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Copper/silver-containing chalcogenides have recently attracted significant interest for their promising thermoelectric applications. In this article, we report the syntheses of five new layered quaternary Cu/Ag-containing chalcogenides: SrScCuSe3, SrScCuTe3, BaScCuSe3, BaScCuTe3, and BaScAgTe3. The single-crystal X-ray diffraction studies show that all the Cu-containing compounds are isostructural and crystallize in the orthorhombic crystal system in the Cmcm space group. Interestingly, the crystal structure of isoelectronic BaScAgTe3 does not adopt the same structure type as observed for AkScCuQ3 (Ak = Sr/Ba; Q = Se/Te). Instead, it crystallizes in the primitive orthorhombic Pnma space group. The crystal structures of all these compounds are two-dimensional consisting of 2∞[ScMQ3]2− layers (M = Cu/Ag) separated by Ak2+ cations. The building blocks of these structures are distorted tetrahedral MQ4 and octahedral ScQ6 units. The arrangement and sharing of the MQ4 and ScQ6 units are slightly different for the two structure types, AkScCuQ3 (Ak = Sr and Ba; Q = Se and Te) and BaScAgTe3. Resistivity study confirms the metallic behavior for BaScCuTe3. The positive sign of thermopower values suggests holes as the charge carriers. The value of the thermoelectric figure of merit (zT) for polycrystalline BaScCuTe3 was found to be enhanced on increasing the temperature with a maximum zT value of 0.34 at 779 K. In addition, we have also studied the structural and electronic properties of BaScCuTe3 and BaScAgTe3 within the framework of density functional theory (DFT).

Three new isostructural quaternary tellurides, Ba2Ln1−xMn2Te5 (Ln = Pr, Gd, and Yb), have been synthesized by the molten-flux method at 1273 K. The single-crystal X-ray diffraction studies at 298(2) K showed that Ba2Ln1−xMn2Te5 crystallize in the space group –C2/m of the monoclinic crystal system. There are six unique crystallographic sites in this structure's asymmetric unit: one Ba site, one Ln site, one Mn site, and three Te sites. The Ln site in the Ba2Ln1−xMn2Te5 structure is partially filled, which leaves about one-third of the Ln sites vacant (□) for Pr and Gd compounds. These structures do not contain any homoatomic or metallic bonding and can be charge-balanced as (Ba2+)2(Gd/Pr3+)2/3(Mn2+)2(Te2−)5. The refined composition for the Yb compound is Ba2Yb0.74(1)Mn2Te5 and can be charge-balanced with a mixed valence state of Yb2+/Yb3+. The crystal structures of Ba2Ln1−xMn2Te5 consist of complex layers of 2∞[Ln1−xMn2Te5]4− stacked along the [100] direction, with Ba2+ cations separating these layers. The Ln atoms are bound to six Te atoms that form a distorted octahedral geometry around the central Ln atom. Each Mn atom in this structure is coordinated to four Te atoms in a distorted tetrahedral fashion. These LnTe6 and MnTe4 units are the main building blocks of the Ba2Ln1−xMn2Te5 structure. The optical absorption study performed on a polycrystalline Ba2Gd2/3Mn2Te5 sample reveals a direct bandgap of 1.06(2) eV consistent with the DFT study. A semiconducting behavior was also observed for polycrystalline Ba2Gd2/3Mn2Te5 from the resistivity study. The temperature-dependent magnetic studies on a polycrystalline sample of Ba2Gd2/3Mn2Te5 did not reveal any long-range magnetic order down to 5 K. The effective magnetic moment (μeff) of 10.37μB calculated using the Curie–Weiss law is in good agreement with the theoretical value (μcal) of 10.58μB.

A new quaternary telluride, Ba

The single crystals of BaIn2Te4 have been synthesized at 1173 K using the sealed tube solid-state method. The polycrystalline sample of the compound was also synthesized by stoichiometric reaction of elements with intermittent grinding and annealing at 1123 K. The crystal structure of BaIn2Te4 has been established by single crystal X-ray diffraction study at 298(2) K. It crystallizes in space group D 2 h 20 −Cccm of the orthorhombic crystal system with unit cell dimensions of a = 7.1417(14) A, b = 12.034(2) A, and c = 12.107(2) A with four formula units. The structure of BaIn2Te4 is related to the tetragonal binary TlSe structure (space group: I4/mcm). The crystal structure of BaIn2Te4 features one-dimensional chains of 1 ∞ [In2Te42−] that are separated by filling of Ba atoms. Indium atoms in these chains are surrounded by four Te atoms that form distorted tetrahedron geometry around In atoms. This compound does not show any homoatomic bonding and is having a closed shell electronic configuration. Thus, it can be easily charge balanced as Ba2+(In3+)2(Te2−)4. The phase purity of the polycrystalline BaIn2Te4 sample was established by the powder X-ray diffraction study. The direct band gap of 1.36(2) eV was estimated from the optical absorption study at room temperature. This ternary compound is an n-type semiconductor as suggested by the negative sign of the Seebeck coefficients (S). The absolute value of S was found to be decreasing on heating the polycrystalline BaIn2Te4 sample. The temperature-dependent resistivity study confirmed the semiconducting nature of the BaIn2Te4 sample. The thermal conductivity (κtot) values of polycrystalline BaIn2Te4 were found to be extremely low and were decreasing with raising the temperature with the lowest value of 0.30 Wm−1K−1 at 965 K. Hence, BaIn2Te4 and its analogs with related structures could be promising materials for thermoelectric applications by further optimization of carrier concentrations to improve electrical transport properties.

