The Sialons MLn[Si4−xAlxOxN7−x] with M = Eu,Sr, Ba and Ln = Ho‐Yb – Twelve Substitution Variants with the MYb[Si4N7] Structure Type

The oxonitridoalumosilicates (so‐called sialons) MLn[Si_4?xAl_xO_xN_7?x] with M = Eu, Sr, Ba and Ln =Ho, Er, Tm, Yb were obtained by the reaction of the respective lanthanoid metal, the alkaline earth carbonates or europium carbonate, resp., AlN, “Si(NH)₂” and MCl₂ as a flux in a radiofrequency furnace at temperatures around 2100 °C. The compounds MLn[Si_4?xAl_xO_xN_7?x] are relevant for the investigation of substitutional effects on the materials properties due to their ability of tolerating a comparatively large phase width up to x ≈ 2.0(5). The crystal structures of the twelve compounds were refined from X‐ray single crystal data and X‐ray powder data and are found to be isotypic to the MYb[Si₄N₇] structure type. The compounds crystallize in space group P6₃mc (no. 186, hexagonal) and are made up of chains of so‐called starlike units [N^[4](SiN₃)₄] or [N^[4]((Si,Al)(O,N)₃)₄], respectively. These units are formed by four (Si,Al)(N/O)₄ tetrahedra sharing a common central nitrogen atom. The structure refinement was performed utilizing an O/N‐distribution model according to Paulings rules, i.e. nitrogen was positioned on the four‐fold bridging site and nitrogen and oxygen were distributed equally on both of the two‐fold bridging sites, resulting in charge neutrality of the compound. The Si and Al atoms were distributed equally on their two crystallographic sites, referring to their elemental proportion in the compound, due to being poorly distinguishable by X‐ray methods. The chemical compositions of the compounds were derived from electron probe micro analyses (EPMA).     

Pr10[Si10−xAlxO9+xN17−x]Cl with x ≈ 1 – an Oxonitridoaluminosilicate Chloride

The oxonitridoaluminosilicate chloride Pr₁₀[Si_10?xAl_xO_9+xN_17?x]Cl was obtained by the reaction of praseodymium metal, the respective chloride, AlN and Al(OH)₃ with “Si(NH)₂” in a radiofrequency furnace at temperatures around 1900 °C. The crystal structure was determined by single‐crystal X‐ray diffraction (Pbam, no. 55, Z = 2,a = 10.5973(8) Å, b = 11.1687(6) Å, c = 11.6179(7) Å, R1 = 0.0337). The sialon crystallizes isotypically to the oxonitridosilicate halides Ce₁₀[Si₁₀O₉N₁₇]Br, Nd₁₀[Si₁₀O₉N₁₇]Br and Nd₁₀[Si₁₀O₉N₁₇]Cl, which represent a new layered structure type. The structure refinement was performed utilizing an O/N‐distribution model according to Paulings rules, i.e. nitrogen was positioned on all bridging sites and mixed O/Noccupation was assumed on the terminal sites resulting in charge neutrality of the compounds. The Si and Al atoms were refined equally distributed on their three crystallographic sites, due to their poor distinguishability by X‐ray analysis. The tetrahedra layers of the structure consist of condensed [(Si,Al)N₂(O,N)₂] and [(Si,Al)N₃(O,N)] tetrahedra of Q² and Q³ type. The chemical composition of the compound was derived from electron probe micro analyses (EPMA).     

