Población

1. Highly Efficient pc-LED Phosphors Sr1-xBaxSi2O2N2:Eu2+ (0 ≤ x ≤ 1) - Crystal

Structures and Luminescence Properties Revisited

All known Sr1-xBaxSi2O2N2:Eu2+ (0 ≤ x ≤ 1) phases represent layered oxonitridosilicates with alternately stacked metal-ion and silicate layers. The corresponding structures are compared to each other, elaborating similarities and differences, mainly focusing on the relative orientation of consecutive silicate layers. The impact of various real-structure effects on diffraction patterns is shown and guidelines are worked out to avoid misinterpretation. As the

homogeneity of such samples is strongly related to the synthesis route, a possible reaction mechanism is postulated and possible impurity phases are discussed. Eu-doped Sr1-xBaxSi2O2N2:Eu2+ (0 ≤ x ≤ 1) samples shown intense luminescence from the blue to the yellow spectral region in dependence of the composition.

2. New Polymorph of the Highly Efficient LED-Phosphor SrSi2O2N2:Eu2+ - Polytypism

of a Layered Oxonitridosilicate

The monoclinic polymorph of SrSi2O2N2 was synthesized by a solid-state reaction (P21, a = 7.1036(14), b = 14.078(3), c = 7.2833(15) Å, β = 95.23(3)°, V = 725.3(3) Å3). The crystal structure is characterized by an alternating stacking sequence of silicate layers and metal-ion layers. The translation period along the stacking direction is doubled compared to the triclinic polymorph. The structure model was confirmed by TEM investigations in combination with HRTEM-image simulations. Single-crystal luminescence measurements yielded an emission wavelength of 532 nm (FWHM ~2600 cm-1) which is equal to a shift to smaller wavelengths by ~5 nm compared to the triclinic polymorph. The monoclinic SrSi2O2N2:Eu2+ polymorph is a very attractive phosphor material for enhancement of color rendition of white-light pc-LEDs.

3. Real Structure and Diffuse Scattering of Sr0.5Ba0.5Si2O2N2:Eu2+ - A Highly Efficient

Yellow Phosphor for pc-LEDs

Sr0.5Ba0.5Si2O2N2:Eu2+ shows intense emission in the yellow spectral range (λem ≈ 560 nm). Rietveld refinement reveals the average structure of Sr0.5Ba0.5Si2O2N2:Eu2+ (P1, a = 7.2059(2), b = 7.3887(3), c = 7.3340(2) Å, α = 88.524(4), β = 84.454(3), γ = 75.980(4)°, V = 377.07(2) Å3) which is isotypic to that of triclinic SrSi2O2N2:Eu2+. The PXRD pattern shows pronounced broad intensity maxima indicative for diffuse scattering from planar defects. In order to elucidate the real structure, PXRD simulations have been calculated based

on a disorder model, taking into account all possible metal-atom positions and silicate layer orientations. These simulations show that crystallites of Sr0.5Ba0.5Si2O2N2:Eu2+ are built up from small anti-phase domains within larger twin domains. Chromaticity coordinates are almost identical to the most frequently used commercial LED phosphor YAG:Ce3+ but a significantly higher luminous efficacy (LE = 495 lm/W) is measured. Therefore, LEDs with this material qualify for applications in e.g. outdoor lighting.

4. Unexpected Luminescence Properties of Sr0.25Ba0.75Si2O2N2:Eu2+ - A Narrow Blue

Emitting Oxonitridosilicate with Cation Ordering

The crystal structure of Sr0.25Ba0.75Si2O2N2:Eu2+ was determined using electron and X-ray diffraction methods (Pna21, a = 5.470(2), b = 14.277(3), c = 4.791(1) Å and V = 374.2(2) Å3). In crystallites suitable for TEM investigations, intergrowth of nanodomains is present, which leads to pronounced diffuse scattering. Corrugated metal-atom layers are present in the structure while lattice parameters are similar to those of the BaSi2O2N2- type. HRTEM image simulations indicate cation ordering, which, in combination with the corrugated metal atom layers, explains the unexpected excellent luminescence properties. Sr0.25Ba0.75Si2O2N2:Eu2+ shows intense blue emission (λem = 472 nm) and the FWHM of the emission band (37 nm) corresponds to the smallest value observed for Eu2+-doped blue emitting (oxo)nitridosilicates so far.