Three new isostructural quaternary tellurides, Ba

A monophasic polycrystalline sample of Ba2Ge2Te5 has been synthesized for the first time using the sealed tube solid-state method. The Rietveld refinement of a polycrystalline Ba2Ge2Te5 and a single crystal X-ray diffraction study confirm that Ba2Ge2Te5 crystallizes in the orthorhombic polar C 2 v 9 -Pna21 space group. Each of the Ge atoms in the Ba2Ge2Te5 structure is covalently connected to one Ge and three Te atoms making one-dimensional (1D) chains of 1 ∞ [Ge2Te5]4− that are separated by Ba2+ cations. The (Ba2+)2(Ge3+)2(Te2−)5 can be charge-balanced as per the Zintl-Klemm concept. A resistivity study of Ba2Ge2Te5 shows a metallic behavior till 18 K below which metal to insulator transition was observed. Thermal conductivity of Ba2Ge2Te5 was found to decrease gradually on heating the sample with a minimum of about 0.41 Wm–1K–1 at 773 K. The DFT studies predict semiconducting nature for Ba2Ge2Te5 with a narrow indirect bandgap of about 0.6 eV.

Standard solid-state methods produced black crystals of the compounds BaCu0.43(3)Te2 and BaAg0.77(1)Te2 at 1173 K; the crystal structures of each were established using single-crystal X-ray diffraction data. Both crystal structures are modulated. The compound BaCu0.43(3)Te2 crystallizes in the monoclinic superspace group P2(αβ1/2)0, having cell dimensions of a = 4.6406(5) A, b = 4.6596(5) A, c = 10.362(1) A, β = 90.000(9)°, and Z = 2 and an incommensurate vector of q = 0.3499(6)b* + 0.5c*. The compound BaAg0.77(1)Te2 crystallizes in the orthorhombic P21212(α00)000 superspace group with cell dimensions of a = 4.6734(1) A, b = 4.6468(1) A, c = 11.1376(3) A, and Z = 2 and an incommensurate vector of q = 0.364(2)a*. The asymmetric unit of the BaCu0.43(3)Te2 structure comprises eight crystallographically independent sites; that for BaAg0.77(1)Te2 comprises four. In these two structures, each of the M (M = Cu, Ag) atoms is connected to four Te atoms to make two-dimensional layers of [MxTe4/4]n- that are separated by layers of Ba atoms and square nets of Te. A Raman spectroscopic study at 298(2) K on a pelletized polycrystalline sample of BaAg0.8Te2 shows the presence of Ag-Te (83, 116, and 139 cm-1) and Ba-Te vibrations (667 and 732 cm-1). A UV-vis-NIR spectroscopic study on a powdered sample of BaAg0.8Te2 shows the semiconducting nature of the compound with a direct band gap of 1.0(2) eV, consistent with its black color. DFT calculations give a pseudo bandgap with a weak value of the DOS at the Fermi level.

Standard solid-state methods produced black crystals of the compounds BaCu

A new ternary telluride, Ba3ScTe5, with a pseudo-one-dimensional structure, was synthesized at 1173 K by standard solid-state methods. A single-crystal X-ray diffraction study at 100(2) K shows the structure to be modulated. The structure of the subcell of Ba3ScTe5 crystallizes with two formula units in the hexagonal space group D6h3-P63/mcm with unit cell dimensions of a = b = 10.1190(5) A and c = 6.8336(3) A. The asymmetric unit of the subcell structure consists of four crystallographically independent sites: Ba1 (site symmetry: m2m), Sc1 (-3.m), Te1 (m2m), and Te2 (3.2). Its structure is made up of chains of ∞1[ScTe33-] that are separated by Ba2+ cations. The Sc atoms are bonded to six Te1 atoms that form a slightly distorted octahedral geometry. The structure of the subcell also contains linear infinite chains of Te2 with intermediate Te···Te interactions. The superstructure of Ba3ScTe5 is incommensurate and was solved in the hexagonal superspace group P-6(00γ)0 with a = 10.1188(3) A and c = 6.8332(3) A and a modulation vector of q = 0.3718(2)c*. The arrangement and coordination geometries of the atoms in the superstructure are very similar to those in the substructure. However, the main difference is that the infinite chains of Te atoms in the superstructure are distorted owing to the formation of long- and short-bonded pairs of Te atoms. The presence of these chains with intermediate Te···Te interactions makes assignment of the formal oxidation states arbitrary. The optical absorption study of a polycrystalline sample of Ba3ScTe5 that was synthesized by the stoichiometric reaction of elements at 1173 K reveals a direct band gap of 1.1(2) eV. The temperature-dependent resistivity study of polycrystalline Ba3ScTe5 shows semiconducting behavior corroborating the optical studies, while density functional theory calculations report a pseudo band gap of 1.3 eV.