Darstellung,Eigenschaften und Kristallstruktur von RuSn6[(Al1/3–xSi3x/4)O4]2 (0 ≤ x ≤ 1/3) – einem Oxid mit isolierten RuSn6‐Oktaedern

Preparation, Properties, and Crystal Structure of RuSn₆[(Al_1/3–xSi_3x/4)O₄]₂ (0 ≤ x ≤ 1/3) – an Oxide with isolated RuSn₆ Octahedra RuSn₆[(Al_1/3–xSi_3x/4)O₄]₂ is obtained by the solid state reaction of RuO₂, SnO₂, Sn, and Si in an Al₂O₃‐crucible at 1273 to 1373 K. The compound is cubic with the space group Fm 3 m (a = 9.941(1) Å, Z = 4, R₁ = 0.0277, wR₂ = 0.0619), a semiconductor and stable in air. Results of Mößbauer measurements as well as bond length‐bond strength calculations justify the ionic formulation Ru²⁺Sn₆²⁺[(Al_1/3–x³⁺Si_3x/4⁴⁺)O₄^2–]₂. The central motif of the crystal structure are separated RuSn₆‐octahedrea. These are interconnected by oxygen atoms, arranged tetrahedrely above the surfaces of the RuSn₆‐octahedrea and partialy filled with Al and Si, respectively. Because of these features the compound can be considered as a variant of the crystal structure type of pentlandite.     

Color Point Tuning in Lu3−x−yMnxAl5−xSixO12:yCe3+ for White LEDs

A series of the solid‐solution phosphors Lu_3?x?yMn_xAl_5?xSi_xO₁₂:yCe³⁺ is synthesized by solid‐state reaction. The obtained phosphors possess the garnet structure and exhibit similar excitation properties as the phosphor Lu₃Al₅O₁₂:Ce³⁺, but with an effectively improved red component in the emission spectrum. This can be attributed to the energy transfer from Ce³⁺ to Mn²⁺. Our investigation reveals that electric dipole–quadrupole interactions dominate the energy‐transfer mechanism and that the critical distance determined by the spectral overlap method is about 9.21 Å. The color‐tunable emissions of the Lu_3?x?yMn_xAl_5?xSi_xO₁₂:yCe³⁺ phosphor as a function of Mn₃Al₂Si₃O₁₂ content are realized by continuously shifting the chromaticity coordinates from (0.354, 0.570) to (0.462, 0.494). They indicate that the obtained material may have potential application as a blue radiation‐converting phosphor for white LEDs with high‐quality white light.     

Site Preference of Cations and Structural Variation in MgAl2–xGaxO4 (0 ≤ x ≤ 2) Spinel Solid Solution

The crystal structures of MgAl_2–xGa_xO₄ (0 ≤ x ≤ 2) spinel solid solutions (x = 0.00, 0.38, 0.76, 0.96, 1.52, 2.00) were refined using ²⁷Al MAS NMR measurements and single crystal X‐ray diffraction technique. Site preferences of cations were investigated. The inversion parameter (i) of MgAl₂O₄ (i = 0.206) is slightly larger than given in previous studies. It is considered that the difference of inversion parameter is caused by not only the difference of heat treatment time but also some influence of melting with a flux. The distribution of Ga³⁺ is little affected by a change of the temperature from 1473 K to 973 K. The degree of order‐disorder of Mg²⁺ or Al³⁺ between the fourfold‐ and sixfold‐coordinated sites is almost constant against Ga³⁺ content (x) in the solid solution. A compositional variable of the Ga/(Mg + Ga) ratio in the sixfold‐coordinated site has a constant value through the whole compositional range: the ratio is not influenced by the occupancy of Al³⁺. The occupancy of Al³⁺ is independent of the occupancy of Ga³⁺, though it depends on the occupancy of Mg²⁺ according to thermal history. The local bond lengths were estimated from the refined data of solid solutions. The local bond length between specific cation and oxygen corresponds with that expected from the effective ionic radii except local Al–O bond length in the fourfold‐coordinated site and local Mg–O bond length in the sixfold‐coordinated site. The local Al–O bond length in the fourfold‐coordinated site (1.92 Å) is about 0.15 Å longer than the expected bond length. This difference is induced by a difference in site symmetry of the fourfold‐coordinated site. The nature that Al³⁺ in spinel structure occupies mainly the sixfold‐coordinated site arises from the character of Al³⁺ itself. The local Mg–O bond length in the sixfold‐coordinated site (2.03 Å) is about 0.07 Å shorter than the expected one. Difference Fourier synthesis for MgGa₂O₄ shows a residual electron density peak of about 0.17 e/ų in height on the center of (Ga_0.59 Mg_0.41)–O bond. This peak indicates the covalent bonding nature of Ga–O bond on the sixfold‐coordinated site in the spinel structure.     

