TIPS IMPORTANTES EN LA FILOSOFÍA SUZUKI

Concluding from the average structure, Sr0.5Ba0.5Si2O2N2:Eu2+ _{is a highly disordered layered} oxonitridosilicate. The simulation of diffuse scattering in the powder pattern using a disorder model supports this thesis. Another approach is HRTEM (Figure 10).

Figure 10: HRTEM image (Fei Titan 80-300) of a Sr0.5Ba0.5Si2O2N2:Eu2+ crystallite showing one-dimensional disorder. In the FT (inset), there are diffuse streaks perpendicular to the layer plane that either interconnect Bragg reflections or appear in between rows of Bragg reflections.

The Fourier transformation (FT) of a high-resolution micrograph of a crystallite of Sr0.5Ba0.5Si2O2N2:Eu2+ shows sharp Bragg reflections interconnected by diffuse streaks and additional strong diffuse streaks in between as a consequence of many domain boundaries per area. To demonstrate the performance of the disorder model described above and also to illustrate the impact of domain size variation on diffraction patterns, SAED patterns were simulated, in dependence on the model compiled for Sr0.5Ba0.5Si2O2N2:Eu2+, and compared to experimental ones of SrSi2O2N2 (varying degree of disorder, Figure 11). In contrast to Sr0.5Ba0.5Si2O2N2:Eu2+, the domain sizes in the pure Sr compound vary more significantly. It also contains larger domains, however, all diffraction patterns can be simulated with the same disorder model assuming slightly different transition probabilities for the stacking possibilities.

For crystallites of SrSi2O2N2 the occurrence and intensity distribution of diffuse streaks varies due to varying domain sizes. To simulate such effects in SrSi2O2N2, the stacking probabilities in a stacking model were slightly changed towards more extreme values. Both rotation-twin (stacking mode 7 and 8; see Table 3) as well as anti-phase domain sizes (stacking mode 2; see Table 3) were varied in order to simulate more or less diffuse intensities while the other transition probabilities were kept on fixed values. In Figure 11, simulated SAED patterns with different stacking probabilities are compared to experimental ones (hkh layer, i.e. zone axis [101]). Corresponding to the chosen values, the appearance of doubled number of reflections

(due to twinning) in every second row and/or intense diffuse streaks connecting the Bragg reflections, respectively, can be observed as a consequence of varying domain sizes.

Figure 11. Comparison of electron diffraction patterns of SrSi2O2N2 (left: a, c, e (experimental, CM30/ST); right: b, d, f (simulations)). Top: calculation with the same stacking sequence used for PXRD simulation (weak parasitic reflections); middle: smaller sizes for anti-phase and rotation-twin domains (see text); bottom: larger sizes for anti-phase and rotation-twin domains (see text).

For SAED simulation (b) the values reported in section “Simulation of the diffuse scattering” were chosen. As a consequence, sharp Bragg reflections are interconnected by diffuse streaks primarily for rows with h = 2n + 1. For the SAED simulation (d) both rotation-twin and anti- phase domain sizes were reduced. Rotation-twin boundaries are now calculated with 10 % probability (domains size about 10 nm) and the occupation probabilities for set A and B positions equals each other (average domain size about few nm). Diffuse streaks are more clearly visible while doubled reflections due to twinning are about to disappear. SAED simulation (f) is contrary because domain sizes are enlarged. In detail rotation-twin boundaries occur with overall 0.1 % probability leading to domains with ≈ 1 μm of size. Anti-

phase boundaries were also reduced meaning that layers with metal atoms on alternative positions are positioned on each other with a probability of only 24 %. Diffuse streaks are therefore rather weak. The observed diffraction pattern resembles that of the ideal structure (EuSi2O2N2) with large twin domains.

2.3.4 Conclusion

A promising way of a systematic optimization of luminescent materials with defined properties is based on the detailed knowledge of the atomic structure and its influence on the luminescence parameters. Understanding and analyzing effects like stacking disorder in layered materials is therefore of great interest. To characterize samples with respect to such effects, the simulation of diffuse scattering in powder XRD patterns is a promising way because the degree of disorder in selected crystals may differ strongly. Thus, “best” crystals used for structural studies might not be representative for macroscopic samples.

Although average structure models for disordered systems do not contain information about the local structure, they are necessary to show e.g. isotypism to other structures. Furthermore, the uncommon red-shifted luminescence of Sr0.5Ba0.5Si2O2N2:Eu2+, compared to SrSi2O2N2:Eu2+, could be explained on the basis of the average structure model. The shift to longer wavelength can be explained by a less restricted local lattice relaxation at the Eu2+ sites because of shorter Eu-O bond length compared to SrSi2O2N2:Eu2+. The present luminescence investigation and also the reported structural data are significantly more reliable than prior results because of fully characterised samples. As shown by Rietveld refinement, the average structure of Sr0.5Ba0.5Si2O2N2:Eu2+ is isotypic to that of SrSi2O2N2, however, the real structures of the compounds differ significantly. Simulations of the powder pattern of Sr0.5Ba0.5Si2O2N2:Eu2+ prove a disorder model with many anti-phase and few twin boundaries, however more twin boundaries are present than reported for SrSi2O2N2. Because of the pseudo-symmetry of both the metal atom layers as well as the silicate layers, the presence of anti-phase domains does not change the metal coordination significantly. Moreover, disorder might influence luminescence properties noticeable, although in the present case this was not observed because of the pronounced pseudosymmetry. Not much is known about such effects. The results derived by detailed fitting of XRD patterns in this work encourage comparable investigations for other layered luminescence materials.

Acknowledgements

The authors thank Viola Duppel (MPI FKF Stuttgart) and Markus Döblinger (LMU Munich) for TEM investigations, Christian Minke for EDX measurements as well as Tobias Rosenthal and Florian Stadler (all at LMU Munich) for preliminary work. Financial support by the Fonds der Chemischen Industrie (FCI), Germany, and Philips Lumileds Lighting Company is gratefully acknowledged.

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