Low thermal conductivity in a new mixed metal telluride Mn1.8(1)In0.8(1)Si2Te6

The design of new complex mixed metal tellurides (containing low toxicity cations) with intrinsic ultralow thermal conductivity is of paramount importance in the field of thermoelectrics. Herein, we report the synthesis and characterization of polycrystalline and single crystals of a new mixed-metal quaternary telluride Mn1.8(1)In0.8(1)Si2Te6. The structural aspects and chemical formula of this phase at room temperature have been established using single crystal X-ray diffraction and EDX studies. The trigonal centrosymmetric (space group: P3̄1m) structure of the title phase has cell constants of a = b = 7.0483(7) Å and c = 7.1277(8) Å. The structure has three independent cationic sites, one mixed (In1/Mn1), one partially filled Mn2, and one Si1, which are bonded with Te1 atoms. Each metal atom (In and Mn) in the structure is octahedrally coordinated with six neighboring Te1 atoms. The structure also features dimers of Si atoms, and each Si atom is bonded to three Te1 atoms to form ethane-like Si2Te6 units. The optical absorption study of a polycrystalline Mn1.8In0.8Si2Te6 sample shows a narrow optical bandgap of 0.6(2) eV. Temperature-dependent resistivity and Seebeck coefficient studies confirmed the p-type semiconducting nature of the sample with high values of S (301 μV K-1 to 444 μV K-1). The total thermal conductivity (ktot) study of the polycrystalline sample shows a decreasing trend on heating with an extremely low value of 0.28 W m-1 K-1 at 773 K. Magnetic measurements indicate a glassy magnetic behavior for the sample below 8 K.

Ba14Si4Sb8Te32(Te3): hypervalent Te in a new structure type with low thermal conductivity

Heavier pnictogen (Sb, Bi) containing chalcogenides are well known for their complex structures and semiconducting properties for numerous applications, particularly thermoelectric materials. Here, we report the syntheses of single crystals and polycrystalline phases of a new complex quaternary polytelluride, Ba14Si4Sb8Te32(Te3), via a high-temperature reaction of elements. A single-crystal X-ray diffraction study showed that it crystallizes in an unprecedented structure type with monoclinic symmetry (space group: P21/c). The crystal structure of Ba14Si4Sb8Te32(Te3) consists of one-dimensional ∞1[Si4Sb8Te32(Te3)]28- stripes, which are separated by the Ba2+ cations. Its complex structure features linear polytelluride units of Te34- having intermediate Te⋯Te interactions. A polycrystalline Ba14Si4Sb8Te32(Te3) sample shows a direct narrow bandgap of 0.8(2) eV, which indicates its semiconducting nature. The electrical resistivity of a sintered pellet of the polycrystalline sample exponentially decreases from ∼39.3 Ωcm to ∼0.57 Ωcm on heating it from 323 K to 773 K, confirming the sample's semiconducting nature. The sign of Seebeck coefficient values is positive in the 323 K to 773 K range confirming the p-type nature of the sintered sample. Interestingly, the sample attains an extremely low thermal conductivity of ∼0.32 Wm-1K-1 at 773 K, which could be attributed to the lattice anharmonicity caused by the lone pair effect of Sb3+ species in its complex pseudo-one-dimensional crystal structure. The electronic band structure of the title phase and the strength of chemical bonding of pertinent atomic pairs have been evaluated theoretically using the DFT method.

Electronic and Thermal Properties of the Cation Substitution-Derived Quaternary Chalcogenide CuInSnSe4

Quaternary chalcogenides continue to be of interest for a variety of technological applications, with physical properties stemming from their structural complexity and stoichiometric variation. In certain structure types, partial vacancies on specific lattice positions present an opportunity to investigate electrical and thermal properties in light of these lattice defects. In this work, we investigated the structural, thermal, and electronic properties of CuInSnSe4, a material that belongs to a relatively unexplored class of quaternary chalcogenides with a defect adamantine crystal structure. First-principles calculations together with experimental measurements revealed a chalcopyrite-like structure with inherent vacancies and characteristic s-p and p-d orbital hybridizations in the electronic structure of the material. Cation disorder and lattice anharmonicity result in very low thermal conductivity with values significantly lower than those for related compositions. This work reveals the fundamental physical properties of a previously uninvestigated quaternary chalcogenide and may aid investigations of similar as well as other quaternary chalcogenide compositions.

Single Crystals of EuScCuSe3: Synthesis, Experimental and DFT Investigations

EuScCuSe₃ was synthesized from the elements for the first time by the method of cesium-iodide flux. The crystal belongs to the orthorhombic system (Cmcm) with the unit cell parameters a = 3.9883(3) Å, b = 13.2776(9) Å, c = 10.1728(7) Å, V = 538.70(7) ų. Density functional (DFT) methods were used to study the crystal structure stability of EuScCuSe₃ in the experimentally obtained Cmcm and the previously proposed Pnma space groups. It was shown that analysis of elastic properties as Raman and infrared spectroscopy are powerless for this particular task. The instability of EuScCuSe₃ in space group Pnma space group is shown on the basis of phonon dispersion curve simulation. The EuScCuSe₃ can be assigned to indirect wide-band gap semiconductors. It exhibits the properties of a soft ferromagnet at temperatures below 2 K.