Origin and Location of Electrons and Protons during the Formation of Intermetalloid Clusters [Sm@Ga3−xH3−2xBi10+x]3− (x=0, 1)

Reaction of [GaBi₃]^2? with [Sm(C₅Me₄H)₃] yielded the first protonated ternary intermetalloid clusters [Sm@Ga_3?xH_3?2xBi_10+x]^3? ( 1 ; x=0,1). The presence of the Ga? H bonds and the transfer of electrons and protons during the formation of 1 were elucidated by a combination of experimental and quantum chemical methods, thereby rationalizing the role of the solvent ethane‐1,2‐diamine as a Brønsted acid. As an organic by‐product, we observed the previously unknown octamethylfulvene ( 2 ) upon C? C coupling of (C₅Me₄H)^?.     

IrIn7GeO8 = [IrIn6](GeO4)(InO4) und Verbindungen der Mischkristallreihe [IrIn6](Ge1+xIn1−4x/3O8) (0 ≤ x ≤ 0.75) – erste Oxide mit [IrIn6]‐Oktaedern

IrIn₇GeO₈ = [IrIn₆](GeO₄)(InO₄) and Compounds of the Solid Solution Series [IrIn₆](Ge_1+xIn_1?4x/3O₈) (0 ≤ x ≤ 0.75): First Oxides containing [IrIn₆] Octahedra The low valent indiumoxides IrIn₇GeO₈ = [IrIn₆](GeO₄)(InO₄) and [IrIn₆](Ge_1+xIn_1?4x/3O₈) (0 x ≤ 0.75) are formed by heating intimate mixtures of Ir, In, In₂O₃ and GeO₂ in corundum crucibles under an atmosphere of argon (1420 K, 70 h). The compounds are black and semiconducting. X‐ray powder diffraction patterns can be indexed on the basis of a face centered cubic unit cell with lattice parameters ranging from a = 1012.3(1) pm (x = 0) to a = 1007.3(1) pm (x = 0.75). Characteristic building units in [IrIn₆](Ge_1+xIn_1?4x/3O₈) are isolated [IrIn₆]⁹⁺ octahedra with short Ir‐In distances of 253.5 pm, which are linked via [GeO₄]^4? and [InO₄]^5? tetrahedra to a three dimensional framework. Starting from IrIn₇GeO₈ = [IrIn₆](GeO₄)(InO₄), the isoelectronic substitution of 4 In³⁺ ions by 3 Ge⁴⁺ ions and one Ge‐vacancy leads to the formation of a solid solution series [IrIn₆](GeO₄)_1+x(O₄)_x/3(InO₄)_1?4x/3, which shows a slight decrease in the cubic lattice parameter with increasing x. According to Rietveld refinements the structure of [IrIn₆](GeO₄)(InO₄) exhibits a statistical distribution of the tetrahedrally coordinated Ge and In atoms ( , R(prof.) = 4.4 %, R(int.) = 2.5 %). The crystal and electronic structures of [IrIn₆](GeO₄)(InO₄) are discussed on the basis of first principles electronic structure calculations.     

Ca3ZnGeO2[Ge4O12]: a Ca–Zn germanate related to the mineral taikanite

The title compound, Ca₃ZnGeO₂[Ge₄O₁₂] (tricalcium zinc germanium dioxide dodecaoxidotetragermanate), adopts a taikanite‐type structure. The tetrahedral [Ge₄O₁₂] chain geometry is very similar to that of the silicate chain of taikanite, i.e. BaSr₂Mn³⁺₂O₂[Si₄O₁₂], while the major difference is found parallel to the c axis. In taikanite, Mn³⁺ octahedra form an infinite zigzag chain, whereas the title compound has a chain of distorted ZnO₆ octahedra, which alternates with distorted GeO₄ tetrahedra connected to each other via two common edges. Eightfold‐coordinated Ca²⁺ polyhedra and ZnO₆ octahedra form a slab parallel to (001) which alternates with another slab containing the tetrahedrally coordinated Ge sites along the c axis.     

