Crystal Structure and Nontypical Deep-Red Luminescence of Ca₃Mg[Li₂Si₂N₆]:Eu²⁺

Rare-earth-doped nitridosilicates exhibit outstanding luminescence properties and have been intensively studied for solid-state lighting. Here, we describe the new nitridolithosilicate Ca₃Mg­[Li₂Si₂N₆]:Eu²⁺ with extraordinary luminescence characteristics. The compound was synthesized by the solid-state metathesis reaction in sealed Ta ampules. The crystal structure was solved and refined on the basis of single-crystal X-ray diffraction data. Ca₃Mg­[Li₂Si₂N₆]:Eu²⁺ crystallizes in the monoclinic space group <i>C</i>2/<i>m</i> (no. 12) [<i>Z</i> = 4, <i>a</i> = 5.966(1), <i>b</i> = 9.806(2), <i>c</i> = 11.721(2) Å, β = 99.67(3)°, <i>V</i> = 675.9(2) ų] and exhibits a layered anionic network made up of edge- and corner-sharing LiN₄ tetrahedra and [Si₂N₆]^10– bow-tie units. The network charge is compensated by Ca²⁺ and Mg²⁺ ions. Upon irradiation with UV to blue light, red emission at exceptionally long wavelengths (λ_em = 734 nm, fwhm ≈2293 cm^–1) is observed. According to emission in the near-infrared, application in LEDs for horticultural lighting appears promising

High-Pressure Polymorph of Phosphorus Nitride Imide HP₄N₇ Representing a New Framework Topology

A new polymorph of phosphorus nitride imide HP₄N₇ has been synthesized under high-pressure/high-temperature conditions from P₃N₅ and NH₄Cl at 6 GPa and temperatures between 800 and 1300 °C. Its crystal structure was elucidated using single-crystal X-ray diffraction data. β-HP₄N₇ (space group <i>C</i>2/<i>c</i>, no. 15, <i>Z</i> = 4, <i>a</i> = 12.873(2) Å, <i>b</i> = 4.6587(4) Å, <i>c</i> = 8.3222(8) Å, β = 102.351(3)°, <i>R</i>₁ = 0.0485, <i>wR</i>₂ = 0.1083) crystallizes in a new framework structure type that is made up of all-side vertex-sharing PN₄ tetrahedra. The topology of the network is represented by the point symbol (3².4².5².6³.7)­(3⁴.4⁴.5⁴.6³), and it has not been identified in other compounds so far. Structural differences between the two polymorphs of HP₄N₇ as well as the topological relationship to the recently discovered high-pressure polymorph β-HPN₂ are discussed. Additionally, FTIR and solid-state NMR spectroscopy are used to corroborate the results of the structure determination

Li₂₄Sr₁₂[Si₂₄N₄₇O]F:Eu²⁺Structure and Luminescence of an Orange Phosphor

The oxonitridosilicate fluoride phosphor Li₂₄Sr₁₂[Si₂₄N₄₇O]­F:Eu²⁺ was synthesized from Si₃N₄, SrH₂, LiNH₂, LiF, and EuF₃ as dopant in a radio frequency furnace. The crystal structure (space group <i>Pa</i>3̅ (no. 205), <i>a</i> = 10.72830(10) Å, <i>R</i>₁ = 0.0401, <i>wR</i>₂ = 0.0885, <i>Z</i> = 1) of the host compound Li₂₄Sr₁₂[Si₂₄N₄₇O]F was solved and refined on the basis of single-crystal X-ray diffraction data. Li₂₄Sr₁₂[Si₂₄N₄₇O]F is homeotypic with the nitridosilicate Li₂SrSi₂N₄ as both compounds are characterized by the same tetrahedra network topology, but Li₂₄Sr₁₂[Si₂₄N₄₇O]F is an oxonitridosilicate and contains an additional F site. The implemented F is verified by EDX measurements as well as through calculations with PLATON. Besides, the electrostatic consistency of the refined crystal structure is proven by lattice energy calculations. The Eu²⁺-doped compound Li₂₄Sr₁₂[Si₂₄N₄₇O]­F:Eu²⁺ shows an orange to red luminescence (λ_max = 598 nm; fwhm = 81 nm) under excitation with blue light, which differs from that of Li₂SrSi₂N₄:Eu²⁺ (λ_em = 613 nm; fwhm = 86 nm) due to the additional F site. According to the blue-shifted emission, application in LEDs for sectors with low CRI is conceivable

