X-ray diffraction study of K3NdSi7O17: a new framework silicate with a linear Si-O-Si bond

Crystal chemistry and the role of ionic radius in rare earth tetrasilicates: Ba2RE2Si4O12F2 (RE = Er3+-Lu3+) and Ba2RE2Si4O13 (RE = La3+-Ho3+)

Structural variations across a series of barium rare earth (RE) tetrasilicates are studied. Two different formulas are observed, namely those of a new cyclo-silicate fluoride, BaRE₂Si₄O₁₂F₂ (RE = Er³⁺-Lu³⁺) and new compounds in the Ba₂RE₂Si₄O₁₃ (RE = La³⁺-Ho³⁺) family, covering the whole range of ionic radii for the rare earth ions. The Ba₂RE₂Si₄O₁₃ series is further subdivided into two polymorphs, also showing a dependence on rare earth ionic radius (space group P{\overline 1} for La³⁺-Nd³⁺, and space group C2/c for Sm³⁺-Ho³⁺). Two of the structure types identified are based on dinuclear rare earth units that differ in their crystal chemistries, particularly with respect to the role of fluorine as a structural director. The broad study of rare earth ions provides greater insight into understanding structural variations within silicate frameworks and the nature of f-block incorporation in oxyanion frameworks. The single crystals are grown from high-temperature (ca 953 K) hydrothermal fluids, demonstrating the versatility of the technique to access new phases containing recalcitrant rare earth oxides, enabling the study of structural trends.

Single-crystal X-ray diffraction study of Cs2Er[Si6O14]F and Cs2Er[Si4O10]F

Single-crystal growth experiments in the system CsF-Er2O3-SiO2 resulted in the simultaneous crystallization of two chemically related compounds within the same run: Cs2Er[Si6O14]F (phase I) and Cs2Er[Si4O10]F (phase II). They represent the first examples for cesium erbium silicates containing fluorine. Basic crystallographic data are – phase I: space group Cmca, a=17.2556(6) Å, b=24.6565(7) Å, c=14.4735(5) Å, V=6157.9(3) Å3, Z=16; phase II: space group Pnma, a=22.3748(7) Å, b=8.8390(2) Å, c=11.9710(4) Å, V=2367.5(1) Å3, Z=8. The structures were determined by direct methods and refined to residuals of R(|F|)=0.0229 for 2920 (phase I) and 0.0231 for 2314 (phase II) independent observed reflections with I>2σ(I). The structure of phase I represents a previously unknown structure type with a three dimensional tetrahedral framework consisting of Q3 and Q4 groups in the ratio 2:1. Basic building units of the network are unbranched sechser single-chains running parallel to [001]. The network can be conveniently built up from the condensation of tetrahedral layers parallel to (010) or (100), respectively. The crystal structure of phase II can be classified as a tubular or columnar chain silicate indicating that the backbones of the structure are multiple chains of silicate tetrahedra. This structure is isotypic to a Cs2Y[Si4O10]F, a compound that has been characterized previously. Alternatively, both compounds can be described as mixed octahedral-tetrahedral frameworks, which can be classified according to their polyhedral microensembles. A topological analysis of both nets is presented.

Atomic order and cluster energetics of a 17 wt% Si-based glass versus the liquid phase

Aerodynamic levitation of a multicomponent 17 wt% Si glass formed by rapid quenching of the melt phase was studied by high resolution x-ray diffraction (XRD) and reverse Monte Carlo (RMC) modelling. The main local atomic order features comprised interactions between Si, Fe and Mg polyhedra, the stereochemistry of which was on a par with the literature. Both the glass and the liquid state appeared to consist of the same fundamental Si-O, Fe-O and Mg-O clusters, with only the relative number of each varying between the two. Transition from liquid to glass involved a three-fold decrease in uncoordinated O (to within the first minimum of the total g(r)) and a marked increase of Fe-Si-Mg polyhedra bridging O. Octahedral Fe coordination was not suggested by the RMC data. All-electron open-shell density functional theory (DFT) calculations of the most prominent clusters suggested independence between the Fe oxidation state and its polyhedra O-coordination. Of secondary thermodynamic importance were indications of network-forming Fe(2+) and Fe(3+) distorted trigonal and tetrahedral polyhedra. In all occasions, the Fe ferrous and ferric states involved comparable binding energies within similar clusters which indicate a dynamic equilibrium between the two.

