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Rüdiger Kniep

Publications and source records attributed to Rüdiger Kniep.

14 recordsLinked to original sources
PubMed2007

Control of channel shapes in a microporous manganese(II)-borophosphate framework by variation of size and shape of organic template cations.

The templated microporous compounds [H2(Templ.)][MnII{B2P3O12(OH)}], [templates: 1,3-diaminopropane, C3H10N2 (DAP); piperazine, C4H10N2 (PIP); 1,4-diazacyclo[2.2.2]octane, C6H12N2 (DABCO)] were prepared under mild hydrothermal conditions. The crystal structures (H2DAP-Mn: Pmc2(1) (no. 26), a=1259.43(5), b=949.86(5), c=1135.92(5) pm, Z=4; H2PIP-Mn: Ima2 (no. 46), a=1257.9(1), b=948.69(8), c=1158.19(8) pm, Z=4; H2DABCO-H2PIP-Mn: Ima2 (no. 46), a=1262.90(7), b=961.05(5), c=1151.42(7) pm, Z=4) are characterized by identical framework connectivities [MnII{B2P3O12(OH)}]2-, but vary in shapes (diameters) of the structural channels depending on the shapes of the templating molecule ions. The situation clearly reflects the directing effect of true templates during endotemplating reactions. The experimental results (preparation, chemical analyses, and X-ray refinements) are supported by detailed ab initio calculations (structure optimizations).

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PubMed2006

Chirality and magnetism in a novel series of isotypic borophosphates: M(II)[BPO4(OH)2] (M(II) = Mn, Fe, Co).

Three novel isotypic borophosphates M(II)[BPO4(OH)2] (M(II) = Mn, Fe, Co) have been hydrothermally synthesized and structurally characterized. The isotypic compounds crystallize in the chiral space group P3(1)21 or P3(2)21. Their crystal structures are characterized by edge-sharing helical M(II)O6-octahedral chains wound around 3(1) or 3(2) screw axes (along [001]) interconnected by borophosphate single chains of alternating BO2(OH)2 and PO4 tetrahedra perpendicular to [001], resulting in three-dimensional framework structures. The helical M(II)O6-octahedral chains lead to low-dimensional magnetic properties.

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PubMed2006

High-pressure chemistry of nitride-based materials.

Besides temperature at one atmosphere, the applied pressure is another important parameter for influencing and controlling reaction pathways and final reaction products. This is relevant not only for the genesis of natural minerals, but also for synthetic chemical products and technological materials. The present critical review (316 references) highlights recent developments that utilise high pressures and high-temperatures for the synthesis of new materials with unique properties, such as high hardness, or interesting magnetic or optoelectronic features. Novel metal nitrides, oxonitrides as well as the new class of nitride-diazenide compounds, all formed under high-pressure conditions, are highlighted. Pure oxides and carbides are not considered here. Moreover, syntheses under high-pressure conditions require special equipment and preparation techniques, completely different from those used for conventional synthetic approaches at ambient pressure. Therefore, we also summarize the high-pressure techniques used for the synthesis of new materials on a laboratory scale. In particular, our attention is focused on reactive gas pressure devices with pressures between 1.2 and 600 MPa, multi-anvil apparatus at P < 25 GPa and the diamond anvil cell, which allows work at pressures of 100 GPa and higher. For example, some of these techniques have been successfully upgraded to an industrial scale for the synthesis of diamond and cubic boron nitride.

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PubMed2006

Two new hybrid organic/inorganic copper(II)-oxovanadate(V) diphosphonates: [Cu2(phen)2(O3PCH2PO3)(V2O5)(H2O)] x H2O and [Cu2(phen)2(O3P(CH2)3PO3)(V2O5)] x C3H8. Synthesis, structure, and magnetic properties.

