Ju-Song Yu

Non-centrosymmetric Selenides AZn4In5Se12 (A=Rb, Cs): Synthesis, Characterization and Nonlinear Optical Properties

Two new non-centrosymmetric polar quaternary selenides, namely, RbZn4 In5 Se12 and CsZn4 In5 Se12 , have been synthesized and structurally characterized. They exhibit a 3D diamond-like framework (DLF) consisting of corner-shared MSe4 (M=Zn/In) tetrahedra, in which the A+ ions are located. Both compounds are thermally stable up to 1300 K and exhibit large transmittance in the infrared region (0.65-25 μm) with measured optical band gaps of 2.06 eV for RbZn4 In5 Se12 and 2.11 eV for CsZn4 In5 Se12 . Inspiringly, they exhibit a good balance between strong second harmonic generation (SHG) efficiency (3.9 and 3.5×AgGaS2 ) and high laser-induced damage thresholds (13.0×AgGaS2 ). Theoretical calculations based on density functional theory (DFT) methods confirm that such strong SHG responses originate from the 3D DLF structure.

Coexistence of Strong Second Harmonic Generation Response and Wide Band Gap in AZn4Ga5S12 (A=K, Rb, Cs) with 3D Diamond‐like Frameworks

Mid-infrared (MIR, 2-20 μm) second-order nonlinear optical (NLO) materials with outstanding performances are of great importance in laser science and technology. However, the enormous challenge to design and synthesize an excellent MIR NLO material lies in achieving simultaneously a strong second harmonic generation (SHG) response [dij >0.6 × AgGaS2 (AGS)] and wide band gap (Eg >3.5 eV). Herein three new MIR NLO materials, AZn4 Ga5 S12 (A=K, Rb, Cs) are reported, which crystallize in the KCd4 Ga5 S12 -type structure and adopt a 3D diamond-like framework (DLF) consisting of MS4 (M=Zn/Ga) tetrahedra; achieving the desired balance with strong powder SHG response (1.2-1.4 × AGS) and wide band gap (Eg ≈3.65 eV). Moreover, they also show large laser induced damage thresholds (LIDTs, 36 × AGS), a wide range of optical transparency (0.4-25 μm) and ultrahigh thermal stability (up to 1400 K). Upon analyzing the structure-property relationship of AXII4 XIII5 Q12 family, these 3D DLF structures can be used as a highly versatile and tunable platform for designing excellent MIR NLO materials.

AXII4XIII5Te12 (A = Rb, Cs; XII = Mn, Zn, Cd; XIII = Ga, In): quaternary semiconducting tellurides with very low thermal conductivities

The discovery of novel materials with very low thermal conductivity is paramount to improving the efficiency of thermoelectric devices. Here we present a series of quaternary semiconducting tellurides AXXTe12 (A = Rb, Cs; XII = Mn, Zn, Cd; XIII = Ga, In) with three-dimensional (3D) diamond-like frameworks (DLFs) and they exhibit a very low thermal conductivity (ca. 0.26-0.42 W m-1 K-1) around 800 K.

Ba6Li2CdSn4S16: lithium substitution simultaneously enhances band gap and SHG intensity

Herein, a new noncentrosymmetric (NCS) compound Ba6Li2CdSn4S16 (1) was discovered. As compared to its isostructural Ba6Ag2CdSn4S16 (2), compound 1 with Li substitution exhibited enlarged band gap (1: 3.02 eV vs.2: 2.70 eV) as well as enhanced second harmonic generation intensity (SHG, 1: 7.6 vs.2: 5.3 × AgGaS2 at 2.05 μm laser radiation). This is rare because the band gap of an NLO compound is usually inversely related to the SHG intensity. Both structures were chararcterized by single crystal X-ray diffraction techniques. The three-dimensional (3D) frameworks are constructed by corner-sharing SnS4 and (Li/Cd)S4 or (Ag/Cd)S4 tetrahedra with Ba2+ as counter cations. Density functional theory calculations (DFT) have confirmed that Li+ plays a significant role in the widening of the band gap and enhancement of the SHG response due to its strong ionicity. Because Li has small electronegativity and strong ionic bond nature, in comparison with that of 2, the top of the VB in 1 is driven down to lower energy regions to widen Eg, and the non-bonding ingredients of S-3p states are increased in 1 to produce stronger SHG intensity. The calculated d23 for 1 and 2 are 27.81 and 17.30 pm V−1 respectively, that agree with the experimental observations.

(Cs6Cl)6Cs3[Ga53Se96]: A Unique Long Period-Stacking Structure of Layers Made from Ga2Se6 Dimers via Cis or Trans Intralayer Linking

The new compound (Cs6Cl)6Cs3[Ga53Se96] with its own structure type has been discovered by high-temperature solid-state reactions. The compound features a unique long period-stacking structure of layers that are built by the commonly observed dimeric Ga2Se6 unit extending in cis or trans intralayer linking. Single-crystal X-ray diffraction analyses show the trigonal space group R3̅m (No. 166) and a = 11.990(5) Å, c = 50.012(4) Å, and V = 6226.5(6) Å(3). The UV-vis-near-IR spectrum reveals a wide band gap of 2.74 eV that agrees well with the electronic structure calculation.

Syntheses, structures, and thermoelectric properties of ternary tellurides: RECuTe2 (RE = Tb–Er)

In this work, four ternary rare-earth copper tellurides with a general formula RECuTe2 (RE = Tb, Dy, Ho, Er) were successfully synthesized by a conventional high-temperature solid-state reaction of the elements. These isostructural materials adopted a known TmCuTe2-structure type and contained a 2D honeycomb-like layer built up from CuTe4 tetrahedral building units along the c-axis, with RE3+ located in layers. Moreover, all of them show a first order structure phase-transition, from the low-temperature-phase (LTP, space group: Pm1) to the high-temperature-phase (HTP, space group: P), due to the Cu atoms re-distribution on the covalent CuTe4-based layer substructure. It is interesting to note the influence of the electronegativity of the constituent rare-earth metals on thermoelectric (TE) properties, e.g., the Seebeck coefficient (S) values decrease while the electrical conductivity (σ) and thermal conductivity (k) values increase with increasing electronegativity for a given RE. Consequently, TbCuTe2 showed the highest dimensionless figure-of-merit ZT value of 1.0 at 750 K in the present RECuTe2 systems, which is a promising TE candidate for medium temperature applications.