Mehmet Somer

The Cluster Anion Si94

Solely on the basis of Raman spectra and quantum chemical calculations, the previously unknown cluster anion Si94- (structure shown) was characterized and its structure determined. The anion is formed as a component of solid phases by the thermal decomposition of alkali metal monosilicides.

Vibrational Spectra of the Cluster Anions [E4]4- in themetallic Sodium and Barium Compounds Na4E4 and Ba2E4 (E = Si, Ge)

The vibrational spectra of the cluster anions [E4]4– (E = Si, Ge) in the metallic compounds Ba2E4 and Na4E4 have been measured and assigned based on the Td symmetry of the discrete tetrahedranide anion. Due to the lower site-symmetries in the respective crystals all degenerate modes are split, but to different extends. The characteristic breathing frequency ν(E–E) of the [E4]4– cluster appears exclusively in the Raman spectrum and is almost unaffected by the nature of counterions: ν(E–E) = 486 cm–1 (E = Si) and 276 and 278 (Ge), respectively. The calculated valence force constants fd (Si–Si) = 1.17 Ncm–1 (Na); 1.15 Ncm–1 (Ba) and fd (Ge–Ge) = 0.98 Ncm–1 (Na); 0.94 Ncm–1 (Ba) are in good agreement with those previously reported.

Structures and Properties of NbOF3 and TaOF3 — with a Remark to the O/F Ordering in the SnF4 Type Structure

Powder samples of NbOF3 und TaOF3 were prepared by heating mixtures of NbO2F and NbF5 or TaO2F and TaF5, respectively, in the corresponding stoichiometric ratio in platinum crucibles under argon atmosphere (180—220 °C). Both oxide fluorides are coulourless with a slight greyish tinge. They are sensitive to moisture and decompose in air at room temperature within hours. Both, NbOF3 and TaOF3 crystallize as a variant of the SnF4 type structure, space group I4/mmm. The structures have been refined from X-ray powder diffraction data using the Rietveld method (a = 3.9675(1) A, c = 8.4033(1) A, RB = 3.60 %, Rp = 4.58 % for NbOF3 and a = 3.9448(1) A, c = 8.4860(1) A, RB = 2.07 %, Rp = 2.44 % for TaOF3). Characteristic building units are sheets of corner sharing MX6 octahedra which are stacked via van der Waals interactions to a three dimensional framework. The occupancy of the two crystallographic sites for the anions by O and F is discussed on the basis of structure refinements, bond order summations, IR and NMR data and calculations of the Madelung parts of the lattice energy. Struktur und Eigenschaften von NbOF3 und TaOF3 — mit einer Bemerkung zur O/F-Ordnung im SnF4-Typ Pulverproben von NbOF3 und TaOF3 bilden sich beim Erhitzen inniger Gemenge von NbO2F und NbF5 bzw. TaO2F und TaF5 im stochiometrischen Verhaltnis im Pt-Rohr unter Argon atmosphare (180—220 °C). Beide Oxidfluoride sind farblos, weisen aber immer grauen Schimmer auf. Sie sind feuchtigkeitsempfindlich und zerfliesen bei Raumtemperatur an Luft innerhalb von Stunden. NbOF3 und TaOF3 sind isotyp und kristallisieren in einer Variante der SnF4-Struktur, Raumgruppe I 4/mmm. Die Strukturen wurden aus Rontgenpulverdaten mittels der Rietveldmethode verfeinert (a = 3.9675(1) A, c = 8.4033(1) A, RB = 3.60 %, Rp = 4.58 % fur NbOF3 und a = 3.9448(1) A, c = 8.4860(1) A, RB = 2.07 %, Rp = 2.44 fur % TaOF3). Charakteristische Baueinheiten sind Schichten eckenverknupfter [MX6]-Oktaeder (M = Nb, Ta) die uber van der Waals Wechselwirkungen zum dreidimensionalen Gerust verknupft sind. Die Verteilung der O- und F-Atome auf die beiden kristallchemisch verschiedenen Lagen wird anhand der Verfeinerungen der Kristallstruktur, Bindungsordnungssummen, IR-, NMR-Daten und Berechungen des Madelunganteils der Gitterenergie diskutiert.

The [Sn5]2– Cluster Compound [K‐(2,2,2‐crypt)]2Sn5 – Synthesis, Crystal Structure, Raman Spectrum, and Hierarchical Relationship to CaIn2

Dark red crystals of [K-(2,2,2)-crypt)]2Sn5 precipitate after the reaction of (2,2,2)-crypt with a solution of K1.33Sn in liquid ammonia at room temperature. The compound is sensitive to oxidation and hydrolysis. The sequence of Raman bands (104, 120, 133 and 180 cm–1) is characteristic for the trigonal bipyramidal closo-[Sn5]2– cluster anion. The wave numbers correspond with the data from Hartree-Fock calculations (114, 128, 142 and 187 cm–1). The compound crystallizes trigonally (a = 11.736 A, c = 22.117 A, Z = 2, space group P3c1 (No. 165); Pearson code hP262), isotypic with [Na-(2,2,2)-crypt)]2Pb5. The atoms of the cluster show strange anisotropic displacements, which are perfectly reducible to a helical rigid-body motion around and along [001] (libration: ± 9.5°; translation ± 0.29 A). The structure can be described as a hierarchical derivative of the initiator CaIn2 (P63/mmc, hP6), generated by an atom-to-aggregate replacement: [Ca][In]2 = [Sn5][K @ C36H72N4O12]2. Thus, the distribution of the [Sn5]2– Zintl anions is hexagonal primitive, and the cation complexes are located close to the centers of trigonal superprisms formed by Sn5 clusters.

