R. Kniep

An NMR Study of Biomimetic Fluorapatite : Gelatine Mesocrystals

The mesocrystal system fluoroapatite—gelatine grown by double-diffusion is characterized by hierarchical composite structure on a mesoscale. In the present work we apply solid state NMR to characterize its structure on the molecular level and provide a link between the structural organisation on the mesoscale and atomistic computer simulations. Thus, we find that the individual nanocrystals are composed of crystalline fluorapatite domains covered by a thin boundary apatite-like layer. The latter is in contact with an amorphous layer, which fills the interparticle space. The amorphous layer is comprised of the organic matrix impregnated by isolated phosphate groups, Ca3F motifs and water molecules. Our NMR data provide clear evidence for the existence of precursor complexes in the gelatine phase, which were not involved in the formation of apatite crystals, proving hence theoretical predictions on the structural pre-treatment of gelatine by ion impregnation. The interfacial interactions, which may be described as the glue holding the composite materials together, comprise hydrogen bond interactions with the apatite PO43− groups. The reported results are in a good agreement with molecular dynamics simulations, which address the mechanisms of a growth control by collagen fibers, and with experimental observations of an amorphous cover layer in biominerals.

SrN and SrN2: Diazenides by Synthesis under High N2-Pressure

Nitride and/or diazenide ions in the octahedral holes of nearly close-packed arrangements of Sr2+ ions characterize the new compounds described herein. They were obtained as single-phase products by oxidation of Sr2 N with molecular nitrogen under high pressure.

Pulse plasma synthesis and chemical bonding in magnesium diboride

A new way of preparation of magnesium diboride by pulse plasma technique is described which results in microcrystalline powder material. The superconducting transition with an onset at 37.3 K was observed in good agreement with the results of other groups obtained on samples prepared by conventional techniques. The phase exists near the nominal composition without a significant homogeneity range. The chemical bonding in magnesium diboride was analyzed by means of electron localization function.

Ca[(Ni1−xLix)N]: limited solid solutions (0 ⩽ x ⩽ 0.58) in the system Ca[NiN] (Y[CoC]-type structure)-Ca[LiN] (modified fluorite-type structure)

Abstract Solid solutions of composition Ca[(Ni 1− x Li x )N] (0 ⩽ x ⩽ 0.58) were prepared as polycrystalline materials by annealing mixtures of the ternary components Ca[NiN] and Ca[LiN]. Single crystals of the limiting composition Ca[(Ni 0.42 Li 0.58 )N]were grown from the melt (tetragonal, P 4 2 / mmc ; a = 372.3(1) pm, c = 665.6(1) pm; Z = 2; D x = 3.03 g cm −3 ). The crystal structure of the mixed crystal series (Y[CoC] type) contains linear ∞ 1 [(Ni 1− x Li x )N 2/2 ] chains with bond lengths ranging from 179.0 pm ( x = 0) to 186.1 pm ( x = 0.58).

Planar Fe6 Cluster Units in the Crystal Structure of RE15Fe8C25 (RE=Y, Dy, Ho, Er)

Carbometalates represent a special class of ternary and higher carbides containing complex anions 1 ðTyCzÞ (n = 0, 1, 2, 3) characterized by covalent bonds between the transition metals (T) and the highly polarizable (monoatomic) carbon species. The negative charge of the carbo ligands (C ) together with the low oxidation states of the transition metals cause high negative charges on the complex carbometalate anions, which have to be balanced by cations bearing a high positive charge, as brought about by rare-earth elements (RE), and resulting in the general formula REx[TyCz]. Carbometalates as electron-precise compounds without a tendency to form perceptible homogeneity ranges and exclusively containing monoatomic carbo ligands are shown to exist in a range of atomic ratios given by the condition (x + y)/z 2. In the systems REFeC, several ternary phases fulfilling this compositional condition have been reported. However, their crystal structures either contain C2 pairs as structural units instead of monoatomic carbo ligands, such as RE2[Fe(C2)4/2] (RE = Y, Tb–Lu), [2] Sc3[Fe(C2)4/2], [3, 4] RE[FeC2] (RE = Sc, Sm, Gd–Er, Lu), [5,6] and RE3.67[Fe(C2)3] (RE = La–Nd, Sm) ), or the crystal structures have not been determined (GdFeC, RE2Fe2C3, RE4Fe4C7 (RE = Ce, Gd)). 10] Our continuous search for carboferrates exclusively containing monoatomic carbo ligands has been unsuccessful so far. Instead, we obtained a new series of isotypic compounds, RE15Fe8C25 (RE = Y, Dy, Ho, Er), containing the novel structural unit of a Fe6 cluster surrounded by C2 pairs and a monoatomic carbon species. Crystal structure and materials properties as well as the chemical bonding situation in these compounds are discussed in detail. The metallic-gray rare-earth iron carbides were prepared by a high-temperature route, and their crystal structures were determined from X-ray diffraction data (single crystals as well as powders). 13] The crystal structure is exemplarily described for the Er compound. The striking feature of the crystal structure is an unusual planar group of six Fe atoms, which are arranged to form a triangle with Fe atoms at the vertices (Fe1) and at the midpoints of the edges (Fe2) as shown in Figure 1. The

