New Structure Types among Copper Chalcogenides by Mixing Tellurium with Sulfur or Selenium
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There is evidence for the existence of non-classical bonding in several binary antimonides, selenides, and tellurides. Owing to such non-classical bonding, some of these solid materials exhibit exciting semiconducting and thermoelectric properties, which make them attractive from a technological view point. However, lack of efficiency is a serious limitation in most of those thermoelectrics. It is very crucial, hence, to find new materials with superior properties and understand the structure and bonding in such materials, in order to facilitate the fine-tuning of the physical properties. With this expectation, several quaternary barium copper chalcogenides are synthesized and characterized in the present study. The chalcogen elements, selenium tellurium, are used in various ratios, in order to understand and tune the binding interactions. Extensive single crystal x-ray diffraction studies are expected to reveal the minute details of the bonding interactions together with electronic structure calculation and physical property measurements. In addition, characterization techniques such as powder x-ray diffraction, electron microscopy, differential scanning calorimetry, thermopower and conductivity measurements are utilized. The ternary and quaternary chalcogenides, Ba₂Cu₄₋ₓSeyTe₅₋y were synthesized from the elements in stoichiometric ratios at 700°C, followed by annealing at 600°C. The ternary telluride Ba₂Cu₄₋ₓTe₅ crystallizes in a new structure type, space group C2/c, with lattice dimensions of a = 9.4428(6) Å, b = 9.3289(6) Å, c = 13.3028(8) Å, β = 101.635(1)°, V = 1147.8(1) Å3, for x = 0.75(1) (Z = 4). The corresponding selenide-telluride adopts another new, but strongly related, structure type, space group P4₁2₁2, with a = 6.5418(3) Å, c = 25.782(2) Å, V = 1103.3(1) Å3, for Ba₂Cu₃.₂₆₍₂₎Se₀.₇₂₉₍₈₎Te₄.₂₇₁ (Z = 4). Between 0.13 and 1.0 Te per formula unit can be replaced with Se, while the Cu content appears to vary only within 0.67 ≤ x ≤ 0.81 for Ba₂Cu₄₋ₓSeyTe₅₋y. Despite crystallizing in different crystal systems, the telluride and the selenide-telluride exhibit topologically equivalent structure motifs, namely chains of Cu(Se,Te)₄ tetrahedra with a Cu atom cis/trans chain as well as an almost linear Te atom chain. All these chalcogenides - as far as measured - are p-doped semiconductors, as determined by Seebeck coefficient and electrical conductivity measurements. Two new orthorhombic chalcogenides, Ba₂Cu₆₋ₓSeyTe₅₋y and Ba₂Cu₆₋ₓSyTe₅₋y were synthesized at 800°C. They are isostructural and crystallize in a new structure type, with space group Pbam. Ba₂Cu₆₋ₓSyTe₅₋y, with a = 9.6560(6) Å, b = 14.0533(9) Å, c = 4.3524(3) Å and Ba₂Cu₅.₆₄Se₁.₀₉Te₃.₉₁with a = 9.7048(6) Å, b = 14.1853(9) Å, c = 4.3840(3) Å. They have Cu6 units extending along c-axis, and two such units are interconnected by S or Se atoms along a-axis. These compounds are nonmetallic with low Seebeck coefficients. Two more new quaternary chalcogenides were uncovered, BaCu₅.₉₂₆₍₁₅₎SeTe₆ and BaCu₅.₇₂₍₁₆₎Se₀.₄₆₄₍₁₅₎Te₆.₅₃₆ with a = 6.9680(2) Å and a = 6.9888(4) Å, respectively, in space group Pm̅3. These compounds have basic Cu₈Te₁₂ frameworks, which can be an important feature for thermoelectric materials. Ba occupies the void. One Cu atom from each cage cluster of eight such cages forms a Cu₈ cube with Se atom occupying it. BaCu₅.₉SeTe₆ was experimentally determined to be p-type doped semiconductor with moderate Seebeck coefficient value.