Zur Kenntnis von LiSi2N3: Synthese und Verfeinerung der Kristallstruktur

On LiSi₂N₃ – Synthesis and Crystal Structure Refinement Single-crystalline LiSi₂N₃ was obtained by the reaction of lithium and silicon diimide under N₂ atmosphere in a radio-frequency furnace at 1300 °C. LiSi₂N₃ is isotypic with Li₂SiO₃ and it crystallizes as an ordered variant of wurtzite. A single-crystal X-ray diffraction analysis (Cmc2₁, a = 922.15(9), b = 529.64(8), c = 477.98(5) pm, Z = 4, R1 = 0.0173, wR2 = 0.0382) confirms and improves the structural data which previously had been obtained from powder diffraction data.     

Silicate und Germanate mit der Struktur des Silicocarnotits

Silicates and Germanates with the Structure of Silicocarnotite A number of lanthanoid-containing compounds Ca₃Ln₂(XO₄)₃ with X = Si, Ge have been prepared, which have the silicocarnotite structure. With the silicates, Ln ranges from Sm to Lu, with the germanates only from Sm to Tb. Ca₃Y₂(SiO₄)₃ crystallizes in this structure type, too. In the case of Ln = La? Nd, mixtures “Ca₃Ln₂(XO₄)₃” are formed containing apatite phases. The germanates Ca₃Ln₂(GeO₄)₃ with Ln = Dy? Lu; Y, Sc and Ca₃Sc₂(SiO₄)₃ have the garnet structure, as already known. In addition, the lead compounds Pb₃La₂(SiO₄)₃, Pb₃Y₂(SiO₄)₃ and Pb₂Y₃(SiO₄)₃O_0,5 have been synthesized. They have an apatite structure where the halogen positions are unoccupied or half-occupied.     

Mechanism of deleading of silicate glass by 0.5 N HNO3

Mechanism of removal of lead from silicate glass containing 68.5 wt% PbO by 0.5 N HNO₃ was investigated by incorporation of the chemical-analyses/weight-loss data into shrinking-core model (SCM) and minimization of the difference. Scanning electron microscopy (SEM), emission spectrometry with inductively coupled plasma (ICP), X-ray diffraction (XRD) and energy dispersive X-ray analysis (EDS) were used to determine the compositional changes of the lead-silicate glass (LSG) samples. Dual inter-diffusion chemical reaction mechanisms having respective activation energies of 83.49 and 47.80 kJ/mol dominated the deleading process.     

Spectroscopic properties and simulation of white-light in Dy-doped silicate glass

Spectroscopic properties of various concentrations Dy³⁺-doped silicate glasses were characterized by excitation and emission spectra. The optimal doping concentration of Dy³⁺ ions was found to be 3.0 wt%, and the nature of resonance energy transfer was confirmed to be electric dipole-dipole interaction according to Huang’s rule. Simulation of white-light for these glasses was also performed by varying the excitation wavelength. The results show that the white-light luminescence color could be tuned to various wavelength excitations, and the present silicate glass is more suitable for generation of white-light for blue LED chips.     

Characterisation of Siliceous Extra‐Framework Species in DAY Zeolites by 29Si MAS NMR and IR Spectroscopic Measurements

Dealuminated Y zeolites (DAY) were obtained by steaming of NH₄NaY at temperatures between 450 °C and 700 °C. They were characterised by means of ²⁷Al and ²⁹Si MAS NMR, IR spectroscopic and XRD measurements. The Si/Al framework ratios of samples were calculated using the ²⁹Si MAS NMR signal intensities, the wave numbers of the double‐ring vibration band w_DR and the asymmetrical TOT valence vibration w_TOT of IR spectra as well as the XRD lattice constant a₀. In contrast to actual Si/Al ratio obtained from w_DR and a₀, the NMR spectroscopic and w_TOT values were determined to be too high because of the superposition of the signals coming from dealuminated zeolite framework and silica gel which forms in the zeolite as a result of steaming. The differently determined Si/Al ratios characterise the siliceous extra‐framework species.     

The effect of changing the TiO2 content (0-6%) on the structure and thermal evolution of glasses obtained from porphyric sands and dolomite

Glasses have been obtained from mixtures of porphyric sands and dolomite. The influence of changes in the TiO₂ content (0-6%) on the glass structure and the formation of crystalline phases on reheating has been studied. The experimental results suggest that in the studied system TiO₂ promotes glass-in-glass phase separation and plays the role of a nucleating agent. The activation energy for crystal growth, E_C = 486 kJ mol⁻¹, and the Avrami parameter (m = 1) have been evaluated by means of thermoanalytical techniques in the case of the base glass (no TiO₂ added). The value of the Avrami parameter (m = 1) agrees well with SEM observations of dendritic crystal growth from surface nuclei. In the other glasses, lath-like crystals were observed, the microstructure being finer the greater the TiO₂%. The first addition of TiO₂ (2%) gives, on quenching, a partially devitrified product; subsequent additions of titania, surprisingly, allow glasses to be formed more easily. The experimental results suggest that Na₂O and K₂O, present in the porphyric sands and therefore in the glasses (to a total amount of ≈ 5.6%), segregate preferentially into the titania-rich phase with respect to MgO. Taking into account that Na₂O and K₂ are useful in lowering the liquidus temperature of glasses but are known to have a negative effect on the mechanical properties, this can be important in the production of glass ceramics.     

