Alexey N. Kuznetsov

Iodobismuthates Containing One-Dimensional BiI4– Anions as Prospective Light-Harvesting Materials: Synthesis, Crystal and Electronic Structure, and Optical Properties

Four iodobismuthates, LiBiI4·5H2O (1), MgBi2I8·8H2O (2), MnBi2I8·8H2O (3), and KBiI4·H2O (4), were prepared by a facile solution route and revealed thermal stability in air up to 120 °C. Crystal structures of compounds 1-4 were solved by a single crystal X-ray diffraction method. 1: space group C2/c, a = 12.535(2), b = 16.0294(18), c = 7.6214(9) Å, β = 107.189(11)°, Z = 4, R = 0.029. 2: space group P21/c, a = 7.559(2), b = 13.1225(15), c = 13.927(4) Å, β = 97.14(3)°, Z = 2, R = 0.031. 3: space group P21/c, a = 7.606(3), b = 13.137(3), c = 14.026(5) Å, β = 97.14(3)°, Z = 2, R = 0.056. 4: space group P21/n, a = 7.9050(16), b = 7.7718(16), c = 18.233(4) Å, β = 97.45(3)°, Z = 4, R = 0.043. All solid state structures feature one-dimensional (BiI4)(-) anionic chains built of [BiI6] octahedra that share two opposite edges in such a fashion that two iodine atoms in cis-positions remain terminal. The calculated electronic structures and observed optical properties confirmed that compounds 1-4 are semiconductors with direct band gaps of 1.70-1.76 eV, which correspond to their intense red color. It was shown that the cations do not affect the optical properties, and the optical absorption is primarily associated with the charge transfer from the I 5p orbitals at the top of the valence band to the Bi 6p orbitals at the bottom of the conduction band. Based on their properties and facile synthesis, the title compounds are proposed as promising light-harvesting materials for all-solid solar cells.

New representative of Bi95+-containing phases: synthesis and crystal structure of the NbIV-containing Bi10Nb3Cl18 compound

The Bi10Nb3Cl18 compound was prepared using the high-temperature ampoule method, and its crystal structure was determined by single-crystal X-ray diffraction. This phase contains the Bi95+ cluster polycations and Bi+ cations surrounded by the NbCl62– complex anions. The latter contain the paramagnetic central NbIV ions, as evidenced by magnetic measurements.

New subvalent bismuth telluroiodides incorporating Bi2 layers : the crystal and electronic structure of Bi2TeI

Two new subvalent bismuth telluroiodides, Bi2TeI and Bi4TeI1.25, were prepared by the gas-phase synthesis. The compositions of these phases were determined by energy-dispersive X-ray spectroscopy. X-ray diffraction study of melt grown Bi2TeI single crystals demonstrated that the compound crystallizes in the monoclinic system (space group C/2m) with the unit cell parameters a = 7.586(1) Å, b = 4.380(1) Å, c = 17.741(3) Å, β = 98.20°. The layered crystal structure of Bi2TeI consists of weakly bonded two dimensional blocks with a stoichiometry of the title compound. The blocks are stacked along the c axis. Each block consists of eight atomic layers alternating in the Te-Bi-I-Bi-Bi-I-Bi-Te order and includes a double layer of bismuth atoms. Based on the results of ab initio quantum-chemical calculations, the title compound is expected to possess a pronounced anisotropy of conductivity.

Galvanomagnetic and Thermoelectric Properties of BiTeBr and BiTeI Single Crystals and their Electronic Structure

BiTeI and BiTeBr single crystals are synthesized by the Bridgman method, and their galvanomagnetic and thermoelectrical properties are investigated. Both semiconductors have n-type conductivity. The thermoelectric efficiency of BiTeBr is much higher than that of BiTeI, which is related mainly to a larger See-beck coefficient for the former compound. For both crystals, the band structure is calculated from the density-functional theory. It is shown that both compounds are semiconductors with an indirect energy band gap.

