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Wolfgang Schnick

Publications and source records attributed to Wolfgang Schnick.

13 recordsLinked to original sources
PubMed2006

Crystal structure, physical properties and HRTEM investigation of the new oxonitridosilicate EuSi2O2N2.

The new layered oxonitridosilicate EuSi(2)O(2)N(2) has been synthesized in a radio-frequency furnace at temperatures of about 1400 degrees C starting from europium(III) oxide (Eu(2)O(3)) and silicon diimide (Si(NH)(2)). The structure of the yellow material has been determined by single-crystal X-ray diffraction analysis (space group P1 (no. 1), a=709.5(1), b=724.6(1), c=725.6(1) pm, alpha=88.69(2), beta=84.77(2), gamma=75.84(2) degrees ,V=360.19(9)x10(6) pm(3), Z=4, R1=0.0631, 4551 independent reflections, 175 parameters). Its anionic Si(2)O(2)N(2) (2-) layers consist of corner-sharing SiON(3) tetrahedra with threefold connecting nitrogen and terminal oxygen atoms. High-resolution transmission electron micrographs indicate both ordered and disordered crystallites as well as twinning. Magnetic susceptibility measurements of EuSi(2)O(2)N(2) exhibit Curie-Weiss behavior above 20 K with an effective magnetic moment of 7.80(5) mu(B) Eu(-1), indicating divalent europium. Antiferromagnetic ordering is detected at 4.5(2) K. EuSi(2)O(2)N(2) shows a field-induced transition with a critical field of 0.50(5) T. The four crystallographically different europium sites cannot be distinguished by (151)Eu Mössbauer spectroscopy. The room-temperature spectrum is fitted by one signal at an isomer shift of delta=-12.3(1) mm s(-1) subject to quadrupole splitting of DeltaE(Q)=-2.3(1) mm s(-1) and an asymmetry parameter of 0.46(3). Luminescence measurements show a narrow emission band with regard to the four crystallographic europium sites with an emission maximum at lambda=575 nm.

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PubMed2006

Nanocrystalline lanthanide nitride materials synthesised by thermal treatment of amido and ammine metallocenes: X-ray studies and DFT calculations.

The decomposition process of ammine lanthanide metallocenes was studied by X-ray diffractometry, spectroscopy and theoretical investigations. A series of ammine-tris(eta(5)-cyclopentadienyl)lanthanide(III) complexes 1-Ln (Lanthanide (Ln)=Sm, Gd, Dy, Ho, Er, Yb) was synthesised by the reaction of [Cp(3)Ln] complexes (Cp=cyclopentadienyl) with liquid ammonia at -78 degrees C and structurally characterised by X-ray diffraction methods, mass spectrometry and vibrational (IR, Raman) spectroscopy. Furthermore, amido-bis(eta(5)-cyclopentadienyl)lanthanide(III) complexes 2-Ln (Ln=Dy, Ho, Er, Yb) were synthesised by heating the respective ammine adduct 1-Ln in an inert gas atmosphere at temperatures of between 240 and 290 degrees C. X-ray diffraction studies, mass spectrometry and vibrational (IR, Raman) spectroscopy were carried out for several 2-Ln species and proved the formation of dimeric mu(2)-bridged compounds. Species 1-Ln are highly reactive coordination compounds and showed different behaviour regarding the decomposition to 2-Ln. The reaction of 1-Ln and 2-Ln with inorganic bases yielded lanthanide nitride LnN powders with an estimated crystallite size of between 40 and 90 nm at unprecedented low temperatures of 240 to 300 degrees C. Temperature-dependent X-ray powder diffraction and transmission electron microscopy (TEM) investigations were performed and showed that the decomposition reaction yielded nanocrystalline material. Structural optimisations were carried out for 1-Ln and 2-Ln by employing density functional (DFT) calculations. A good agreement was found between the observed and calculated structural parameters. Also, Gibbs free energies were calculated for 1-Ln, 2-Ln and the pyrolysis reaction to the nitride material, and were found to fit well with the expected ranges.