A new ternary telluride, Ba

Single crystals and a polycrystalline sample of Ba

Single crystals and a polycrystalline sample of Ba2Ag2Se2(Se2) were synthesized by standard solid-state chemistry methods at 1173 and 973 K, respectively. The crystal structure of this ternary comp...

Single crystals of two new chalcogenides CsSc5Te8 and Cs0.6(1)Ti6Se8 were synthesized by the high-temperature solid-state sealed tube method using a reactive flux (CsCl) at 1223 K. The crystal structures of CsSc5Te8 and Cs0.6(1)Ti6Se8 were determined by single-crystal X-ray diffraction method. The compound CsSc5Te8 crystallizes in the monoclinic space group C 2 h 3 −C2/m with the lattice parameters a = 21.3376(15) A, b = 4.1434(3) A, c = 10.2853(7) A, and β = 103.925(2)° having the two formula units (Z = 2). The asymmetric unit of CsSc5Te8 contains eight crystallographically independent atomic sites: Cs1 (site symmetry: 2/m), Sc1 (m), Sc2 (m), Sc3 (2/m), Te1 (m), Te2 (m), Te3 (m), and Te4 (m). The structure of CsSc5Te8 is built up of the three-dimensional anionic framework of 3 ∞ [ Sc 5 Te 8 ] 1 − where the Sc atoms are octahedrally coordinated with six Te atoms forming the one-dimensional tunnels approximately along the b-axis where the Cs+ cations are present. The compound Cs0.6(1)Ti6Se8 crystallizes in the Nb3Te4 structure type with Z = 1, in the hexagonal space group C 6 h 2 −P63/m having cell dimensions of a = b = 9.9520(1) A and c = 3.5710(1) A. The asymmetric unit of Cs0.6(1)Ti6Se8 structure is composed of four crystallographically independent sites with atoms Cs1 (site symmetry: 6 ‾ ..), Ti1 (m..), Se1 (m..), and Se2 ( 6 ‾ ..). The Ti atoms are making distorted octahedral units by coordinating with six Se atoms. These TiSe6 distorted octahedra share edges and corners with the adjacent TiSe6 units to form three-dimensional anionic networks that generate one-dimensional tunnels approximately along the c-direction. The optical absorption measurements show that CsSc5Te8 is a semiconductor having a direct bandgap of 1.2(1) eV at room temperature consistent with the DFT studies.

The monophasic natrophilite form of NaMnPO4 was synthesized using a simple and scalable one-step hydrothermal method. The heat treatment of the natrophilite NaMnPO4 in the air resulted in the formation of the high-temperature maricite polymorph of NaMnPO4. The crystal structures of these two polymorphs were confirmed by the Rietveld refinements and Raman spectroscopy. The DSC study of the natrophilite form shows a broad exothermic peak indicative of a second-order phase transition to high-temperature maricite form. The band gaps (Eg) of 5.18(2) and 1.88(2) eV were estimated for natrophilite and maricite forms, respectively, by the UV–vis absorption studies. Magnetic studies on polycrystalline natrophilite NaMnPO4 show a complex temperature dependence of magnetic susceptibility, which could be attributed to a canted antiferromagnetic behavior with the Neel’s temperature (TN) of 31 K and an anomaly temperature (T*) of 29 K. Maricite NaMnPO4 also show antiferromagnetic interaction of Mn2+ spins with TN of 18 K.

The single crystals of Cs3FeCl5 were synthesized at 973 K using the sealed tube solid-state molten flux method using CsCl as a reactive flux. The polycrystalline sample of Cs3FeCl5 was obtained by the stoichiometric reaction of CsCl and FeCl2 powders at 823 K by the sealed tube solid-state method. The crystal structure of Cs3FeCl5 was determined by single-crystal X-ray diffraction study at 298 (2) K. This ternary halide crystallizes in the body-centered tetragonal crystal system in I4/mcm space group with cell constants of a = b = 9.279 (1) A and c = 14.824 (3) A with four formula units per cell. The asymmetric unit of Cs3FeCl5 contains five crystallographically independent atomic sites: Cs1 (site symmetry: m.2 m), Cs2 (422), Fe1 ( 4 ¯ 2 m), Cl1 (..m), and Cl2 (4/m..). Each Fe atom in Cs3FeCl5 structure is bonded to four Cl1 atoms in a slightly distorted tetrahedral fashion to form isolated FeCl42− units. These FeCl42− units are separated by the Cs+ cations and infinite [CsCl] linear chains. Charge balance in this closed-shell compound can be achieved by 3 × Cs+, 1 × Fe2+, and 5 × Cl−. Bond valence sum (BVS) calculation also supports this assignment of formal oxidation states of elements in Cs3FeCl5 structure. The electronic structure calculation for Cs3FeCl5 performed within a density functional theoretical framework predicts a band gap of 3.5 eV, which is in good agreement with the experimental band gap of 3.71 (2) eV, that was estimated from the UV–vis absorption edge study of polycrystalline Cs3FeCl5.