P2‐Na0.67AlxMn1−xO2: Cost‐Effective,Stable and High‐Rate Sodium Electrodes by Suppressing Phase Transitions and Enhancing Sodium Cation Mobility

Sodium layered P2‐stacking Na_0.67MnO₂ materials have shown great promise for sodium‐ion batteries. However, the undesired Jahn–Teller effect of the Mn⁴⁺/Mn³⁺ redox couple and multiple biphasic structural transitions during charge/discharge of the materials lead to anisotropic structure expansion and rapid capacity decay. Herein, by introducing abundant Al into the transition‐metal layers to decrease the number of Mn³⁺, we obtain the low cost pure P2‐type Na_0.67Al_xMn_1?xO₂ (x=0.05, 0.1 and 0.2) materials with high structural stability and promising performance. The Al‐doping effect on the long/short range structural evolutions and electrochemical performances is further investigated by combining in situ synchrotron XRD and solid‐state NMR techniques. Our results reveal that Al‐doping alleviates the phase transformations thus giving rise to better cycling life, and leads to a larger spacing of Na⁺ layer thus producing a remarkable rate capability of 96 mAh g^‐1 at 1200 mA g^‐1.     

P2‐Na0.67AlxMn1−xO2: Cost‐Effective,Stable and High‐Rate Sodium Electrodes by Suppressing Phase Transitions and Enhancing Sodium Cation Mobility

Sodium layered P2‐stacking Na_0.67MnO₂ materials have shown great promise for sodium‐ion batteries. However, the undesired Jahn–Teller effect of the Mn⁴⁺/Mn³⁺ redox couple and multiple biphasic structural transitions during charge/discharge of the materials lead to anisotropic structure expansion and rapid capacity decay. Herein, by introducing abundant Al into the transition‐metal layers to decrease the number of Mn³⁺, we obtain the low cost pure P2‐type Na_0.67Al_xMn_1?xO₂ (x=0.05, 0.1 and 0.2) materials with high structural stability and promising performance. The Al‐doping effect on the long/short range structural evolutions and electrochemical performances is further investigated by combining in situ synchrotron XRD and solid‐state NMR techniques. Our results reveal that Al‐doping alleviates the phase transformations thus giving rise to better cycling life, and leads to a larger spacing of Na⁺ layer thus producing a remarkable rate capability of 96 mAh g^‐1 at 1200 mA g^‐1.     

Composition optimization and UV‐annealing dependence on the electrical properties of Hf1−xSixO2/Si gate stacks

Hf_1?xSi_xO₂ gate dielectrics grown by UV‐photo‐induced chemical vapor deposition (UV‐CVD) using Hf(OBu^t)₂(mmp)₂ and tetraethoxysilane as precursors have been deposited on Si substrate. Composition dependence of the interfacial microstructure of the Hf_1?xSi_xO₂/Si gate stacks has been investigated via Fourier transform infrared spectroscopy (FTIR) systematically. It has been indicated that the physical properties of the Hf_1?xSi_xO₂ films can be effectively optimized by adjusting the silicon contents incorporated in the films. In order to evaluate its potential implementation as an alternative dielectric in future devices, detailed electrical characterization of Au/Hf_1?xSi_xO₂/Si capacitor has been performed as functions of the silicon contents and the UV‐annealing time. Copyright © 2010 John Wiley & Sons, Ltd.     