Electron paramagnetic resonance and Raman spectroscopy studies on carbon-doped MgB2 superconductor nanomaterials

Undoped and carbon-doped magnesium diboride (MgB2) samples were synthesized using two sets of mixtures prepared from the precursors, amorphous nanoboron, and as-received amorphous carbon-doped nanoboron. The microscopic defect structures of carbon-doped MgB2 samples were systematically investigated using X-ray powder diffraction, Raman and electron paramagnetic resonance spectroscopy. Mg vacancies and C-related dangling-bond active centers could be distinguished, and sp3-hybridized carbon radicals were detected. A strong reduction in the critical temperature Tc was observed due to defects and crystal distortion. The symmetry effect of the latter is also reflected on the vibrational modes in the Raman spectra.

Defect structure of ultrafine MgB2 nanoparticles

Defect structure of MgB2 bulk and ultrafine particles, synthesized by solid state reaction route, have been investigated mainly by the aid of X-band electron paramagnetic resonance spectrometer. Two different amorphous Boron (B) precursors were used for the synthesis of MgB2, namely, boron 95 (purity 95%–97%, 98.5%, <250 nm), which revealed bulk and nanosized MgB2, respectively. Scanning and transmission electron microscopy analysis demonstrate uniform and ultrafine morphology for nanosized MgB2 in comparison with bulk MgB2. Powder X-ray diffraction data show that the concentration of the by-product MgO is significantly reduced when nanoboron is employed as precursor. It is observed that a significant average particle size reduction for MgB2 can be achieved only by using B particles of micron or nano size. The origin and the role of defect centers were also investigated and the results proved that at nanoscale MgB2 material contains Mg vacancies. Such vacancies influence th...

Vertically oriented hexagonal mesoporous zirconia thin films by block copolymer templating

We report the synthesis of vertically oriented, long-range ordered hexagonal mesoporous zirconia thin films. The orientation of hexagonally ordered cylindrical mesopores in thin films was effectively controlled by taking advantage of the temperature dependent hydrophobicity of the templating block copolymer PEO–PPO–PEO. Vertical orientation was obtained when temperature was 30 °C or above throughout the process. Dehydration and enhanced chemical incompatibility between the PEO and PPO blocks at slightly elevated temperatures are proposed to play a key role in the vertical orientation of the mesopores. The temperature changes act on the entire film rather than just at the interfaces and the process can be applied to any mesoporous inorganic film.

Orthorhombic Ba3[BN2]2: a new structure type for an alkaline earth metal nitrido borate

Abstract Yellow, transparent single crystals of Ba 3 [BN 2 ] 2 (P2 1 2 1 2 1 (No. 19), a =424.73(2), b =1105.60(4) and c =1475.72(6) pm, Z =4) are obtained by the reaction of Ba 2 N and BN in arc-welded Nb ampoules at 1300 K. The title compound crystallizes, despite its familiar 3:2:4 stoichiometry, in a new structure type. The vibrational spectra of orthorhombic Ba 3 [BN 2 ] 2 have been measured and assigned. The IR spectrum reveals strong band splitting arising from the low site symmetry and the presence of two different sets of crystallographically independent [BN 2 ] 3− groups. The new structure and the vibrational data are compared with those previously reported for the cubic form of Ba 3 [BN 2 ] 2 .

Al-doped MgB2 materials studied using electron paramagnetic resonance and Raman spectroscopy

Undoped and aluminum (Al) doped magnesium diboride (MgB2) samples were synthesized using a high-temperature solid-state synthesis method. The microscopic defect structures of Al-doped MgB2 samples were systematically investigated using X-ray powder diffraction, Raman spectroscopy, and electron paramagnetic resonance. It was found that Mg-vacancies are responsible for defect-induced peculiarities in MgB2. Above a certain level of Al doping, enhanced conductive properties of MgB2 disappear due to filling of vacancies or trapping of Al in Mg-related vacancy sites.

Direct Experimental Determination of the Optimum Chromium Concentration in Continuous-Wave Cr

We employed several experimental techniques to measure the concentration dependence of the important laser parameters, and directly determine the optimum ion concentration for continuous-wave (CW) operation in room temperature Cr2+:ZnSe lasers. By using diffusion doping, 40 polycrystalline Cr2+:ZnSe samples with ion concentrations in the range of 0.8 times 1018 to 66 times 1018 ions/cm3 were prepared and used in this paper. Based on the spectroscopic measurements, empirical formulae showing the concentration dependence of the passive laser losses, fluorescence lifetime, and the fluorescence efficiency were obtained. By using the fluorescence efficiency data, the optimum chromium concentration, which maximizes the 2400-nm fluorescence intensity at a fixed excitation power, was determined to be 6 times 1018 ions/cm3. The dependence of the optimum concentration on sample length was further discussed. The CW power performance of the samples was also evaluated. At an incident pump power of 2.1 W, the optimum concentration for lasing was determined to be 8.5 times 1018 ions/cm3 that was in good agreement with the fluorescence measurements. The predictions of the fluorescence analysis and laser power measurements were in good agreement at low chromium concentrations. The observed discrepancy at higher doping levels was attributed to thermal loading.