Crystal Structure and Nontypical Deep-Red Luminescence of Ca₃Mg[Li₂Si₂N₆]:Eu²⁺

Rare-earth-doped nitridosilicates exhibit outstanding luminescence properties and have been intensively studied for solid-state lighting. Here, we describe the new nitridolithosilicate Ca₃Mg­[Li₂Si₂N₆]:Eu²⁺ with extraordinary luminescence characteristics. The compound was synthesized by the solid-state metathesis reaction in sealed Ta ampules. The crystal structure was solved and refined on the basis of single-crystal X-ray diffraction data. Ca₃Mg­[Li₂Si₂N₆]:Eu²⁺ crystallizes in the monoclinic space group <i>C</i>2/<i>m</i> (no. 12) [<i>Z</i> = 4, <i>a</i> = 5.966(1), <i>b</i> = 9.806(2), <i>c</i> = 11.721(2) Å, β = 99.67(3)°, <i>V</i> = 675.9(2) ų] and exhibits a layered anionic network made up of edge- and corner-sharing LiN₄ tetrahedra and [Si₂N₆]^10– bow-tie units. The network charge is compensated by Ca²⁺ and Mg²⁺ ions. Upon irradiation with UV to blue light, red emission at exceptionally long wavelengths (λ_em = 734 nm, fwhm ≈2293 cm^–1) is observed. According to emission in the near-infrared, application in LEDs for horticultural lighting appears promising

LiPr₂P₄N₇O₃: Structural Diversity of Oxonitridophosphates Accessed by High-Pressure Metathesis

The structural diversity of tetrahedra networks of phosphates can greatly be enhanced by introduction of mixed N/O anion positions. LiPr₂P₄N₇O₃ exemplifies the benefits of N/O mixed anion positions as it is the first rare-earth (oxo)­nitridophosphate with a single-layered structure and a degree of condensation (atomic ratio of tetrahedra centers (P) to tetrahedra corners (N/O atoms)) of 2/5. The compound was prepared through high-pressure metathesis starting from PrF₃, LiPN₂, Li₂O, and PON using a hydraulic 1000t press and the multianvil technique. LiPr₂P₄N₇O₃ crystallizes as pale-green single-crystals, from which its structure was determined (space group <i>P</i>2₁/<i>c</i> (no. 14), <i>a</i> = 4.927(1), <i>b</i> = 7.848(2), <i>c</i> = 10.122(2) Å, β = 91.55(3)°, <i>Z</i> = 2, <i>R</i>₁ = 0.020, <i>wR</i>₂ = 0.045). The structure consists of single-layers of vertex-sharing Q³-type P­(N/O)₄ tetrahedra forming four- and eight-membered rings arranged in the fashion of the Archimedean <i>fes</i> net. UV–vis spectroscopy revealed the typical Pr³⁺ f<i>–</i>f transitions, leading to a pale-green color of the crystals. Moreover, the optical band gap was determined to 4.1(1) eV, assuming a direct transition. High-temperature powder X-ray diffraction showed the beginning of a gradual decomposition starting at ca. 500 °C

Synthesis of Alkaline Earth Diazenides M_AEN₂ (M_AE = Ca, Sr, Ba) by Controlled Thermal Decomposition of Azides under High Pressure

The alkaline earth diazenides M_AEN₂ with M_AE = Ca, Sr and Ba were synthesized by a novel synthetic approach, namely, a controlled decomposition of the corresponding azides in a multianvil press at high-pressure/high-temperature conditions. The crystal structure of hitherto unknown calcium diazenide (space group <i>I</i>4/<i>mmm</i> (no. 139), <i>a</i> = 3.5747(6) Å, <i>c</i> = 5.9844(9) Å, <i>Z</i> = 2, <i>wR</i>_p = 0.078) was solved and refined on the basis of powder X-ray diffraction data as well as that of SrN₂ and BaN₂. Accordingly, CaN₂ is isotypic with SrN₂ (space group <i>I</i>4/<i>mmm</i> (no. 139), <i>a</i> = 3.8054(2) Å, <i>c</i> = 6.8961(4) Å, <i>Z</i> = 2, <i>wR</i>_p = 0.057) and the corresponding alkaline earth acetylenides (M_AEC₂) crystallizing in a tetragonally distorted NaCl structure type. In accordance with literature data, BaN₂ adopts a more distorted structure in space group <i>C</i>2<i>/c</i> (no. 15) with <i>a</i> = 7.1608(4) Å, <i>b</i> = 4.3776(3) Å, <i>c</i> = 7.2188(4) Å, β = 104.9679(33)°, <i>Z</i> = 4 and <i>wR</i>_p = 0.049). The N–N bond lengths of 1.202(4) Å in CaN₂ (SrN₂ 1.239(4) Å, BaN₂ 1.23(2) Å) correspond well with a double-bonded dinitrogen unit confirming a diazenide ion [N₂]^2–. Temperature-dependent <i>in situ</i> powder X-ray diffractometry of the three alkaline earth diazenides resulted in formation of the corresponding subnitrides M_AE₂N (M_AE = Ca, Sr, Ba) at higher temperatures. FTIR spectroscopy revealed a band at about 1380 cm^–1 assigned to the N–N stretching vibration of the diazenide unit. Electronic structure calculations support the metallic character of alkaline earth diazenides