Ca2+‐Induced Oxygen Generation by FeO42− at pH 9–10

Although FeO₄^2? (ferrate(IV)) is a very strong oxidant that readily oxidizes water in acidic medium, at pH 9–10 it is relatively stable (<2 % decomposition after 1 h at 298 K). However, FeO₄^2? is readily activated by Ca²⁺ at pH 9–10 to generate O₂. The reaction has the following rate law: d[O₂]/dt=k_Ca[Ca²⁺][FeO₄^2?]². ¹⁸O‐labeling experiments show that both O atoms in O₂ come from FeO₄^2?. These results together with DFT calculations suggest that the function of Ca²⁺ is to facilitate O–O coupling between two FeO₄^2‐ions by bridging them together. Similar activating effects are also observed with Mg²⁺ and Sr²⁺.     

Structural links between zeolite-type and clathrate hydrate-type materials: Synthesis and crystal Structure of [N(CH3)4]6[AlxSi8−xO18−x(OH)2+x] · 44H2O (x = 3–4)

A novel tetramethylammonium aluminosilicate hydrate with the approximate composition [NMe₄]₆[Al_xSi_8?xO_18?x(OH)_2+x] · 44H₂O (x = 3–4) has been identified by powder X-ray diffraction as a component in a polyphasic solid mixture which crystallized at room temperature from an aqueous NMe₄OH? Al₂O₃? SiO₂ solution. Large crystals of the novel hydrate phase could be mechanically selected from that mixture. The crystal structure has been determined from 1 196 unique MoKα diffraction data measured at 180 K: Tetragonal crystal system, cell constants a = 16.181(4) and c = 17.450(4) Å, space group P4/mnc with Z = 2 formula units per unit cell, R = 0.072. The host-guest compound is of polyhedral clathrate type with a mixed three-dimensional, (mainly) four-connected network composed of oligomeric aluminosilicate anions [Al_xSi_8?xO_18?x(OH)_2+x]^6? and H₂O molecules linked via hydrogen bonds O? H …? O. The aluminosilicate anions possess a cube-shaped (double four-ring) structure. Orientationally disordered cationic guest species NMe₄⁺ are enclosed in the large [4⁶6⁸] and [4¹5¹⁰6⁷] polyhedral voids of the host framework; the small [4⁶] cages (i.e. the double four-ring anions) and [4³5⁶] cages are empty. The hydrate is a further member in a recently discovered series of clathrates with mixed tetrahedral networks, which provides a structure-chemical link between zeolite- and clathrate hydrate-type host-guest compounds.     

“114”‐Type Nitrides LnAl(Si4−xAlx)N7Oδ with Unusual [AlN6] Octahedral Coordination

Aluminum–nitrogen six‐fold octahedral coordination, [AlN₆], is unusual and has only been seen in the high‐pressure rocksalt‐type aluminum nitride or some complex compounds. Herein we report novel nitrides LnAl(Si_4−x Al_x)N₇O_δ (Ln=La, Sm), the first inorganic compounds with [AlN₆] coordination prepared via non‐high‐pressure synthesis. Structure refinements of neutron powder diffraction and single‐crystal X‐ray diffraction data show that these compounds crystallize in the hexagonal Swedenborgite structure type with P 6₃mc symmetry where Ln and Al atoms locate in anticuboctahedral and octahedral interstitials, respectively, between the triangular and Kagomé layers of [SiN₄] tetrahedra. Solid‐state NMR data of high‐purity La‐114 powders confirm the unusual [AlN₆] coordination. These compounds are the first examples of the “33‐114” sub‐type in the “114” family. The additional site for over‐stoichiometric oxygen in the structure of 114‐type compounds was also identified.     

Neue Vertreter des Er6[Si11N20]O‐Strukturtyps Hochtemperatur‐Synthesen und Kristallstrukturen von Ln(6+x/3)[Si(11–y)AlyN(20+x–y)]O(1–x+y) mit Ln = Nd,Er, Yb,Dy und 0 ≤ x ≤ 3, 0 ≤ y ≤ 3

