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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.

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We have studied transport properties (resistivity, Hall effect, thermopower, and thermal conductivity) of Bi2S3 and two new ternary bismuth sulfides, KBi6.33S10 and K2Bi8S13, to explore their potential for thermoelectric applications. While both Bi2S3 and K2Bi8S13 display an essentially metallic conduction, KBi6.33S10 has a distinctly semiconducting character. From the Hall measurements it follows that the dominant carriers in all three compounds are electrons and that the carrier density in K2Bi8S13 is almost 2 orders of magnitude larger than in Bi2S3 and KBi6.33S10 over the whole temperature range from 4 to 300 K. While the room temperature thermopowers of KBi6.33S10 and K2Bi8S13 are comparable (∼−100 μV/K), the thermopower of Bi2S3 is a factor of 3 larger. All three compounds are poor conductors of heat, in particular the thermal conductivity of KBi6.33S10 is some 25% lower than the thermal conductivity of Bi2Te3, the benchmark thermoelectric material. To make these materials useful thermoelectrics, ways must be found how to significantly increase their electrical conduction while preserving the reasonably large thermopower and low thermal conductivity.

The thickness-dependent electronic states and physical properties of two-dimensional materials suggest great potential applications in electronic and optoelectronic devices. However, the enhanced surface effect in ultra-thin materials might significantly influence the structural stability, as well as the device reliability. Here, we report a spontaneous phase transformation of gallium telluride (GaTe) that occurred when the bulk was exfoliated to a few layers. Transmission electron microscopy (TEM) results indicate a structural variation from a monoclinic to a hexagonal structure. Raman spectra suggest a critical thickness for the structural transformation. First-principle calculations and thermodynamic analysis show that the surface energy and the interlayer interaction compete to dominate structural stability in the thinning process. A two-stage transformation process from monoclinic (m) to tetragonal (T) and then from tetragonal to hexagonal (h) is proposed to understand the phase transformation. The results demonstrate the crucial role of interlayer interactions in the structural stability, which provides a phase engineering strategy for device applications.

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KBi 6.33 S 10 and K 2 Bi 8 S 13 were synthesized by the direct combination of K 2 S/Bi 2 S 3 at high temperature (>700°C). The reaction of K 2 S/3.3Bi 2 S 3 at 800°C revealed the presence of a new ternary sulfide KBi 6.33 S 10 (I, 92% yield). The shiny, silver polycrystalline material crystallizes in the orthorhombic space group Pnma (No. 62) with a ) 24.05(1) Å, b ) 4.100-(2) Å, c ) 19.44(1) Å, V ) 1917(3) Å 3 , Z ) 4, and d c ) 5.828 g/cm 3 . Data with F o 2 > 3σ(F o 2 ), 862; no. of variables 108, 2θ max 50°. The final R/R w ) 4.3/4.7%. The structure consists of blocks of Bi 2 Te 3 -and CdI 2 -type units that are connected to form a three-dimensional network with K + ions located in the channels that run along the b axis. The same reaction but with a different reactant ratio at 750°C gave the new ternary sulfide K 2 Bi 8 S 13 (II, 94% yield). This compound crystallizes in the monoclinic space group P2 1 /m (No. 11) with a ) 16.818(2) Å, b ) 4.074(5) Å, c ) 17.801(3) Å, ) 90.48(1)°, V ) 1220(2) Å 3 , Z ) 2, and d c ) 5.900 g/cm 3 . Data with F o 2 > 3σ(F o 2 ), 1924; no. of variables 131, 2θ max 50°. The final R/R w ) 7.3/8.2%. The structure of the shiny rodlike crystals is closely related to that of I. As in I, it also consists of Bi 2 Te 3 -and CdI 2 -type fragments that connect to form K + -filled channels. The two potassium atoms and one bismuth atom are disordered over three sites. Electrical conductivity on I show semiconducting behavior with 10 2 S/cm at 300 K. Compound II possesses an electrical conductivity of 10 2 S/cm at 300 K. The optical bandgaps of I and II (0.06-0.24 eV) were estimated by infrared diffuse reflectance measurements. Thermal analysis and thermal conductivity data for I and II are reported. The thermal conductivity of KBi 6.33 S 10 is found to be substantially lower than that of Bi 2 Te 3 . a The estimated standard deviations in the mean bond lengths and the mean bond angles are calculated by the equations σl ) {∑n(lnl) 2 /n(n -1)} 1/2 , where ln is the length (or angle) of the nth bond, l the mean length (or angle), and n the number of bonds.

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