermoelectric material $\beta $ -K2Bi8Se13

“…However, its resistivity ratio between 300K and 2K, i.e.(300K)/(2K), is as small as 2.8, suggesting bad metallic behavior. This is consistent with the result reported previously by Chung et al [12]. As noted above, strong SOC is expected in -K 2 Bi 8 Se 13 owing to the presence of heavy Bi and Se atoms.…”

“…As seen in Fig. 2, -K2Bi8Se13 is metallic despite its relatively large band gap ~0.59 eV [12], which may be due to non-stoichiometric self-doping effects. However, its resistivity ratio between 300K and 2K, i.e.(300K)/(2K), is as small as 2.8, suggesting bad metallic behavior.…”

“…The realization of TIs requires a gap with inverted band structure at a timereversal invariant momentum point in Brillouin zone [22]. -K2Bi8Se13 is a narrow gap semiconductor with band gap ~0.59 eV [12] which is larger than those of TIs [23][24][25][26][27][28][29]. The bad metallicity seen in temperature dependence of resistivity (Fig.…”

“…Although our transport studies could not provide any evidences for topological surface states, the strong SOC revealed in our MR measurements may play an important role in determining thermoelectric properties of - respectively. Due to large S and small , -K2Bi8Se13 exhibits relatively large ZT [12]. It has been suggested that the complex unit cell with weakly bonded K + ion and mixed occupancy of K/Bi strongly scatter phonons and thereby reduce phonon conductivity of -K2Bi8Se13 [36].…”

LiBi3S5—A lithium bismuth sulfide with strong cation disorder

“…This class of compounds has proven to be remarkably large and has contributed many complex compositions and structures favorable for high thermoelectric performance [3], e.g., K 2.5 Bi 8.5 Se 14 [4], β-K 2 Bi 8 Se 13 [4], A 2 Bi 8 Se 13 (A = Rb, Cs) [5], and CsBi 4 Te 6 [6]. In comparison to selenides and tellurides, most of the alkali-metal bismuth sulfides exhibit wide energy band gaps and strong ionic interactions between the alkali-metal ions and the [Bi x S y ] z-framework, e.g., in β-and γ-CsBiS 2 [7], γ-RbBi 3 S 5 [8], and KBi 3 S 5 [9].…”