New Representatives of the Er₆[Si₁₁N₂₀]O Structure Type. High‐Temperature Synthesis and Single‐Crystal Structure Refinement of Ln_(6+x/3)[Si_(11–y)Al_yN_(20+x–y)]O_(1–x+y) with Ln = Nd, Er, Yb, Dy and 0 ≤ x ≤ 3, 0 ≤ y ≤ 3 According to the general formula Ln_(6+x/3)[Si_(11–y)Al_yN_(20+x–y)]O_(1–x+y) (0 ≤ x ≤ 3, 0 ≤ y ≤ 3) four nitridosilicates, namely Er₆[Si₁₁N₂₀]O, Yb_6.081[Si₁₁N_20.234]O_0.757, Dy_0.33Sm₆[Si₁₁N₂₀]N, and Nd₇[Si₈Al₃N₂₀]O were synthesized in a radiofrequency furnace at temperatures between 1300 and 1650 °C. The homeotypic crystal structures of all four compounds were determined by single‐crystal X‐ray diffraction. The nitridosilicates are trigonal with the following lattice constants: Er₆[Si₁₁N₂₀]O: a = 978.8(4) pm, c = 1058.8(3) pm; Yb_6.081[Si₁₁N_20.243]O_0.757: a = 974.9(1) pm, c = 1055.7(2) pm; Dy_0.33Sm₆[Si₁₁N₂₀]N: a = 989.8(1) pm, c = 1078.7(1) pm; Nd₇[Si₈Al₃N₂₀]O: a = 1004.25(9) pm, c = 1095.03(12) pm. The crystal structures were solved and refined in the space group P31c with Z = 2. The compounds contain three‐dimensional networks built up by corner sharing SiN₄ and AlN₄ tetrahedra, respectively. The Ln³⁺ and the “isolated” O^2– ions are situated in the voids of the structures. According to Ln_(6+x/3)[Si_(11–y)Al_yN_(20+x–y)]O_(1–x+y) an extension of the Er₆[Si₁₁N₂₀]O structure type has been found.     

A New Cu, Al Fluoride Disilicate CuAl2F2(Si2O7) and its Relations to Topaz

CuAl₂F₂(Si₂O₇) has been prepared by hydrothermal synthesis and its crystal structure was determined by single crystal X‐ray diffraction: space group Pnma, a = 8.8697(9), b = 14.084(2), c = 4.7553(5) Å, wR₂ = 0.056, R = 0.022. Cu²⁺ shows elongated square pyramidal coordination. Edge‐ and corner‐sharing [AlO₄F₂] octahedra with fluorine atoms in cis position form layers parallel to the ac plane. Along b these layers are linked by Si₂O₇ groups to form a three‐dimensional framework [Al₂F₂(Si₂O₇]^2–. In addition, the [CuO₅] pyramides connect two Al octahedra of neighbouring layers. The crystal structure is discussed as a derivative from topaz structure. The modular (or polysomatic) approach is used for this purpose, and for modelling hypothetical related compounds.     

AlIII-Phthalocyanine: Darstellung,Eigenschaften und Kristallstruktur von Tetra(n-butyl)ammonium-trans-di(nitrito(O))phthalocyaninato(2−)aluminat(III)

Al^III Phthalocyanines: Synthesis, Properties, and Crystal Structure of Tetra(n-butyl)-ammonium-trans-di(nitrito(O))phthalocyaninato(2?)aluminate(III) [Al(Cl)Pc^2?] reacts with excess (ⁿBu₄N)NO₂ in dimethylformamide yielding less soluble blue tetra(n-butyl)ammonium-trans-di(nitrito(O))phthalocyaninato(2?)aluminate(III), (ⁿBu₄N)^trans[Al(ONO)₂Pc^2?], which crystallizes in the monoclinic space group C2/c (No. 15) with Z = 4. The Al atom is in the special position 4 d in the center of the Pc^2? ligand and the two nitrit ions are monodentate O-coordinated in a mutually trans arrangement to the Al atom. The Al? O and average Al? N_iso bond distances are 1.927(2) and 1.956 Å, respectively. The geometric data of the coordinated nitrite ion are: d(N? O) = 1.277(4) Å; d(N? O) = 1.221(4) Å; ?(O? N? O) = 114.3(3)°; ?(Al? O? N) = 121.3(2)°. The non-bonded O atoms are trans to the Al atom. The Pc^2? ligand is slightly ruffled. The UV-VIS-NIR spectra and the vibrational spectra are discussed.     