Two new hybrid organic/inorganic copper oxovanadium diphosphonates [Cu2(phen)2(O3PCH2PO3)(V2O5)(H2O)] x H2O (1) and [(Cu2(phen)2(O3P(CH2)3PO3)(V2O5)] x C3H8 (2) have been obtained by hydrothermal synthesis. The compounds are monoclinic, and they crystallize in the space group P2(1)/n with cell parameters of a = 11.788(2) A, b = 17.887(3) A, c = 14.158(2) A, and beta = 93.99(0) degrees and in the space group C2/c with cell parameters of a = 11.025(1) A, b = 18.664(2) A, c = 15.054(2) A, and beta = 90.06(0) degrees, respectively. Both compounds present two-dimensional frameworks built up from infinite chains of corner-sharing vanadium tetrahedra and diphosphonate groups connected by copper tetramers for (1) and copper dimers for (2). The remarkable feature of (2) is the encapsulation of propane molecules, stabilized by strong hydrogen bonding between the layers. The magnetic properties of the compounds have been investigated showing antiferromagnetic coupling with Tmax = 64 K for (1) and Curie-like paramagnetic behavior for (2).

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PubMed2006

Ba2[Ni3N2]: a low-valent nitridonickelate-synthesis, crystal structure, and physical properties.

The ternary alkaline-earth nitridonickelate Ba2[Ni3N2] (Ba2[NiI2Ni0N2]) was prepared by the reaction of mixtures of Ba2N and Ni in nitrogen gas of ambient back-pressure at 1173 K. The crystal structure determined by X-ray single-crystal and powder diffraction methods as well as from neutron diffraction data at various temperatures between 2 and 298 K is orthorhombic (Cmca (no. 64), 298 K: a=715.27(18) pm, b=1032.99(21) pm, c=740.12(20) pm) and provides the first example of a nitridonickelate with a two-dimensional complex anion. The Ni2 atom is described with a split position and the corresponding superstructure variants are investigated by theoretical full-potential nonorthogonal local-orbital calculations (FPLO). The average oxidation state of Ni in Ba2[Ni3N2] is +0.67, the lowest average value observed in nitridonickelates so far. Investigations of the physical properties demonstrate that Ba2[Ni3N2] acts as a "poor" metal with a large resistivity of approximately 2.7 mOmega cm at 300 K and exhibits low-dimensional magnetism with antiferromagnetic ordering at T approximately 90 K. XAS spectra correspond with low-valent Ni states.

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PubMed2006

An atomistic simulation scheme for modeling crystal formation from solution.

We present an atomistic simulation scheme for investigating crystal growth from solution. Molecular-dynamics simulation studies of such processes typically suffer from considerable limitations concerning both system size and simulation times. In our method this time-length scale problem is circumvented by an iterative scheme which combines a Monte Carlo-type approach for the identification of ion adsorption sites and, after each growth step, structural optimization of the ion cluster and the solvent by means of molecular-dynamics simulation runs. An important approximation of our method is based on assuming full structural relaxation of the aggregates between each of the growth steps. This concept only holds for compounds of low solubility. To illustrate our method we studied CaF2 aggregate growth from aqueous solution, which may be taken as prototypes for compounds of very low solubility. The limitations of our simulation scheme are illustrated by the example of NaCl aggregation from aqueous solution, which corresponds to a solute/solvent combination of very high salt solubility.

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PubMed2005

Chain structures in alkali metal borophosphates: synthesis and characterization of K3[BP3O9(OH)3] and Rb3[B2P3O11(OH)2].

Two new alkali metal borophosphates, K3[BP3O9(OH)3] and Rb3[B2P3O11(OH)2], were synthesized by applying solvothermal techniques using ethanol as solvent. The crystal structures were solved by means of single-crystal X-ray diffraction (K3[BP3O9(OH)3], monoclinic, C2/c (No. 15), a = 2454.6(8) pm, b = 736.3(2) pm, c = 1406.2(4) pm, beta = 118.35(2) degrees , Z = 8; Rb3[B2P3O11(OH)2], monoclinic, P2(1)/c (No. 14), a = 781.6(2) pm, b = 667.3(2) pm, c = 2424.8(5) pm, beta = 92.88(1) degrees , Z = 4). Both crystal structures comprise borophosphate chain anions. While for the rubidium compound a loop-branched chain motif is found as common for most of the chain anions in alkali metal borophosphates, the crystal structure of the potassium phase comprises the first open-branched chain with the highest phosphate content found so far in this group of compounds. Both chain anions are closely related to known anhydrous or hydrated phases, and the structural relations are discussed in terms of how the presence of OH groups and hydrogen bonds as well as number, charge, and size of charge balancing cations influence the 3D structural arrangement. The anionic entities are classified in terms of general principles of structural systematics for borophosphates.

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