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PubMed2006

Crystal structure of the high-pressure phase of the oxonitridosilicate chloride Ce4[Si4O(3 + x)N(7 - x)]Cl(1 - x)O(x), x approximately = 0.2.

The structural compression mechanism of Ce4[Si4O(3 + x)N(7 - x)]Cl(1 - x)O(x), x approximately = 0.2, was investigated by in situ single-crystal synchrotron X-ray diffraction at pressures of 3.0, 8.5 and 8.6 GPa using the diamond-anvil cell technique. On increasing pressure the low-pressure cubic structure first undergoes only minor structural changes. Between 8.5 and 8.6 GPa a first-order phase transition occurs, accompanied by a change of the single-crystal colour from light orange to dark red. The main structural mechanisms, leading to a volume reduction of about 5% at the phase transition, are an increase in and a rearrangement of the Ce coordination, the loss of the Ce2, Ce3 split position, and a bending of some of the inter-polyhedral Si-N-Si angles in the arrangement of the corner-sharing Si tetrahedra. The latter is responsible for the short c axis of the orthorhombic high-pressure structure compared with the cell parameter of the cubic low-pressure structure.

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PubMed2004

Rubidium metaborate, Rb3B3O6.

Rubidium metaborate, Rb3B3O6, was obtained by the reaction of Rb2CO3 and BN using a radiofrequency furnace at a maximum reaction temperature of 1173 K. The crystal structure has been determined by single-crystal X-ray diffraction. The space group is R3c, with all atoms positioned on a twofold axis (Wyckoff site 18e). The ionic compound is isotypic with Na3B3O6, K3B3O6 and Cs3B3O6.

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PubMed2004

Characterization of the thermally induced topochemical solid-state transformation of NH4[N(CN)2] into NCN[double bond]C(NH2)2 by means of X-ray and neutron diffraction as well as Raman and solid-state NMR spectroscopy.

The mechanism of the solid-solid transformation of NH(4)[N(CN)(2)] into NCN[double bond]C(NH(2))(2), which represents the isolobal analogue of Wöhler's historic conversion of ammonium cyanate into urea, has been investigated by temperature-dependent single-crystal and powder X-ray diffraction, neutron powder diffraction, and Raman and solid-state NMR spectroscopy as well as thermoanalytical measurements. The transformation of the ionic dicyanamide into its molecular isomer upon controlled thermal treatment was found to proceed topochemically in the solid state with little molecular motion, giving rise to a single-crystal to single-crystal transformation which manifests itself by a defined metric relation between the unit cells of the two isomers. The exothermic phase transition is thermally activated and was observed to commence at temperatures > or =80 degrees C. The pronounced temperature dependence of the onset of the transformation may be assessed as an indication for the metastability of ammonium dicyanamide at elevated temperatures. Thermal analyses reveal a decrease in the reaction enthalpy (56-13 kJ mol(-1)) at higher heating rates and an average mass loss of 10% gaseous ammonia. Evidence was found for crucial mechanistic steps of the transformation, which is likely to proceed via proton transfer from the ammonium ion to one of the terminal nitrogen atoms of the anion. The protonation is followed by nucleophilic attack of the in situ generated ammonia at the electrophilic nitrile carbon. The proposed mechanistic pathway is based on the results of combined Raman and solid-state NMR spectroscopic as well as neutron powder diffraction measurements.

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PubMed2003

Melem (2,5,8-triamino-tri-s-triazine), an important intermediate during condensation of melamine rings to graphitic carbon nitride: synthesis, structure determination by X-ray powder diffractometry, solid-state NMR, and theoretical studies.

