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R Nesper

Publications and source records attributed to R Nesper.

7 recordsLinked to original sources
PubMed2001

Ba4Si3Br2: a double salt of barium bromide and barium silicide containing a novel cyclotrisilicide unit.

A new compound of the composition Ba4Si3Br2 was obtained by reacting BaSi with a melt of BaBr2. Ba4Si3Br2 may be described as a double salt between a Zintl phase and a halide. The structure was determined by single-crystal X-ray diffraction (P21c, a= 1504.1(2), b = 884.5(1), c = 880.2(1) pm, beta = 101.93(2) degrees, R = 0.041 Rw = 0.079). The crystal structure contains isolated barium cations and bromide anions as well as novel singly bonded cyclotrisilicid units. The electronic structure has been studied by linear Muffin tin orbital (LMTO) band structure calculations, which reveal semiconducting behavior with a band gap of about 0.1 eV, and by an analysis of the electron localization function.

Journal Article↗
PubMed2000

Ce4

The yellow-orange oxonitridosilicate oxide Ce4[Si4O4N6]O was obtained by the reaction of cerium metal with Si(NH)2 and SiO2 in a radiofrequency furnace at 1560 degrees C. The crystal structure was determined by single-crystal X-ray diffraction (a = 1033.67(6) pm, P2,3, Z = 4, R1 = 0.0412, wR2 = 0.0678) and powder neutron diffraction. In the solid there are complex cations [Ce4O]10+ that are enveloped by a hyperbolical layer structure [Si4O4N6]10-. The layer is built up by corner-sharing SiON3 tetrahedra of Q3 type. The oxygen atoms of the SiON3 tetrahedra are terminally bound to Si, while all nitrogen atoms bridge two neighboring Si centres. The crystallographic differentiation of O and N was unequivocally possible by a careful evaluation of the single-crystal X-ray diffraction data combined with lattice-energy calculations by using the MAPLE concept (Madelung part of lattice energy). Furthermore the results were confirmed by the chemical analyses. Subsequently, the determined N/O distribution and their crystallographic ordering was proved by neutron powder diffraction. In accordance with the molar ratio Si:(O,N) = 2:5 the [Si4O4N6]10- network may be classified as a layer silicate. In this specific case a hyperbolically corrugated topology of the layers is observed; this is correlated to periodic nodal surface (PNS) representatives.

Journal Article↗
PubMed2000

Infinite, Linear, Unbranched Borynide Chains in LiB(x)-Isoelectronic to Polyyne and Polycumulene The financial support by the Swiss National Science Foundation (projects 21-36586.92 and 4030-032775) is gratefully acknowledged. We thank Dr. V. Shklover for SEM pictures, Dr. M. Spahr for magnetic measurements, Dr. T. Chatterij and Dr. E. Souard for the neutron diffraction experiments, and Prof. Dr. S. Roth, Prof. Dr. H. Kuzmany, Prof. H. D. Lutz, and Dr. E. Suchanek for spectroscopic investigations.

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Journal Article↗
PubMed2000

Topological relationships and building blocks in Zintl phases of the composition Ba(n+1)(Mg,Li)2nSi2(n+1).

The structures of two novel Zintl phases, Ba6Mg5.2Li2.8Si12 and BaMg0.1Li0.9Si2, are presented. Both compounds contain chains in cis-trans conformation. The silicon partial structure of Ba6Mg5.2Li2.8Si12 (C2/m; a = 1212.0(1), b = 459.78(4), c = 1129.10(9) pm; beta = 91.77(2) degrees; Z = 1) is built of unbranched, planar Si6 chains while BaMg0.1Li0.9Si2 (Pnma; a = 725.92(5), b = 461.36(3), c = 1169.08(8) pm; Z = 4) consists of infinite Si(n) chains. The compounds show all electronic and structural characteristics that are typical for the special subset of Zintl phases with highly charged planar anions. The structures of the new compounds, as well as that of Ba2Mg3Si4, can be derived from the common parent type BaMg2Si2. It is shown that a comprehensive picture of a chemical twinning based on BaMg2Si2 can be derived.

Journal Article↗
PubMed2000

Elucidation of simple pathways for reconstructive phase transitions using periodic equi-surface (PES) descriptors. The silica phase system. I. Quartz-tridymite

A method of estimating short topological pathways for solid-solid reconstructive phase transitions is proposed. To screen the simplest pathways out of the infinite manifold in configurational space, a Fourier function approach is used, based on periodic nodal (PNS) and periodic equi-surface (PES) descriptors. The simplicity of the chosen functions representing the structures in question and the linear transition approach provide for most simple relevant transition models. Here it is shown that the tetrahedral networks of quartz and tridymite are represented topologically and transformed into each other by this approach. A trigonal network related to alpha-ThSi2 and B2O3 appears as intermediate during the transition model of the periodic functions. The transition path found in this way seems to be of exciting directness and of fundamental topological interest. The presented approach is not restricted to this specific case and is expected to be applicable to a wide variety of reconstructive phase transitions of solids.

Journal Article↗
PubMed1999

Conformation of peroxynitrite: determination by crystallographic analysis.

Peroxynitrite is an inorganic toxin of biological importance. It is formed in vivo from the diffusion-limited reaction of nitrogen monoxide with superoxide. Due to the partial double bond between the nitrogen and the first peroxide oxygen, peroxynitrite can occur in two conformations, cis and trans. The synthesis of tetramethylammonium peroxynitrite in ammonia [Bohle, D. S., et al. (1994) J. Am. Chem. Soc. 116, 7423-7424] yields small crystals if the ammonia is left to evaporate slowly. X-ray structure analysis shows that peroxynitrite crystallizes in the cis form, relative to the N-O bond. Crystal twinning or disorder prevents the determination of accurate bond lengths and bond angles. However, a nearly flat (torsion angle of 22 degrees ) molecule with O=N, N-O, and O-O bond lengths of 1.16, 1.35, and 1.41 A, respectively, would fit the observed electron density best. The space group of tetramethylammonium peroxynitrite is Pmmn (59), and it has the following unit cell dimensions: a = 7.1778(11) A, b = 8.6893(13) A, and c = 5.7266(9) A.

Crystallography↗