Crystal structure of sodium gallium [monohydrogenmonophosphate-dihydrogenmonoborate-monophosphate],NaGa[BP2O7(OH)3]

BGaH3NaO10P2, monoclinic, C12/c1 (No. 15), a = 10.408(3) A, b = 8.094(2) A, c = 9.099(2) A, = 116.64(2)°, V = 685.2 A, Z = 4, Rgt(F) = 0.025, wRref(F ) = 0.068, T = 293 K. Source of material NaGa[BP2O7(OH)3] was synthesized under mild hydrothermal conditions. The reaction was carried out with mixtures of Ga metal (0.139 g) dissolved in 1ml of HCl (18%) with Na2HPO4·12H2O (1.075 g) and Na2B4O7·10H2O (0.4767 g) (molar ratio of Ga:P:B = 2:3:6) in aqueous solution. The mixture was sealed in glass tubes (after adding 1 ml H2O to achieve a degree of filling of 30%) with subsequent heating at 408 K for 60 days. The starting materials are all of analytical grade. Discussion With the increasing interest in microporous materials, synthesis of compounds like borophosphates with open framework structure have drawn much attention during the past few years and show a rich crystal chemistry [1]. Systems including a p-block metal have not been widely explored up to now besides one Al compound [2] reported only recently. The structure of the title compound is isotypic to the Fe [3] and Al analogues [2] and is characterized by isolated GaO4(OH)2 octahedra sharing common O-corners with phosphate and common O(OH)-corners with hydrogenborate groups from the ol igomeric uni ts [B2P2O7(OH)3]. The condensation of the borophosphate oligomers with Ga-coordination octahedra via common corners results in an overall three-dimensional framework which contains elliptical channels running along the [001] direction. The cross section of the channels is defined by eight-membered octahedral/tetrahedral rings (four Ga coordination octahedra, two phosphateand two borate-groups). Sodium ions are distributed within the open channels. The Ga—O bond distances are 1.925 and 1.965 A, while the Ga—OH value is increased to 1.995 A. The bond distances P—O and B—O in the oligomeric borophosphate groups correspond to respective values in the Feand Al-analogues [2,3]. Z. Kristallogr. NCS 216 (2001) 15–16 15

Thermo-chemical properties and electrical resistivity of Zr-based arsenide chalcogenides

abstract Ternary phases in the systems Zr–As–Se and Zr–As–Te were studied using single crystals of ZrAs1.40(1)Se0.50(1) and ZrAs1.60(2)Te0.40(1) (PbFCl-type of structure, space group P4/nmm) as well as ZrAs0.70(1)Se1.30(1) and ZrAs0.75(1)Te1.25(1) (NbPS-type of structure, space group Immm). The characterization covers chemical compositions, crystal structures, homogeneity ranges and electrical resistivities. At 1223 K, the Te-containing phases can be described with the general formula ZrAsxTe2–x, with 1.53(1)≤x≤1.65(1) (As-rich) and 0.58(1)≤x≤0.75(1) (Te-rich). Both phases are located directly on the tie-line between ZrAs2 and ZrTe2, with no indication for any deviation. Similar is true for the Se-rich phase ZrAsx–ySe2–x with 0.70(1)≤x≤0.75(1). However, the compositional range of the respective As-rich phase ZrAsx–ySe2–x (0.03(1)≤y≤0.10(1); 1.42(1)≤x≤1.70(1)) is not located on the tie-line ZrAs2–ZrSe2, and exhibits a triangular region of existence with intrinsic deviation of the composition towards lower non-metal contents. Except for ZrAs0.75Se1.25, from the homogeneity range of the Se-rich phase, all compounds under investigation show metallic characteristics of electrical resistivity at temperatures >20 K. Related uranium and thorium arsenide selenides display a typical magnetic field-independent rise of the resistivity towards lower temperatures, which has been explained by a non-magnetic Kondo effect. However, a similar observation has been made for ZrAs1.40Se0.50, which, among the Zr-based arsenide chalcogenides, is the only system with a large concentration of intrinsic defects in the anionic substructure.