Single-phase melem (2,5,8-triamino-tri-s-triazine) C(6)N(7)(NH(2))(3) was obtained as a crystalline powder by thermal treatment of different less condensed C-N-H compounds (e.g., melamine C(3)N(3)(NH(2))(3), dicyandiamide H(4)C(2)N(4), ammonium dicyanamide NH(4)[N(CN)(2)], or cyanamide H(2)CN(2), respectively) at temperatures up to 450 degrees C in sealed glass ampules. The crystal structure was determined ab initio by X-ray powder diffractometry (Cu K alpha(1): P2(1)/c (No. 14), a = 739.92(1) pm, b = 865.28(3) pm, c = 1338.16(4) pm, beta = 99.912(2) degrees, and Z = 4). In the solid, melem consists of nearly planar C(6)N(7)(NH(2))(3) molecules which are arranged into parallel layers with an interplanar distance of 327 pm. Detailed (13)C and (15)N MAS NMR investigations were performed. The presence of the triamino form instead of other possible tautomers was confirmed by a CPPI (cross-polarization combined with polarization inversion) experiment. Furthermore, the compound was characterized using mass spectrometry, vibrational (IR, Raman), and photoluminescence spectroscopy. The structural and vibrational properties of molecular melem were theoretically studied on both the B3LYP and the MP2 level. A structural optimization in the extended state was performed employing density functional methods utilizing LDA and GGA. A good agreement was found between the observed and calculated structural parameters and also for the vibrational frequencies of melem. According to temperature-dependent X-ray powder diffractometry investigations above 560 degrees C, melem transforms into a graphite-like C-N material.

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PubMed2002

Transformation of ammonium dicyanamide into dicyandiamide in the solid.

Ammonium dicyanamide NH(4)[N(CN)(2)] was synthesized through aqueous ion exchange. The crystal structure was investigated by single-crystal X-ray diffraction (P2(1)/c, a = 378.67(6) pm, b = 1240.9(3) pm, c = 911.84(14) pm, beta = 91.488(18) degrees, Z = 4). It derives from the CsCl structure type. Medium strong hydrogen bonds between NH(4)(+) and [N(CN)(2)](-) ions are indicative of the observed formation of dicyandiamide H(4)C(2)N(4) during heating. According to DSC and temperature-dependent X-ray powder diffractometry, this isomerization is exothermic and occurs between 102 and 106 degrees C in the solid. The reaction represents the isolobal analogue to the classical synthesis of urea by heating NH(4)OCN. While other alkali and alkaline earth dicyanamides undergo trimerization or polymerization of their anions during heating, ammonium dicyanamide thus shows a different reactivity.

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PubMed2002

A density functional study of phosphorus nitride P3N5: refined geometries, properties, and relative stability of alpha-P3N5 and gamma-P3N5 and a further possible high-pressure phase delta-P3N5 with kyanite-type structure.

The crystal structures and the enthalpy-pressure phase diagram of P(3)N(5) were investigated by using density functional methods. Applying both approximations to the electron exchange and correlation gives a consistent picture for the two known polymorphs, alpha-P(3)N(5) and gamma-P(3)N(5). The calculated zone-center phonon modes compare very well with the experimental results. They indicate low-frequency bending modes for two-coordinate N atoms of alpha-P(3)N(5), which are responsible for a C2/c --> C c structural modulation of alpha-P(3)N(5) at moderate pressures. alpha-P(3)N(5) transforms into gamma-P(3)N(5) at pressures of about 6 GPa. We propose gamma-P(3)N(5) transforms into a delta-P(3)N(5) with Kyanite-type structure at pressures exceeding 43 GPa. Upon quenching, this triclinic modification of P(3)N(5) is likely to distort into a more symmetric monoclinic structure.

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PubMed2001

High-Pressure Synthesis of gamma-P(3)N(5) at 11 GPa and 1500 degrees C in a Multianvil Assembly: A Binary Phosphorus(V) Nitride with a Three-Dimensional Network Structure from PN(4) Tetrahedra and Tetragonal PN(5) Pyramids This work was supported by the Fonds der Chemischen Industrie and the Deutsche Forschungsgemeinschaft (Gottfried-Wilhelm-Leibniz-Programm). The authors thank Dr. D. Frost and Prof. Dr. D. C. Rubie, Bayerisches Geoinstitut, Universität Bayreuth, as well as Dr. P. Ulmer, ETH Zürich, for help in adapting the multianvil-technique. Also many thanks to W. Wünschheim, Department Chemie, LMU München for the development and programming of software for the high-pressure experiments.

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