New approach for measuring gelation rate during synthesis of ZnO/SiO2 gel

The synthesis of mixed ZnO/SiO₂ oxides has been carried out using sol gel technique. Gelation time of the produced oxides gel has been measured experimentally by using turbidity change with time using Turbidimeter. In addition, gelation time was estimated visually. It is found that the gelation time is decreased by increasing the concentration of ZnO and SiO₂. Correlations between gelation times and concentrations of products are also discussed. Surface energy between the formed gel and solution is calculated as 12.1 mJ/m². The gelation rate is increased and the Gibbs free energy for the formation of critical nucleus is decreased by increasing the concentration product of ZnO · SiO₂. The critical radius of the nucleus is decreased from 5.75 to 5.02 Å when the concentration product is increased from 6.42% to 15.72%. On the other hand, the number of molecules in the critical nucleus is decreased from 11 to 8 when the concentration product changed under the same conditions. This approach can be used as a model to discuss the effect of any additives on the enhancing or inhibition the gelation rate for any gel.     

La3F3[Si3O9]: Das erste Fluoridsilicat aus dem ternären System LaF3/La2O3/SiO2

La₃F₃[Si₃O₉]: The First Fluoride Silicate in the Ternary System LaF₃/La₂O₃/SiO₂ By reacting La₂O₃ with LaF₃ and SiO₂ (silica gel) using CsCl as a flux (molar ratio 1 : 1 : 3 : 6; 700 °C, 21 d) it was possible to obtain single crystals of La₃F₃[Si₃O₉] (hexagonal, space group: P 6 2c (no. 190); a = 708.32(3), c = 1089.48(6) pm; Z = 2) as colourless, hexagonal platelets. The crystal structure comprises discrete cyclic [Si₃O₉]^6– anions of three corner-linked [SiO₄] tetrahedra along with a graphite-like network of the composition {[LaF_3/3]²⁺}. Shorter reaction times even produced single crystals of tysonite-type LaF₃ (trigonal, space group: P 3 c1 (no. 165); a = 718.80(6), c = 735.94(6) pm; Z = 6) on which a X-ray structure analysis was achieved, too. The structures of both compounds, each of which show an elevenfold anionic coordination (CN = 9 + 2) for the La³⁺ cations, are compared. The influence of the reactivity of the educts and the temperature on the reaction as well as the difficulties in the X-ray differentiation of fluorine and oxygen will be discussed.     

Alkalimetall‐Stannid‐Silicate und ‐Germanate: ‘Doppelsalze’ mit dem Zintl‐Anion [Sn4]4—

Alkaline Metal Stannide‐Silicates and ‐Germanates: ‘Double Salts’ with the Zintl Anion [Sn₄]^4— The crystal structures of the tetrelid tetrelates A₁₂[Sn₄]₂[GeO₄] (A = Rb/Cs: monoclinic, P2₁/c, a = 1289.1(2) / 1331.72(7), b = 2310.1(4)/ 2393.6(1), c = 1312.6(2)/ 1349.21(7) pm, β = 119.007(3)/ 118.681(1)°, Z = 4, R1 = 0.1049/0.0803) and Cs₂₀[Sn₄]₂[SiO₄]₃ (monoclinic, Cc, a = 2331.9(1), b = 1340.1(2), c = 1838.9(2) pm, β= 102.61(3)°, R1 = 0.0763) contain the Zintl anions [Sn₄]^4— and isolated oxotetrelate ions [MO₄]^4— (M = Si, Ge). The high temperature form of CsSn crystallizes with the KGe type (cubic, P4¯3n, a = 1444.7(1) pm, R1 = 0.0395).     

The role of organic acids in mineral weathering

Organic acids and their anions (for brevity we shall use the term “acids” to include both) may affect mineral weathering rates by at least three mechanisms: by changing the dissolution rate far from equilibrium through decreasing solution pH or forming complexes with cations at the mineral surface; by affecting the saturation state of the solution with respect to the mineral; and by affecting the speciation in solution of ions such as Al³⁺ that themselves affect mineral dissolution rate. In this paper we review the effects of organic acids on the dissolution rates of silicate minerals, particularly feldspars, under conditions approximating the natural weathering environment −25°C, pH 4–7 and with concentrations of organic acids comparable to those measured in soil solutions.

Feldspar dissolution rates far from equilibrium increase with decreasing pH below pH 4–5. They appear to be independent of pH between pH 4–5 and about 8, and above pH 8 feldspar dissolution rates increase with increasing pH.

Small chelating ligands such as oxalate appear to accelerate feldspar dissolution through complexation of Al at the surface of the mineral. Feldspar dissolution rates in the presence of 1 mM oxalic acid show effects ranging from no enhancement to enhancements of a factor of 15, depending upon the data set, pH, and aluminum content of the mineral: there is a great deal of scatter in the available data. In general, concentrations of oxalate of the order of 1 mM are necessary to cause a significant effect. Humic acids do not appear to increase feldspar dissolution rates significantly.

Dissolution rates must decrease as the solution approaches saturation with respect to the primary phase (the chemical affinity effect). Organic acids will influence chemical affinity by complexing Al (and possibly other elements) in solution and hence decreasing the chemical activity of Al³⁺. There are essentially no data on the effect of chemical affinity on feldspar dissolution rate at 25°C and mildly acid pH, so it is hard to evaluate the importance of organic acids in accelerating silicate dissolution through the chemical affinity effect. The effect of complexation of dissolved Al does not appear to be an important determinant of silicate dissolution rates in nature.

Observed rates of silicate weathering in the field are typically much slower than predicted from laboratory experiments far from equilibrium, suggesting control by transport of solutes between “micropores” and “macropores” (“micropores” include fractures and crystal defects within grains). If such transport is rate-controlling, analysis of the effect of organic acids on weathering rates in nature in terms of dissolution rates far from equilibrium may be misleading.