Ba4FeAgS6: a new antiferromagnetic and semiconducting quaternary sulfide

The single crystals of a quaternary sulfide, Ba₄FeAgS₆, have been synthesized by reacting elements at 873 K inside a sealed fused silica tube. The title phase is the first ordered quaternary compound of the Ba-Ag-Fe-S system. The crystal structure of Ba₄FeAgS₆ is characterized by a single-crystal X-ray diffraction study at 298(2) K. It crystallizes in the space group C52h - P2₁/n of the monoclinic crystal system with unit cell dimensions of a = 8.6367(5) Å, b = 12.0291(7) Å, c = 13.2510(7) Å, and β = 109.015(2)°. This compound is stoichiometric, and its structure contains twelve unique crystallographic sites: four Ba, one Fe, one Ag, and six S sites. All atoms of the structure occupy the general positions. The Ba₄FeAgS₆ structure consists of one-dimensional chains of 1∞[FeAgS₆]^8- that are extended in the [100] direction. The negative charges on these chains are counterbalanced by the filling of Ba²⁺ cations in between the 1∞[FeAgS₆]^8- chains. The Fe atoms are bonded to four S atoms that form a distorted tetrahedral geometry around the central Fe atom. Each Ag atom in this structure is coordinated with four S atoms in a distorted tetrahedral fashion. These FeS₄ and AgS₄ motifs are the main building blocks of the Ba₄FeAgS₆ structure. The corner-sharing of FeS₄ and AgS₄ tetrahedra creates one-dimensional chains of 1∞[FeAgS₆]^8-. This structure does not contain any homoatomic or metallic bonds and can be charge-balanced as (Ba²⁺)₄(Fe³⁺)₁(Ag¹⁺)₁(S^2-)₆. The optical absorption study performed on a polycrystalline Ba₄FeAgS₆ sample reveals a direct bandgap of 1.2(1) eV. The magnetic studies reveal the antiferromagnetic behavior of Ba₄FeAgS₆ below 50 K. The thermal conductivity and theoretical electronic structure of Ba₄FeAgS₆ are also studied in detail.

A Challenge toward Novel Quaternary Sulfides SrLnCuS3 (Ln = La, Nd, Tm): Unraveling Synthetic Pathways, Structures and Properties

We report on the novel heterometallic quaternary sulfides SrLnCuS₃ (Ln = La, Nd, Tm), obtained as both single crystals and powdered samples. The structures of both the single crystal and powdered samples of SrLaCuS₃ and SrNdCuS₃ belong to the orthorhombic space group Pnma but are of different structural types, while both samples of SrTmCuS₃ crystallize in the orthorhombic space group Cmcm with the structural type KZrCuS₃. Three-dimensional crystal structures of SrLaCuS₃ and SrNdCuS₃ are formed from the (Sr/Ln)S₇ capped trigonal prisms and CuS₄ tetrahedra. In SrLaCuS₃, alternating 2D layers are stacked, while the main backbone of the structure of SrNdCuS₃ is a polymeric 3D framework [(Sr/Ln)S₇]_n, strengthened by 1D polymeric chains (CuS₄)_n with 1D channels, filled by the other Sr²⁺/Ln³⁺ cations, which, in turn, form 1D dimeric ribbons. A 3D crystal structure of SrTmCuS₃ is constructed from the SrS₆ trigonal prisms, TmS₆ octahedra and CuS₄ tetrahedra. The latter two polyhedra are packed together into 2D layers, which are separated by 1D chains (SrS₆)_n and 1D free channels. In both crystal structures of SrLaCuS₃ obtained in this work, the crystallographic positions of strontium and lanthanum were partially mixed, while only in the structure of SrNdCuS₃, solved from the powder X-ray diffraction data, were the crystallographic positions of strontium and neodymium partially mixed. Band gaps of SrLnCuS₃ (Ln = La, Nd, Tm) were found to be 1.86, 1.94 and 2.57 eV, respectively. Both SrNdCuS₃ and SrTmCuS₃ were found to be paramagnetic at 20–300 K, with the experimental magnetic characteristics being in good agreement with the corresponding calculated parameters.

