Andrei V. Mironov

Carbon substitution in MgB2 single crystals: Structural and superconducting properties

The growth of carbon-substituted magnesium diboride MgsB1˛xCxd2 single crystals with 0 x 0.15 is reported, and the structural, transport, and magnetization data are presented. The superconducting transition temperature decreases monotonically with increasing carbon content in the full investigated range of substitution. By adjusting the nominal composition, Tc of substituted crystals can be tuned in a wide temperature range between 10 and 39 K. Simultaneous introduction of disorder by carbon substitution and significant increase of the upper critical field Hc2 is observed. Comparing with the nonsubstituted compound, Hc2 at 15 K for x = 0.05 is enhanced by more than a factor of 2 for H oriented both perpendicular and parallel to the ab plane. This enhancement is accompanied by a reduction of the Hc2-anisotropy coefficient g from 4.5 sfor the nonsubstituted compoundd to 3.4 and 2.8 for the crystals with x = 0.05 and 0.095, respectively. At temperatures below 10 K, the single crystal with larger carbon content shows Hc2 sdefined at zero resistanced higher than 7 and 24 T for H oriented perpendicular and parallel to the ab plane, respectively. Observed increase of Hc2 cannot be explained by the change in the coherence length due to the disorder-induced decrease of the mean free path only.

MgB2 single crystals: high pressure growth and physical properties

Single crystals of MgB2 with a size up to 1.5 × 0.9 × 0.2 mm3 have been grown with a high pressure cubic anvil technique. The crystal growth process is very peculiar and involves an intermediate nitride, namely MgNB9. Single crystals of BN and MgB2 grow simultaneously by a peritectic decomposition of MgNB9. Magnetic measurements with SQUID magnetometry in fields of 1–5 Oe show sharp transitions to the superconducting state at 37–38.6 K with a width of ~0.5 K. The high quality of the crystals allowed the accurate determination of magnetic, transport (electric and heat) and optical properties as well as scanning tunnelling spectroscopy (STS) and decoration studies. Investigations of crystals with torque magnetometry show that H//cc2 for high quality crystals is very low (24 kOe at 15 K) and saturates with decreasing temperature, while H//abc2 increases up to 140 kOe at 15 K. The upper critical field anisotropy γ = H//abc2/H//cc2 was found to be temperature dependent (decreasing from γ 6 at 15 K to 2.8 at 35 K). The effective anisotropy γeff, as calculated from reversible torque data near Tc, is field dependent (increasing roughly linearly from γeff 2 in zero field to 3.7 in 10 kOe). The temperature and field dependence of the anisotropy can be related to the double gap structure of MgB2 with a large two-dimensional gap and small three-dimensional gap, the latter of which is rapidly suppressed in a magnetic field. Torque magnetometry investigations also show a pronounced peak effect, which indicates an order–disorder phase transition of vortex matter. Decoration experiments and STS visualize a hexagonal vortex lattice. STS spectra in zero field evidence two gaps 3 meV and 6 meV with a weight depending on the tunnelling direction. Magneto-optic investigations in the far-infrared region with H//c show a clear signature of the smaller of the two superconducting gaps, completely disappearing only in fields higher than H//cc2.

Mixed Valence Copper(I,II) Binuclear Complexes with Unexpected Structure: Synthesis, Biological Properties and Anticancer Activity

We have synthesized and characterized a panel of new binuclear mixed valence Cu(I,II) complexes containing substituted 2-alkylthio-5-arylmethylene-4H-imidazolin-4-ones with unusual structure. These complexes are shown to be cytotoxic for various cell lines. We have found that these compounds did not intercalate DNA, inhibited number of polymerases (telomerase predominantly), accumulated in the cell nucleus, and caused DNA degradation. Preliminary studies revealed that lead compound inhibited human breast adenocarcinoma growth in mice model.

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.

The structures of Nd1+xBa2−xCu3O7−δ (x = 0.05, 0.22, 0.42) phases refined from X-ray single-crystal data

Abstract Crystal structures of Nd 1+ x Ba 2− x Cu 3 O 7−δ phase ( x =0.05, 0.22 and 0.42) were refined from X-ray single-crystal data (tetragonal system, space group P4/mmm, Z =1, R =0.027, 0.027, 0.025 respectively). The main differences between these similar structures deal with the cation and oxygen sites in the (Ba/NdO) layer. Joint occupation of the same site by Ba and Nd ions with essentially different ionic radii results in the splitting of that site along the c -axis and the shift of the oxygen in this layer from the ideal position at the four-fold axis to achieve appropriate coordination for the smaller rare earth cation. These changes may be the structural reason for the suppression of superconductivity in R 1+ x Ba 2− x Cu 3 O 7−δ (R is rare earth) phases.

MgNB9, a new magnesium nitridoboride

The structure of a new magnesium nitridoboride, MgNB(9), has been refined from single-crystal X-ray data. The Mg and N atoms lie on sites with crystallographic 3m symmetry. The structure consists of two layers alternating along the c axis. The NB(6) layer, with B(12) icosahedra, has the C(2)B(13) structure type. Within this layer, boron icosahedra are bonded to N atoms, each coordinating to three boron polyhedra. Another MgB(3) layer, with B(6) octahedra, does not belong to any known structure type. The boron icosahedra and octahedra are connected to each other, thus forming a three-dimensional boron framework.

Single crystal growth and properties of MgB2 and Mg(B1−xCx)2

Abstract Single crystals of MgB2 and Mg(B1−xCx)2 have been grown using cubic anvil technique. Tc values vary in a wide range (39–9 K) with carbon content varying from 0% up to 16%. Using SiC as the precursor leads to C and not to Si substituted crystals. Micro-hardness measurements performed on MgB2 single crystals give average value of 1100 kg/mm2.

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.

Structure determination of bis{(4Z)-1-(2-azidoethyl)-4-[(pyridin-2-yl)methylidene]-2-thiolatoimidazol-5(4H)-one}dicopper chloride from X-ray powder diffraction data

The new tridentate organic N2S-type ligand, (4Z)-1-(2-azidoethyl)-4-[(pyridin-2-yl)-methylidene]-2-thioxoimidazol-5(4H)-one (LH), was synthesized. The reaction of LH with copper(II) chloride in a CH2Cl2/MeOH mixture afforded the coordination compound of the composition L2Cu2Cl (3). The crystal structure of 3 was solved and refined from X-ray powder diffraction data. Complex 3 has a binuclear structure with a short interatomic Cu-Cu distance and one bridging chlorine atom, which is located almost symmetrically with respect to both metal atoms. The planar organic ligands are noncoplanar and are at an angle of 53.56(1)° to each other.

The high-pressure form of cadmium vanadate, CdV2O6

The crystal structure of a new high-pressure modification of cadmium divanadium hexaoxide, CdV(2)O(6), was refined from X-ray single-crystal data. It contains zigzag chains of edge-sharing VO(6) octahedra. Octahedra in adjacent chains share corners and form corrugated layers. Octahedrally coordinated Cd atoms, which lie on twofold axes, are situated between the layers. The columbite-like structure results in a strong distortion of the CdO(6) octahedra which may be stabilized only at high pressure.