Structure Elucidation of Complex Endotaxially Intergrown Lanthanum Barium Oxonitridosilicate Oxides by Combination of Microfocused Synchrotron Radiation and Transmission Electron Microscopy.

Single-crystalline domains in intergrown microcrystalline material of the new compounds Ba22.5+x La55-x [Si129 N240-x Ox ]O3 :Ce3+ and Ba25.5+x La77-x [Si170 N312-x O9+x ]O4 :Ce3+ were identified by transmission electron microscopy (TEM). Precise diffraction data from these domains were collected with microfocused synchrotron radiation so that crystal structure elucidation of the complex disordered networks became possible. They are composed of two different interconnected slabs of which one is similar in both compounds, which explains their notorious intergrowth. The distribution of Ba and La is indicated by the analysis of bond-valence sums and by comparison with isostructural Sr28.5+x La75-x [Si170 N312-x O9+x ]O4 . Ce3+ doping leads to yellow luminescence. This is a showcase that highlights the discovery and accurate characterization of new compounds relevant for luminescence applications from heterogeneous microcrystalline samples by exploiting the capability of the combination of TEM and diffraction using the latest focusing techniques for synchrotron radiation.

The Long-Periodic Loop-Branched Chain Structure of the Oxonitridophosphate La21 P40 O46 N57 , Elucidated by a Combination of TEM and Microfocused Synchrotron Radiation.

The lanthanum oxonitridophosphate La21 P40 O46 N57 was synthesized by high-pressure metathesis from partially hydrolysed LiPN2 and LaCl3 at 750-950 °C and 7-9 GPa. The combination of transmission electron microscopy (TEM) and diffraction using microfocused synchrotron radiation revealed a monoclinic crystal structure (space group P21 /n, a=14.042(4), b=7.084(3), c=41.404(10) Å, ß=97.73(3)° and Z=2), which is characterized by loop-branched 21 member single chains of P(O,N)4 tetrahedra that extend along [2 0 1]. These chains are related to the loop-branched dreier single chains with dreier-ring loops in stillwellite (CeBSiO5 ). In La21 P40 O46 N57 , these chains are characterized by a complex long-periodic conformation and exhibit disorder that involves La/N and P split positions. This is an extraordinarily long periodicity with respect to branched single chains of tetrahedra. La21 P40 O46 N57 constitutes the first rare-earth oxonitridophosphate exhibiting a chain structure. Single-crystal data are consistent with electron and powder X-ray diffraction.

Orange-Emitting Li4Sr4[Si4O4N6]O:Eu2+-a Layered Lithium Oxonitridosilicate Oxide.

We report on the structure and properties of the lithium oxonitridosilicate oxide Li4Sr4[Si4O4N6]O:Eu2+ obtained from solid-state metathesis. The crystal structure was solved and refined from single-crystal X-ray data in the space group P42/ nmc (No. 137) [ Z = 2, a = 7.4833(6), c = 9.8365(9) Å, and R1(obs) = 0.0477]. The structure of Li4Sr4[Si4O4N6]O:Eu2+ is built up from a layered 2D network of SiN3O tetrahedra and exhibits stacking disorder. The results are supported by transmission electron microscopy and energy-dispersive X-ray spectroscopy as well as lattice energy, charge distribution, and density functional theory (DFT) calculations. Optical measurements suggest an indirect band gap of about 3.6 eV, while DFT calculations on a model free of stacking faults yield a theoretical electronic band gap of 4.4 eV. Samples doped with Eu2+ exhibit luminescence in the orange spectral range (λem ≈ 625 nm; full width at half-maximum ≈ 4164 cm-1; internal quantum efficiency at room temperature = 24%), extending the broad field of phosphor materials research toward the sparsely investigated materials class of lithium oxonitridosilicate oxides.

Highly Symmetric AB2 Framework Related to Tridymite in the Disordered Nitridosilicate La24Sr14-7x[Si36N72](O1-xFx)14 (x = 0.489).

La24Sr14-7x[Si36N72](O1-xFx)14 with x = 0.489 was obtained as a microcrystalline product by metathesis at 1500 °C in a radio-frequency furnace starting from Si(NH)2, La(NH2)3, SrH2, LaF3, and CeF3. The structure of the new nitridosilicate oxide fluoride was determined by combining transmission electron microscopy (TEM) and single-crystal X-ray diffraction using a microfocused synchrotron beam. The structure model with pronounced disorder [P63/mmc, Z = 1, a = 16.2065(3), c = 9.4165(1) Å, R1(obs) = 0.0436] was confirmed by electron diffraction and aberration-corrected Z-contrast scanning TEM. The highly symmetric AB2 framework, which was theoretically predicted but not yet realized, consists of all-side vertex-sharing SiN4 tetrahedra that form channels along [001] filled with La, Sr, O, and F atoms. The connectivity pattern is related to that of tridymite. X-ray spectroscopy and bond-valence-sum calculations were further taken into account for assignment of the N, O, and F atoms.

Puzzling Intergrowth in Cerium Nitridophosphate Unraveled by Joint Venture of Aberration-Corrected Scanning Transmission Electron Microscopy and Synchrotron Diffraction.