Quaternary Selenides EuLnCuSe3: Synthesis, Structures, Properties and In Silico Studies

In this work, we report on the synthesis, in-depth crystal structure studies as well as optical and magnetic properties of newly synthesized heterometallic quaternary selenides of the Eu⁺²Ln⁺³Cu⁺¹Se₃ composition. Crystal structures of the obtained compounds were refined by the derivative difference minimization (DDM) method from the powder X-ray diffraction data. The structures are found to belong to orthorhombic space groups Pnma (structure type Ba₂MnS₃ for EuLaCuSe₃ and structure type Eu₂CuS₃ for EuLnCuSe₃, where Ln = Sm, Gd, Tb, Dy, Ho and Y) and Cmcm (structure type KZrCuS₃ for EuLnCuSe₃, where Ln = Tm, Yb and Lu). Space groups Pnma and Cmcm were delimited based on the tolerance factor t', and vibrational spectroscopy additionally confirmed the formation of three structural types. With a decrease in the ionic radius of Ln³⁺ in the reported structures, the distortion of the (LnCuSe₃) layers decreases, and a gradual formation of the more symmetric structure occurs in the sequence Ba₂MnS₃ → Eu₂CuS₃ → KZrCuS₃. According to magnetic studies, compounds EuLnCuSe₃ (Ln = Tb, Dy, Ho and Tm) each exhibit ferrimagnetic properties with transition temperatures ranging from 4.7 to 6.3 K. A negative magnetization effect is observed for compound EuHoCuSe₃ at temperatures below 4.8 K. The magnetic properties of the discussed selenides and isostructural sulfides were compared. The direct optical band gaps for EuLnCuSe₃, subtracted from the corresponding diffuse reflectance spectra, were found to be 1.87-2.09 eV. Deviation between experimental and calculated band gaps is ascribed to lower d states of Eu²⁺ in the crystal field of EuLnCuSe₃, while anomalous narrowing of the band gap of EuYbCuSe₃ is explained by the low-lying charge-transfer state. Ab initio calculations of the crystal structures, elastic properties and phonon spectra of the reported compounds were performed.

Syntheses and characterization of two new layered ternary chalcogenides NaScQ2 (Q = Se and Te)

Two new metal ternary chalcogenides, NaScSe2 and NaScTe2, have been synthesized via high-temperature reaction.

Five coordinated Mn in Ba4Mn2Si2Te9: synthesis, crystal structure, physical properties, and electronic structure

A new structure type Ba4Mn2Si2Te9 containing unique MnTe5 units is synthesized. The structure comprises two independent Mn atoms, each with 50% occupancy. It is a narrow bandgap semiconductor (Eg = 0.6(1) eV) consistent with the DFT studies.

Ba3Zr2Cu4S9: the first quaternary phase of the Ba–Zr–Cu–S system

We report the synthesis of red-colored crystals of Ba3Zr2Cu4S9 via a high-temperature reaction of elements at 1223 K. The title compound is the first quaternary phase of the Ba–Zr–Cu–S system....

Germanium Antimony Bonding in Ba4Ge2Sb2Te10 with Low Thermal Conductivity

A new quaternary telluride, Ba₄Ge₂Sb₂Te₁₀, was synthesized at high temperature via the reaction of elements. A single-crystal X-ray diffraction study shows that the title compound crystallizes in its own structure type in the monoclinic P2₁/c space group having cell dimensions of a = 13.984(3) Å, b = 13.472(3) Å, c = 13.569(3) Å, and β = 90.16(3)° with four formula units per unit cell (Z = 4). The pseudo-one-dimensional crystal structure of Ba₄Ge₂Sb₂Te₁₀ consists of infinite ₁^∞[Ge₂Sb₂Te₁₀]^8- stripes, which are separated by Ba²⁺ cations. Each of the Ge(1) atoms is covalently bonded to four Te atoms, whereas the Ge(2) atom is covalently bonded with one Sb(2) and three Te atoms in a distorted tetrahedral geometry. The title compound is the first example of a chalcogenide that shows Ge-Sb bonding. The Sb(1) atom is present at the center of the seesaw geometry of four Te atoms. In contrast, the Sb(2) atom forms a seesaw geometry by coordinating with one Ge(2) and three Te atoms. Condensation of these Ge and Sb centered polyhedral units lead to the formation of ₁^∞[Ge₂Sb₂Te₁₀]^8- stripes. The temperature-dependent resistivity study suggests the semimetallic/degenerate semiconducting nature of polycrystalline Ba₄Ge₂Sb₂Te₁₀. The positive sign of Seebeck coefficient values indicates that the predominant charge carriers are holes in Ba₄Ge₂Sb₂Te₁₀. An extremely low lattice thermal conductivity of ∼0.34 W/mK at 773 K was observed for polycrystalline Ba₄Ge₂Sb₂Te₁₀, which is presumably due to the lattice anharmonicity induced by the stereochemically active 5s² lone pair of Sb. The electronic structure of Ba₄Ge₂Sb₂Te₁₀ and the bonding of atom pairs in the structure have been analyzed by means of ELF analysis and crystal orbital Hamilton population (COHP) analysis.

Reactive molten-flux assisted syntheses of single crystals of Cs19Ln19Mn10Te48 (Ln = Pr and Gd) crystallizing in a new structure type

Two new complex layered quaternary tellurides, Cs19Ln19Mn10Te48 (Ln = Pr and Gd), were synthesized by the reactive molten flux method. The title compounds represent an unprecedented structure type.