Preparation of the Solid Electrolytes Li4+xAlxSi1‐xO4‐yLi3PO4 and Their Ionic Conductivity

The ion conductors Li_4+xAl_xSi_1‐xO₄‐yLi₃PO₄ (x = 0 to 0.5, y = 0 to 0.6) were prepared by the Sol‐Gel method. The powder and sintered samples were characterized by DTA‐TG, XRD, SEM, and AC impedance techniques. The conductivity and sinterability increased when y increased from 0 to 0.4 in the Li_4+xAl_xSi_1‐xO₄‐yLi₃PO₄. The particle size of the powder samples is about 0.13 μm. The maximum conductivity at 20 °C is 3.128 × 10^?5s cm^?1 for Li_4.4Al_0.4Si_0.6O₄‐0.4 Li₃PO₄.     

[{μ‐Cy8Si8O13}2Ca(DME)Ca(THF)2] – The First Metallasilsesquioxane Derivative of a Heavier Alkaline Earth Metal

[{μ‐Cy₈Si₈O₁₃}₂Ca(DME)Ca(THF)₂] ( 2 ), the first metallasilsesquioxane derivative of a heavier alkaline earth metal, has been prepared by a reaction of Cy₇Si₇O₉(OH)₃ ( 1 ) with metallic Ca in liquid ammonia / THF followed by recrystallization from DME. In the course of the reaction ligand rearrangement under formation of the (Cy₈Si₈O₁₃)^? dianion takes place. In the dinuclear calcium complex 2 the anionic silsesquioxane cages act as bridging ligands. The Ca²⁺ ions are unsymmetrically coordinated by THF and DME molecules.     

Na6Si2O7 – The Missing Structural Link among Alkali Pyrosilicates

The crystal structure of sodium pyrosilicate (Na₆Si₂O₇) was solved from single crystal diffraction data and refined to an R index of 0.051 for 17034 independent reflections. The compound is triclinic with space group P (a = 5.8007(8) Å, b = 11.5811(15) Å, c = 23.157(3) Å, α = 89.709(10)°, β = 88.915(11)°, γ = 89.004(11)°, V = 1555.1(4) Å³, Z = 8, D_x = 2.615 g · cm^–3, μ(Mo‐K_α) = 7.94 cm^–1). A characteristic feature of the crystals is a twinning by reticular pseudo‐merohedry, which simulates a much larger monoclinic C centered lattice (V′ = 6220 Å³, Z = 32). The twin element corresponds to a twofold rotation axis running parallel to the [0 direction of the triclinic cell. The compound belongs to the group of sorosilicates, i.e. it is based on [Si₂O₇] groups, which are arranged in layers parallel to (100). Charge compensation within the structure is accomplished by monovalent sodium cations distributed among 24 crystallographically independent positions. They are coordinated by four to six nearest oxygen neighbors. Most of the coordination polyhedra can be approximately described as distorted tetrahedra or tetragonal pyramids. An alternative understanding of Na₆Si₂O₇ can be gained if the tetrahedrally coordinated sodium atoms are considered for the construction of a framework. Actually, each four of the dimers within a single slice are linked by a more or less distorted [NaO₄] tetrahedron. The resulting structural motif is similar to the one that can be observed in melilites, where linkage between the T₂O₇ (T: Al, Si) moieties is provided by [MgO₄]‐ (as in akermanite, Ca₂Mg[Si₂O₇]) or [AlO₄] tetrahedra (as in gehlenite, Ca₂Al[AlSiO₇]). By sharing common edges, the [NaO₄] tetrahedra in Na₆Si₂O₇ are forming columns running parallel to 25 . The resulting framework contains tunnels in which the more irregularly coordinated sodium cations are incorporated.