Levantamiento de la silla de ruedas a la ducha

A comparison between the results of the DFT and the MAPLE calculations (cf. Table 4) leads to the conclusion that MAPLE can provide reasonable starting models, however, a prediction of the best model is not possible. MAPLE calculations of the optimized models revealed no significant differences. All LDA-optimized models had worse MAPLE values due to the fact that covalent bonding is overestimated by LDA and therefore slightly shorter, which has great influence on the MAPLE calculations. The GGA-optimized structures exhibit all less than 0.94% deviation from the theoretical MAPLE sums. In Table 3, the MAPLE values of the optimized model 1 are given in comparison to the values for the model derived by group- subgroup transformation. All partial values are in the expected range. Only the lithium position neighboring the void has a partial MAPLE value slightly below the expected range.

However, respective values of other compounds, which exhibit low coordination for Li+ _range

around 540 kJ/mol as well.[34,41]

2.5.3 Conclusions

In this contribution we present the first nitridosilicate nitride, namely Li2Sr4[Si2N5]N, with a

layered [Si2N5]7– substructure, which can be derived from the apophyllite structure type. The

title compound could be synthesized in closed tantalum ampoules using liquid lithium as fluxing agent. Apparently, the usage of cesium iodide as catalyst extends the spectrum of known quaternary lithium nitridosilicates. Therefore, we expect that a few more compounds can be afforded using lithium as fluxing agent and additional inorganic salts. To corroborate

the half-occupied N3– _{site MAPLE and DFT calculations were performed. In this context no}

higher ordering of the N3– _{site could be found either in the X-ray diffraction or the DFT}

calculations. Moreover, Li2Sr4[Si2N5]N is another example for the efficiency to develop new

quaternary lithium nitridosilicates using liquid lithium. With respect to the highly anisotropic layered nitridosilicate structure and the concomitant presence of isolated N3– _{ions, the high-}

2.5 Li2Sr4[Si2N5]N – A Layered Lithium Nitridosilicate Nitride 57 pressure behavior of Li2Sr4[Si2N5]N is of special interest as it might lead to a nucleophilic

attack of the nitride ions at the Si tetrahedral centers.

2.5.4 Experimental Section

All manipulations were performed with rigorous exclusion of oxygen and moisture in flame- dried Schlenk-type glassware on a Schlenk line interfaced to a vacuum line (10–3 mbar) or in an argon-filled glove box (Unilab, MBraun, Garching, O2 < 0.1 ppm, H2O < 0.1 ppm). Argon

(Messer–Griessheim, 5.0) was purified by passage through columns of silica gel (Merck),

molecular sieves (Fluka, 4 Å), KOH (Merck ≥ 85 %), P4O10 (Roth ≥ 99%, granulate) and

titanium sponge (Johnson Matthey, 99.5%, grain size ≤ 0.8 cm) at 700 °C. For the syntheses

of the title compound tantalum tubes (wall thickness 0.5 mm, internal diameter 10 mm, length

300 mm) were cleaned and the oxide layer removed by treatment with a mixture of HNO3

(concd.) and aqueous HF (40 %). The tubes were weld-shut in an arc furnace under 1 bar of purified argon. During this procedure, the crucible holder was water cooled in order to avoid chemical reactions during welding.

For the synthesis of Li2Sr4[Si2N5]N, silicon diimide (38.7 mg, 0.67 mmol, synthesized

according to literature[42]), LiN3 (16.3 mg, 0.33 mmol, Sigma Aldrich 99.9 %), Sr (59.9 mg,

0.68 mmol, Sigma Aldrich, 99.99%) and CsI (43.0 mg, 0.17 mmol, Sigma Aldrich, dried under vacuum) were mixed in an agate mortar, filled in a tantalum ampoule and covered with lithium (12 mg, 1.7 mmol, Alfa Aesar, 99.9%) in a glove box. The sealed tantalum ampoule was placed into a silica tube under argon, which was placed in the middle of a tube furnace.

The temperature was raised to 900 °C (rate 5 °C min–1), maintained for 24 h and cooled down

to 500 °C (rate 0.2 °C min–1). Subsequently, the sample was quenched to room temperature by

switching off the furnace. The sample contained Li2Sr4[Si2N5]N as orange highly air - and

58 2.5 Li2Sr4[Si2N5]N – A Layered Lithium Nitridosilicate Nitride

Table 5. Crystallographic data of Li2Sr4[Si2N5]N.

Formula Li2Sr4[Si2N5]N

Formula mass /g ∙ mol-1 _504.60

Crystal system tetragonal Space group I4m2 (no. 119) Cell parameters / pm a = 751.46(11)

c = 1508.9(3) Cell volume / 106 _pm3 _{V = 852.0(2)}

Formula units / cell 4

Crystal size / mm3 _{0.09 x 0.04 x 0.01}

ρcalcd. / g ∙ cm-3 3.934

μ / mm-1 25.136

F(000) 912

Diffractometer Stoe IPDS I

Temperature / K 293(2)

Radiation, monochr. Mo-Kα, (λ = 71.073 pm), graphite Absorption correction semi-empirical[21]

θ range / ° 2.7 - 27.5 Measured reflections 3500 Independent reflections 554 Observed reflections 499 Refined parameters 41 Flack parameter 0.50(4) GoF 1.091 R indices (Fo2≥ 2σ (Fo2)) R1 = 0.0499, wR2 = 0.1364

R indices (all data) R1 = 0.0534, wR2 = 0.1413 [a]

Max. / min. residual electron density/ eÅ-3

3.72/-2.26

[a] w = 1/[σ2_(F

2.5 Li2Sr4[Si2N5]N – A Layered Lithium Nitridosilicate Nitride 59 2.5.4.2 Chemical Analyses

Scanning electron microscopy was performed on a JEOL JSM-6500F equipped with a field emission gun at an acceleration voltage of 10 kV with a Si/Li EDX detector (Oxford Instruments, model 7418). Samples were prepared by placing single crystals on adhesive conductive pads and subsequently coating them with a thin conductive carbon film. Each EDX spectrum (Oxford Instruments) was recorded with the analyzed area limited onto one single crystal to avoid the influence of possible contaminating phases. EDX-analysis showed an atomic ratio Sr/Si = 1:1.7 which corroborates the formation of Li2Sr4[Si2N5]N.

2.5.4.3 Single Crystal X-ray Analysis

Single crystals of Li2Sr4[Si2N5]N were isolated under a microscope, which is integrated in a

glove box, enclosed in glass capillaries (Ø = 0.3 mm), and sealed under argon. Single-crystal X-ray diffraction data were collected at room temperature on a STOE IPDS I diffractometer

(Stoe and Cie GmbH, Darmstadt) with Mo-Kα radiation (λ = 0.71073 Å, graphite

monochromator). The structure was solved by direct methods after semi-empirical absorption correction. For the structure solution and refinement (see Table 5) the program package SHELX97 was used.[43] _{Further details on the crystal structure investigation may be obtained}

from the Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (Fax: +49-7247-808-666; E-mail: [email protected]), on quoting the depository number CSD-422596.