Cheriyedath Raj Sankar

Crystal structures, electronic structures, and physical properties of Tl4MQ4 (M = Zr or Hf; Q = S or Se).

The ternary thallium chalcogenides of the general formula Tl(4)MQ(4) (M = Zr or Hf; Q = S or Se) were obtained from high-temperature reactions without air. These sulfides and selenides are isostructural, crystallizing in the triclinic system with space group P1 and Z = 5, in contrast to Tl(4)MTe(4) compounds that adopt space group R3. The unit cell parameters for Tl(4)ZrS(4) are as follows: a = 9.0370(5) Å, b = 9.0375(5) Å, c = 15.4946(9) Å, α = 103.871(1)°, β = 105.028(1)°, γ = 90.138(1)°, and V = 1183.7(1) Å(3). In contrast to the corresponding tellurides, the sulfides and selenides exhibit edge-shared MQ(6) octahedra, propagating along the c axis in a zigzag manner. All elements occur in the most common oxidation states, according to the formulation (Tl(+))(4)M(4+)(Q(2-))(4). Electronic structure calculations predict energy band gaps of 1.7 eV for Tl(4)ZrS(4) and 1.3 eV for Tl(4)ZrSe(4), which are in accordance with the large resistivity values observed experimentally.

Syntheses, crystal structures and thermoelectric properties of two new thallium tellurides: Tl4ZrTe4 and Tl4HfTe4

Three new isostructural tellurides Tl4MTe4 with M = Zr and Hf were prepared from the constituent elements. Single crystal X-ray diffraction data analyses showed that these compounds belong to a new structure type, adopting the space group R with the unit cell dimensions of a = 14.6000(5) A and c = 14.189(1) A when M = Zr, and a = 14.594(1) A and c = 14.142(3) A when M = Hf (Z = 9). The structure consists of M atoms in distorted octahedra formed by Te atoms, which are face-condensed to oligomeric M3Te12 units. The structure refinement of the Zr compound revealed an additional site, occupied by 10% Zr, leading to a refined formula of Tl4Zr1.03Te4. The electronic structure calculations predict semiconducting behavior for the stoichiometric compounds, which is in accordance with the experimental results. The thermoelectric properties of both compounds were determined, and the maximum values of the dimensionless figure-of-merit, ZT, were found to be 0.16 at 420 K for Tl4ZrTe4 and 0.09 at 540 K for Tl4HfTe4 in the measured temperature regime.

New barium copper chalcogenides synthesized using two different chalcogen atoms: Ba2Cu(6-x)STe4 and Ba2Cu(6-x)Se(y)Te(5-y).

Ba(2)Cu(6-x)STe(4) and Ba(2)Cu(6-x)Se(y)Te(5-y) were prepared from the elements in stoichiometric ratios at 1123 K, followed by slow cooling. These chalcogenides are isostructural, adopting the space group Pbam (Z = 2), with lattice dimensions of a = 9.6560(6) Å, b = 14.0533(9) Å, c = 4.3524(3) Å, and V = 590.61(7) Å(3) in the case of Ba(2)Cu(5.53(3))STe(4). A significant phase width was observed in the case of Ba(2)Cu(6-x)Se(y)Te(5-y) with at least 0.17(3) ≤ x ≤ 0.57(4) and 0.48(1) ≤ y ≤ 1.92(4). The presence of either S or Se in addition to Te appears to be required for the formation of these materials. In the structure of Ba(2)Cu(6-x)STe(4), Cu-Te chains running along the c axis are interconnected via bridging S atoms to infinite layers parallel to the a,c plane. These layers alternate with the Ba atoms along the b axis. All Cu sites exhibit deficiencies of up to 26%. Depending on y in Ba(2)Cu(6-x)Se(y)Te(5-y), the bridging atom is either a Se atom or a Se/Te mixture when y ≤ 1, and the Te atoms of the Cu-Te chains are partially replaced by Se when y > 1. All atoms are in their most common oxidation states: Ba(2+), Cu(+), S(2-), Se(2-), and Te(2-). Without Cu deficiencies, these chalcogenides were computed to be small gap semiconductors; the Cu deficiencies lead to p-doped semiconducting properties, as experimentally observed on selected samples.

Improvements of the thermoelectric properties of PbTe via simultaneous doping with indium and iodine

The thermoelectric properties of n-type InxPb1−xTe1−yIy (with x = 0.005, 0.01, 0.015; y = 0.001, 0.002, 0.004, 0.006) were investigated at elevated temperatures up to 655 K. This co-doping significantly affected the Seebeck coefficient and electrical conductivity of all samples within the measured temperature regime except for the sample with the largest concentration of In, wherein the effects of I-doping are comparatively minor. For a given concentration of In, the sample with the largest amount of iodine possesses the highest electrical conductivity, which is consistent within all three sets of samples in our present study. Thermal conductivity values are generally lower than those of undoped PbTe. An increasing iodine concentration at fixed In content was found to gradually increase the dimensionless figure-of-merit, ZT, an effect most significantly observed when x = 0.01.