Thorough investigation of nitridophosphates has rapidly accelerated through development of new synthesis strategies. Here we used the recently developed high-pressure metathesis to prepare the first rare-earth metal nitridophosphate, Ce4Li3P18N35, with a high degree of condensation >1/2. Ce4Li3P18N35 consists of an unprecedented hexagonal framework of PN4 tetrahedra and exhibits blue luminescence peaking at 455 nm. Transmission electron microscopy (TEM) revealed two intergrown domains with slight structural and compositional variations. One domain type shows extremely weak superstructure phenomena revealed by atomic-resolution scanning TEM (STEM) and single-crystal diffraction using synchrotron radiation. The corresponding superstructure involves a modulated displacement of Ce atoms in channels of tetrahedra 6-rings. The displacement model was refined in a supercell as well as in an equivalent commensurate (3 + 2)-dimensional description in superspace group P63(α, ß, 0)0(-α - ß, α, 0)0. In the second domain type, STEM revealed disordered vacancies of the same Ce atoms that were modulated in the first domain type, leading to sum formula Ce4-0.5xLi3P18N35-1.5xO1.5x (x ≈ 0.72) of the average structure. The examination of these structural intricacies may indicate the detection limit of synchrotron diffraction and TEM. We discuss the occurrence of either Ce displacements or Ce vacancies that induce the incorporation of O as necessary stabilization of the crystal structure.

An unusual nitride network of aluminum-centered octahedra and phosphorus-centered tetrahedra and structure determination from microcrystalline samples.

A new imidooxonitridophosphate AlP6O3x(NH)3-3xN9 with x ≈ 0.33 was synthesized under high-pressure high-temperature conditions. The crystal structure determination of the microcrystalline product involved a combination of electron microscopy, synchrotron X-ray diffraction and solid-state NMR. In the solid there are discrete AlN6 octahedra that interconnect imidophosphate layers. The network topology is unprecedented but is related to other nitride structures.

Ammonothermal Synthesis of Novel Nitrides: Case Study on CaGaSiN3.

The first gallium-containing nitridosilicate CaGaSiN3 was synthesized in newly developed high-pressure autoclaves using supercritical ammonia as solvent and nitriding agent. The reaction was conducted in an ammonobasic environment starting from intermetallic CaGaSi with NaN3 as a mineralizer. At 770 K, intermediate compounds were obtained, which were subsequently converted to the crystalline nitride at temperatures up to 1070 K (70-150 MPa). The impact of other mineralizers (e.g., LiN3 , KN3 , and CsN3 ) on the product formation was investigated as well. The crystal structure of CaGaSiN3 was analyzed by powder X-ray diffraction and refined by the Rietveld method. The structural results were further corroborated by transmission electron microscopy, 29 Si MAS-NMR, and first-principle DFT calculations. CaGaSiN3 crystallizes in the orthorhombic space group Cmc21 (no. 36) with lattice parameters a=9.8855(11), b=5.6595(1), c=5.0810(1) Å, (Z=4, Rwp =0.0326), and is isostructural with CaAlSiN3 (CASN). Eu2+ doped samples exhibit red luminescence with an emission maximum of 620 nm and FWHM of 90 nm. Thus, CaGaSiN3 :Eu2+ also represents an interesting candidate as a red-emitting material in phosphor-converted light-emitting diodes (pc-LEDs). In addition to the already known substitution of alkaline-earth metals in (Ca,Sr)AlSiN3 :Eu2+ , inclusion of Ga is a further and promising perspective for luminescence tuning of widely used red-emitting CASN type materials.

Layered germanium tin antimony tellurides: element distribution, nanostructures and thermoelectric properties.

In the system Ge-Sn-Sb-Te, there is a complete solid solution series between GeSb2Te4 and SnSb2Te4. As Sn2Sb2Te5 does not exist, Sn can only partially replace Ge in Ge2Sb2Te5; samples with 75% or more Sn are not homogeneous. The joint refinement of high-resolution synchrotron data measured at the K-absorption edges of Sn, Sb and Te combined with data measured at off-edge wavelengths unambiguously yields the element distribution in 21R-Ge(0.6)Sn(0.4)Sb2Te4 and 9P-Ge(1.3)Sn(0.7)Sb2Te5. In both cases, Sb predominantly concentrates on the position near the van der Waals gaps between distorted rocksalt-type slabs whereas Ge prefers the position in the middle of the slabs. No significant antisite disorder is present. Comparable trends can be found in related compounds; they are due to the single-side coordination of the Te atoms at the van der Waals gap, which can be compensated more effectively by Sb(3+) due to its higher charge in comparison to Ge(2+). The structure model of 21R-Ge(0.6)Sn(0.4)Sb2Te4 was confirmed by high-resolution electron microscopy and electron diffraction. In contrast, electron diffraction patterns of 9P-Ge(1.3)Sn(0.7)Sb2Te5 reveal a significant extent of stacking disorder as evidenced by diffuse streaks along the stacking direction. The Seebeck coefficient is unaffected by the Sn substitution but the thermal conductivity drops by a factor of 2 which results in a thermoelectric figure of merit ZT = ~0.25 at 450 °C for both Ge(0.6)Sn(0.4)Sb2Te4 and Ge(1.3)Sn(0.7)Sb2Te5, which is higher than ~0.20 for unsubstituted stable layered Ge-Sb-Te compounds.