5. Material Properties and Structural Characterization of M3Si6O12N2:Eu2+ (M=Ba, Sr) -

A Comprehensive Study on a Promising Green Phosphor for pc-LEDs

The crystal structure of efficient green phosphor Ba3-xSrxSi6O12N2 (0 ≤ x ≤ 1) was refined on the basis of single-crystal and powder X-ray diffraction data (Ba3Si6O12N2, P3, a = 7.5218(1) Å, c = 6.4684(1) Å, V = 316.94(1)Å3_{). The layered oxonitridosilicate consists} of vertex-sharing SiO3N-tetrahedra forming 6er- and 4er-rings as fundamental building units. High- quality single crystals were the basis for the detailed analysis of structure-property relationships for the mixed phases. The availability of reliable crystallographic data was the basis for further theoretical investigations. The band gap was measured to be 7.05 ± 0.25 eV and therefore agrees well with calculated value of 6.93 eV. Ba3Si6O12N2:Eu2+ exhibits excellent luminescence properties (λ≈ 527 nm, FWHM ≈ 65 nm), which provides potential for application in pc-LEDs. For an increasing Sr- ratio a shift of the emission wavelength to lower energies is observed.

6. Magnesium Double Nitride Mg3GaN3 as New Host Lattice for Eu2+-Doping -

Synthesis, Structural Studies, Luminescence and Band-Gap Determination

The double nitride Mg3GaN3 and binary nitride Mg3N2 were synthesized from the elements at 760 °C in welded shut Ta-ampules. Mg3GaN3 (R3m, a = 3.3939(5), c = 25.854(5) Å, V = 257.91(7) Å3) consists of an electroneutral three-dimensional network of MgN4- and mixed (Mg/Ga)N4-tetrahedra which share common corners and edges. The determination of a structure model for Mg3GaN3 was the basis for first-principles DFT calculations. The most challenging part was the precise characterization of the electron density on the mixed

occupied metal site. For Mg3GaN3 a direct band gap of 3.0 eV was calculated and confirmed by soft X-ray spectroscopy measurements. As expected, the band gap is between the values for GaN and Mg3N2. Mg3GaN3:Eu2+ exhibits yellow luminescence (λmax. = 578 nm, FWHM = 132 nm), while Mg3N2:Eu2+ also shows luminescence (λmax. = 589 nm, FWHM = 145 nm) at room temperature.

7. High-Pressure Synthesis and Characterization of Li2Ca3[N2]3 - An Uncommon

Metallic Diazenide with [N2]2– Ions

Li2Ca3(N2)3 (Pmma, a = 4.7747(1), b = 13.9792(4), c = 8.0718(4) Å, V = 538.77(3)Å3) is the first representative of a ternary diazenide. The compound was synthesized by controlled thermal decomposition of lithium- and calcium azide mixtures in a multi-anvil press (9 GPa, 1023 K). Due to pseudo-hexagonal metrics, PXRD investigations initially led to incorrect structure models. Especially, SAED simulations significantly differed from experimental ones. Detailed crystallographic analysis, i.e. the stepwise reduction to orthorhombic symmetry (requires threefold twinning), was the basis for a reliable structure model and finally made quantum-theoretical investigations possible. PXRD analysis in correct space group results in N–N bonds of 1.34(2)-1.35(3) Å exceeding the values of known binary diazenides. Moreover, refined N–N distances rather match reported values for [N2]3- radical anions. The true character of the [N2]-entity was finally settled by a variety of complementary analyses including electron- diffraction methods, electron spin resonance spectroscopy (ESR), magnetic and conductivity measurements as well as density-functional theory. Li2Ca3[N2]3 contains solely diazenide [N2]2- units and is therefore better described as (Li+)2(Ca2+)3([N2]2-)3·(e-)2.