From Isolated Anions to Polymer Structures through Linking with I2: Synthesis, Structure, and Properties of Two Complex Bismuth(III) Iodine Iodides

We report the synthesis, crystal structures, and optical properties of two new compounds, K18Bi8I42(I2)0.5·14H2O (1) and (NH4)7Bi3I16(I2)0.5·4.5H2O (2), as well as the electronic structure of the latter. They crystallize in tetragonal space group P4/ mmm with the unit cell parameters a = 12.974(1) and c = 20.821(3) Å for 1 and a = 13.061(3) and c = 15.162(7) Å for 2. Though 1 and 2 are not isomorphous, their crystal structures display the same structural organization; namely, the BiI6 octahedra are linked by I2 units to form disordered layers in 1 and perfectly ordered chains in 2. The I-I bond distances in the thus formed I-I-I-I linear links are not uniform; the central bond is only slightly longer than in a standalone I2 molecule, whereas the peripheral bonds are significantly shorter than longer bonds typical for various polyiodides, which is confirmed by Raman spectroscopy. The analysis of the electronic structure shows that the atoms forming the I-I-I-I subunits transfer electron density from their occupied 5p orbitals onto their vacant states as well as onto 6s orbitals of bismuth atoms that center the BiI6 octahedra. This leads to low direct band gaps that were found to be 1.57 and 1.27 eV for 1 and 2, respectively, by optical absorption spectroscopy. Luminescent radiative relaxation was observed in the near-IR region with emission maxima of 1.39 and 1.24 eV for 1 and 2, respectively, in good agreement with the band structure, despite the strong quenching propensity of I2 moieties.

New mixed tellurides of nickel and Group 13–14 metals Ni3−δMTe2 (M = Sn, In, Ga)

Three new mixed tellurides of nickel and group 13–14 metals Ni3−δMTe2 (M = Sn, In, Ga) were prepared by high-temperature ampoule synthesis and studied by powder X-ray diffraction analysis. The compound Ni3−δSnTe2 was also studied by single crystal X-ray diffraction analysis. The structural model of this phase and two analogs was described as consisting of layers with nickel-main group metal bonds confined from the above by tellurium atoms. The van der Waals gap formed through contacts between the tellurium atoms of neighboring layers is partially occupied by nickel atoms.

Thermoelectric properties of BiTeI with addition of BiI3, CuI, and overstoichiometric Bi

BiTeI samples are synthesized, and their galvanomagnetic and thermoelectric properties are investigated when adding BiI3, CuI, and overstoichiometric Bi. The Seebeck coefficient of BiTeI samples considerably increases with addition of CuI, while their heat conductivity decreases. The same properties are observed for the BiTeI samples with BiI3. The presence of overstoichiometric Bi decreases the heat and electrical conductivities almost without affecting the thermopower.

Evaluation of Ce-doped Pr2CuO4 for potential application as a cathode material for solid oxide fuel cells

Pr2−xCexCuO4 (x = 0.05; 0.1; 0.15) samples were synthesized and systematically characterized towards application as a cathode material for solid oxide fuel cells (SOFCs). High-temperature electrical conductivity, thermal expansion, and electrocatalytic activity in the oxygen reduction reaction (ORR) were examined. The electrical conductivity of Pr2−xCexCuO4 oxides demonstrates semiconducting behavior up to 900 °C. Small Ce-doping (2.5 at%) allows an increase in electrical conductivity from 100 to 130 S cm−1 in air at 500–800 °C. DFT calculations revealed that the density of states directly below the Fermi level, comprised mainly of Cu 3d and O 2p states, is significantly affected by atoms in rare earth positions, which might give an indication of a correlation between calculated electronic structures and measured conducting properties. Ce-doping in Pr2−xCexCuO4 slightly increases TEC from 11.9 × 10−6 K−1 for x = 0 to 14.2 × 10−6 K−1 for x = 0.15. Substitution of 2.5% of Pr atoms in Pr2CuO4 by Ce is effective to enhance the electrochemical performance of the material as a SOFC cathode in the ORR (ASR of Pr1.95Ce0.05CuO4 electrode applied on Ce0.9Gd0.1O1.95 electrolyte is 0.39 Ω cm2 at 750 °C in air). The peak power density achieved for the electrolyte-supported fuel cell with the Pr1.95Ce0.05CuO4 cathode is 150 mW cm−2 at 800 °C.