New quaternary chalcogenides, Tl18Pb2M7Q25 (M = Ti, Zr, and Hf; Q = S and Se): crystal structure, electronic structure, and electrical transport properties.

We have synthesized new quaternary chalcogenides of the general formula Tl(18)Pb(2)M(7)Q(25) (M = Ti, Zr and Hf, Q = S, Se), and studied their crystal and electronic structures. They are all isostructural, with a large cubic unit cell of space group Pa3, and a = 17.0952(6) Å in case of Tl(18)Pb(2)Ti(7)S(25) (with four formula units per cell). The structure is composed of several interesting subunits such as isolated M(7)Q(24) entities, weakly connected Tl(9)Pb supertetrahedra (or 4-capped distorted octahedra) and STl(6) distorted octahedra. The finite unit M(7)Q(24) is formed by seven edge-shared MQ(6) octahedra wherein all except the central one are distorted because of the neighborhood of Tl(+) ions that carry a lone pair of electrons. These materials are semiconductors with all elements in their common oxidation states, for example, (Tl(+))(18)(Pb(2+))(2)(Ti(4+))(7)(S(2-))(25). The calculations yielded band gaps of 0.64 eV for the sulfides Tl(18)Pb(2)Ti(7)S(25) and 1.0 eV for Tl(18)Pb(2)Zr(7)S(25). The selenide Tl(18)Pb(2)Ti(7)Se(25) was calculated to have a band gap of 0.44 eV. Electrical conductivity measurements and reflectance spectroscopy also revealed the semiconducting nature of these samples, with experimentally determined gaps between 0.10 and 0.50 eV.

New layered-type quaternary chalcogenides, Tl2PbMQ4 (M = Zr, Hf; Q = S, Se): structure, electronic structure, and electrical transport properties.

We have synthesized and characterized new thallium chalcogenides of the general formula Tl2PbMQ4 (M = Zr, Hf; Q = S, Se) from the constituent elements via high-temperature reaction conditions. These sulfides and selenides crystallize in the monoclinic crystal system (space group C2/c). The unit cell parameters refined from single-crystal X-ray diffraction data for Tl2PbZrS4 are a = 15.455(4) Å, b = 8.214(2) Å, c = 6.751(2) Å, β = 109.093(3)°, and V = 809.9(4) Å(3), with Z = 4. No corresponding tellurides were obtained from similar reaction conditions. The isostructural quaternary chalcogenides form a layered structure, composed of alternating metal and chalcogen layers. The latter are packed along the a axis as in the face-centered cubic packing (ABC), while the metal layers alternate between Tl layers and mixed Pb/Zr layers. All metal atoms are located in differently distorted Q6 octahedra, with the TlQ6 polyhedra being the least regular ones. Density functional theory based electronic structure calculations with inclusion of relativistic spin-orbit interactions predict (indirect) energy band gaps of 0.66 and 0.33 eV for Tl2PbZrS4 and Tl2PbHfSe4, respectively. Optical spectroscopy revealed significantly larger (direct) band gaps of 1.2 and 1.6 eV. The semiconducting character is in agreement with the charge-balanced formula (Tl(+))2Pb(2+)M(4+)(Q(2-))4. The electrical transport properties also show the semiconducting nature of these materials. For Tl2PbHfSe4, the Seebeck coefficient increases from +190 μV K(-1) at room temperature to +420 μV K(-1) at 520 K.

Crystal structure, electronic structure and physical properties of the new quaternary chalcogenides Tl2NdAg3Se4 and Tl2NdAg3Te4

The chalcogenides Tl2NdAg3Q4 (Q = Se, Te) were prepared by heating the elements in stoichiometric ratios under exclusion of air between 973 K and 1073 K. Both compounds adopt new structure types, with Tl2NdAg3Se4 crystallizing in the space group Pnma with a = 15.9937(14) A, b = 4.3420(4) A, c = 14.3676(12) A, V = 997.75(15) A3, Z = 4, while Tl2NdAg3−xTe4 adopts a different structure with related motifs, crystallizing in the noncentrosymmetric space group Cmc21 with a = 4.5968(2) A, b = 16.8620(6) A, c = 44.7131(15) A, V = 3465.8(2) A3, Z = 12, when x = 0.05. In both cases, linear chains of [AgQ4] tetrahedra are interconnected via common corners and edges to build a three-dimensional network, which encompasses the Tl and Nd atoms in linear channels. The differences lie in their three-dimensional connection, resulting in a larger, noncentrosymmetric unit cell of the telluride. Electronic structure calculations indicate semiconducting properties, in accord with the transport property measurements in both cases. A special feature of both materials is the extraordinarily low thermal conductivity.