Crystallographic disorder and electron scattering on structural two-level systems in ZrAs1.4Se0.5

Single crystals of ZrAs1.4Se0.5 (PbFCl-type structure) were grown by chemical vapour transport. While their thermodynamic and transport properties are typical for ordinary metals, the electrical resistivity exhibits a shallow minimum at low temperatures. Application of strong magnetic fields does not influence this anomaly. The minimum of the resistivity in ZrAs1.4Se0.5 apparently originates from interaction between the conduction electrons and structural two-level systems. Significant disorder in the As–Se substructure is inferred from x-ray diffraction and electron microprobe studies.

Crystal structure of sodium indium (monohydrogenmonophosphate- dihydrogenmonoborate-monophosphate), NaIn(BP2O7(OH)3)

B4H12In4Na4O40P8, monoclinic, C12/c1 (No. 15), a = 10.368(2) A, b = 8.520(1) A, c = 9.415(2) A, = 115.951(5)°, V = 747.8 A, Z = 1, Rgt(F) = 0.076, wRref(F) = 0.202, T = 293 K. Source of material NaIn[BP2O7(OH)3] was synthesized under mild hydrothermal conditions. The reactions were carried out with mixtures of In metal (0.230 g) dissolved in 1 ml of HCl (18%), Na2HPO4 · 12H2O (1.074 g) and Na2B4O7 · 10H2O (1.525 g) (molar ratio of In:P:B = 2:3:16) in aqueous solution. The mixtures were sealed in glass tubes (after adding 1 ml H2O to achieve a degree of filling of 30%) with subsequent heating at 408 K for 90 days. The starting materials were all of analytical purity grade. Experimental details The quality of the crystal studied was not good enough, which is reflected in the resulting high R-values. Discussion The synthesis of compounds like borophosphates with open framework structures has drawn much attention during the past few years due to their potential applications as microporous materials. The variety of crystal structures known up to now already shows a rich crystal chemistry [1]. Systems including a p-block metal have not been widely explored besides one Al [2] and one Ga [3] compound reported only recently. Two In containing phases are published in this issue [4,5]. The structure of the title compound is isotypic to the Fe [6], Al and Ga analogues [2,3] and is characterized by isolated InO4(OH)2 octahedra sharing common O-corners with hydrogen phosphate and common O(OH)-corners with hydrogenborate groups from the oligomeric units [BP2O7(OH)3]. The condensation of the borophosphate oligomers with In-coordination octahedra via common corners results in a three-dimensional framework which contains elliptical channels running along the [001] direction. The cross section of the channels is defined by eight-membered octahedral/tetrahedral rings (four In coordination octahedra, two hydrogen phosphateand two hydrogen borate-groups). Sodium ions are distributed within the open channels. The In—O bond distances 2.082 A and 2.110 A are shorter than the In—OH value (2.189 A). The bond distances P—O and B—O in the oligomeric borophosphate groups correspond to respective values in the Gaand Al-analogues [2,3] and other In compounds [4,5]. Z. Kristallogr. NCS 217 (2002) 7–8 7

Dimers (Al2N6)12- and Chains 1∞ (AlN 3-4/2) in the Crystal Structures of Ca6(Al2N6) and Ba3(Al2N4)

Pale yellow transparent single crystals of Ca6[Al2N6] (P21/c, No. 14, a = 693.7(3), b = 614.9(3), c = 987.1(5) pm, ß = 94.01(5)°; Z = 4) and colourless transparent single crystals of Ba3[Al2N4] (Pnna, No. 52, a = 617.9(2), b = 1005.2(4), c = 1023.0(4) pm; Z = 4) were obtained from reactions of mixtures of the representative metals with nitrogen at Tmax = 1000 °C. The crystal structure of Ca6[Al2N6] contains isolated units [Al2N6]12- built of two edge-sharing tetrahedra. Ba3[Al2N4] is an isotype of Sr3[Al2N4]. The crystal structure contains infinite chains 1∞ [AlN4/23-] of trans edge-sharing tetrahedra.