On the stability boundaries of the LaOAgS structure type

Among layered inorganic structure types, that of LaOAgS, numbering over 200 representatives, has been studied extensively due to their promising physical properties (magnetic, thermoelectric, ion conducting, semiconducting, or superconducting). The relative simplicity of this structure type formed by anti-isostructural litharge and mackinawite-derived slabs suggests even larger number of representatives. However, out of over 500 possible candidates suggested from structure modeling, less than 50% could be realized synthetically. In order to gain deeper insight into the “crystal chemical stability boundaries”, we employed a complex approach based on synthesis, geometry analysis, and computational estimates of relative structural stability. Two groups of representatives were studied: i) LaOAgCh (Ch = S, Se, Te) chalcogenides, where stability was estimated (by calculating DFT ground-state energies) against La2O2Ch and Ag2Ch, and ii) AFTPn (A = Ca, Sr, Ba, Eu; T = Zn, Mn, Cd; Pn = P, As, Sb, Bi) pnictides and AeFAlTt (Ae = Sr, Ba; Tt = Si, Ge, Sn) tetrelides where stability was studied against binary alkaline-earth fluorides and ternary AT2X2 compounds (X = Group 14, 15, or 16 element). For the group i), we found that these three compounds were energetically more preferable. Indeed, our attempts to prepare both known S and Se representatives and yet unknown telluride were successful, although the latter has not yet been obtained phase-pure. For the case ii), the calculated pattern agrees well with the experimental results and predictions from geometrical considerations. No Ca compounds were found to exist, and neither do the proposed SrFCdPn. The most interesting case is the AFZnP group (A = Eu, Sr), where, apparently, just a subtle difference in the radii of Sr2+ and Eu2+ allows only the Sr compound to exist. The same pattern is also observed for AFAgS with A = Sr and Eu. Quite interesting findings are the first fluoride tetrelides BaFAlTt (Tt = Si, Ge) which are, in addidion, first examples that are free from transition metals; the latter is also true for an elusive BaFMgAs pnictide. Despite the complexity of the considered systems, our approach permitted us to successfully predict and prepare over 20 new LaOAgS-type compounds. Moreover, we can likely extend it to other related groups, e.g. iron pnictides, which can probably serve as parent compounds for iron-based superconductors. These results together will be presented in the report. This work was supported by Russian Foundation for Basic Researches under Grant No. 16-03-00661. [1] Plokhikh, I. V. (2016) Inorg. Chem. 55, 12409-12418. [2] Charkin, D. O. (2014) J. Alloys Compd. 585, 644-649. [3] Charkin, D. O. (2015) J. Alloys Compd. 627, 451-454.

New family of mixed nickel and group 13–14 metal tellurides with incommensurate structures

Hollandite-type compounds are showed with chemical formula as AxMxM’ 8-xO16. In A-site enter to alkali metal or an alkaline earth metal. In M-site enter to divalent or trivalent metal, and in M’-site enter to tetravalent metal ion. Metal-oxygen octahedron builds onedimensional (1-D) tunnel taken along c-axis. The 1-D tunnel has cavity and bottle neck, and it is known that A-site will be settled in each cavity. In this study, the condition of guest ion sites of K1.88Ga1.88Sn6.12O16 was examined by single X-ray diffraction from 293K to 93K. And, the refinement used the following resiraint conditions; (a) full occupation at the metal site in a host structure, (b) charge neutrality in a whole crystal, (c) exist two K2-site (0, 0, z) shifted from cavity center <K1-site (0, 0, 0.5)> for one vacancy [1]. In refinement of K1.88Ga1.88Sn6.12O16 at 293K and 223K, the ADPs (Atomic Displacement Parameters) and the site occupation were optimized by using these restraint conditions. On the other hands, in refinement at 173K, 130K and 93K, the atomic coordinate of K1-site had to shift from cavity center for optimization. Furthermore, it was shown that the site occupation of K2-site increases from a theoretical value. Therefore, the newly following constraint conditions were introduced for optimizing refinement; (a’) K2-site (0, 0, z) is the guest ion site which adjacent to vacancy, (b’) the guest ion site next to K2-site is K3-site (0, 0, z’), and (c’) other guest ion sites in cavity center are K1-site. By using these equations it was showed the degradation of ADPs and reliability factor of guest ion site. From these results, it was thought that two or more guest ion sites near vacancy would shift from cavity center. Reference [1] Y, Michiue, Acta.Cryst. B63 (2007) 577-583.