Structural, spectroscopic, and computational studies on Tl4Si5O12: a microporous thallium silicate

Single crystals of the previously unknown thallium silicate Tl4Si5O12 have been prepared from hydrothermal crystallization of a glassy starting material at 500 °C and 1 kbar. Structure analysis resulted in the following basic crystallographic data: monoclinic symmetry, space group C2/c, a = 9.2059(5) Å, b = 11.5796(6) Å, c = 13.0963(7) Å, β = 94.534(5)°. From a structural point of view the compound can be classified as an interrupted framework silicate with Q(3)- and Q(4)-units in the ratio 2:1. Within the framework 4-, 6-, and 12-membered rings can be distinguished. The framework density of 14.4 T-atoms/1000 Å(3) is comparable with the values observed in zeolitic materials like Linde type A, for example. The thallium cations show a pronounced one-sided coordination each occupying the apex of a distorted trigonal TlO3 pyramid. Obviously, this reflects the presence of a stereochemically active 6s(2) lone pair electron. The porous structure contains channels running along [110] and [-1 1 0], respectively, where the Tl(+) cations are located for charge compensation. Structural investigations have been completed by Raman spectroscopy. The interpretation of the spectroscopic data and the allocation of the bands to certain vibrational species have been aided by DFT calculations, which were also employed to study the electronic structure of the compound.

High-pressure synthesis and structural investigation of H3P8O8N9: a new phosphorus(V) oxonitride imide with an interrupted framework structure

The first crystalline phosphorus oxonitride imide H(3)P(8)O(8)N(9) (=P(8)O(8)N(6)(NH)(3)) has been synthesized under high-pressure and high-temperature conditions. To this end, a new, highly reactive phosphorus oxonitride imide precursor compound was prepared and treated at 12 GPa and 750 °C by using a multianvil assembly. H(3)P(8)O(8)N(9) was obtained as a colorless, microcrystalline solid. The crystal structure of H(3)P(8)O(8)N(9) was solved ab initio by powder X-ray diffraction analysis, applying the charge-flipping algorithm, and refined by the Rietveld method (C2/c (no. 15), a=1352.11(7), b=479.83(3), c=1820.42(9) pm, β=96.955(4)°, Z=4). H(3)P(8)O(8)N(9) exhibits a highly condensed (κ=0.47), 3D, but interrupted network that is composed of all-side vertex-sharing (Q(4)) and only threefold-linking (Q(3)) P(O,N)(4) tetrahedra in a Q(4)/Q(3) ratio of 3:1. The structure, which includes 4-ring assemblies as the smallest ring size, can be subdivided into alternating open-branched zweier double layers {oB,2(2)(∞)}[(2)P(3)(O,N)(7)] and layers containing pairwise-linked Q(3) tetrahedra parallel (001). Information on the hydrogen atoms in H(3)P(8)O(8)N(9) was obtained by 1D (1)H MAS, 2D homo- and heteronuclear (together with (31)P) correlation NMR spectroscopy, and a (1)H spin-diffusion experiment with a hard-pulse sequence designed for selective excitation of a single peak. Two hydrogen sites with a multiplicity ratio of 2:1 were identified and thus the formula of H(3)P(8)O(8)N(9) was unambiguously determined. The protons were assigned to Wyckoff positions 8f and 4e, the latter located within the Q(3) tetrahedra layers.

The dehydrated copper silicate Na(2)[Cu(2)Si(4)O(11)]: a three-dimensional microporous framework with a linear Si-O-Si linkage

The structure of the title dehydrated copper silicate, disodium dicopper undeca­oxide tetra­silicate, Na₂(Cu₂O₁₁Si₄), was determined by single-crystal X-ray diffraction from a non-merohedral twin. It exhibits an effective three-dimensional microporous framework with the major channels, in which the Na⁺ cations are placed, running along the a-axis direction and smaller channels observed along the b-axis direction. The structure is unusual in that it contains a symmetry-constrained Si—O—Si angle of 180°. The Cu centre is coordinated to five O atoms, exhibiting a slightly distorted square-pyramidal coordination geometry. The Na cation is interacting with five neighbouring O atoms, exhibiting an uncharacteristic coordination environment.