Li₁₄Ln₅[Si₁₁N₁₉O₅]O₂F₂ with Ln = Ce, NdRepresentatives of a Family of Potential Lithium Ion Conductors

The isotypic layered oxonitridosilicates Li₁₄Ln₅[Si₁₁N₁₉O₅]­O₂F₂ (Ln = Ce, Nd) have been synthesized using Li as fluxing agent and crystallize in the orthorhombic space group <i>Pmmn</i> (<i>Z</i> = 2, Li₁₄Ce₅[Si₁₁N₁₉O₅]­O₂F₂: <i>a</i> = 17.178(3), <i>b</i> = 7.6500(15), <i>c</i> = 10.116(2) Å, <i>R</i>1 = 0.0409, <i>wR</i>2 = 0.0896; Li₁₄Nd₅[Si₁₁N₁₉O₅]­O₂F₂: <i>a</i> = 17.126(2), <i>b</i> = 7.6155(15), <i>c</i> = 10.123(2) Å, <i>R</i>1 = 0.0419, <i>wR</i>2 = 0.0929). The silicate layers consist of <i>dreier</i> and <i>sechser</i> rings interconnected via common corners, yielding an unprecedented silicate substructure. A topostructural analysis indicates possible 1D ion migration pathways between five crystallographic independent Li positions. The specific Li-ionic conductivity and its temperature dependence were determined by impedance spectroscopy as well as DC polarization/depolarization measurements. The ionic conductivity is on the order of 5 × 10^–5 S/cm at 300 °C, while the activation energy is 0.69 eV. Further adjustments of the defect chemistry (e.g., through doping) can make these compounds interesting candidates for novel oxonitridosilicate based ion conductors

La₆Ba₃[Si₁₇N₂₉O₂]ClAn Oxonitridosilicate Chloride with Exceptional Structural Motifs

The oxonitridosilicate chloride La₆Ba₃[Si₁₇N₂₉O₂]Cl was synthesized by a high-temperature reaction in a radiofrequency furnace starting from LaCl₃, BaH₂, and the ammonolysis product of Si₂Cl₆. Diffraction data of a micrometer-sized single crystal were obtained using microfocused synchrotron radiation at beamline ID11 of the ESRF. EDX measurements on the same crystal confirm the chemical composition. The crystal structure [space group <i>P</i>6₃/<i>m</i> (no. 176), <i>a</i> = 9.8117(14), <i>c</i> = 19.286(6) Å, <i>Z</i> = 2] contains an unprecedented interrupted three-dimensional network of vertex-sharing SiN₄ and SiN₃O tetrahedra. The SiN₄ tetrahedra form <i>dreier</i> rings. Twenty of the latter condense in a way that the Si atoms form icosahedra. Each icosahedron is connected to others via six SiN₄ tetrahedra that are part of <i>dreier</i> rings and via six Q³-type SiN₃O tetrahedra. Rietveld refinements confirm that the final product contains only a small amount of impurities. Lattice energy (MAPLE) and bond-valence sum (BVS) calculations show that the structure is electrostatically well balanced. Infrared spectroscopy confirms the absence of N–H bonds

From Minor Side Phases to Bulk Samples of Lanthanum Oxonitridosilicates: An Investigation with Microfocused Synchrotron Radiation

Microcrystals of the oxonitridosilicate oxide La₁₁Si₁₃N_27.636O_1.046:Ce³⁺ were obtained by exploratory high-temperature synthesis starting from La, La­(NH₂)₃, Si­(NH)₂, BaH₂, and CeF₃. Owing to the small size of the crystals, microfocused synchrotron radiation was used for structure investigations (space group <i>Cmc</i>2₁ (No. 36), <i>a</i> = 9.5074(4) Å, <i>b</i> = 32.0626(9) Å, <i>c</i> = 18.5076(8) Å, Z = 8, R1­(all) = 0.0267). The crystal structure consists of an unprecedented interrupted three-dimensional network of vertex-sharing SiN_4–<i>x</i>O_<i>x</i> tetrahedra that form channels of <i>siebener</i> rings along [100]. Moreover, the structure is characterized by layers of condensed <i>sechser</i> rings in a boat conformation and <i>vierer</i> rings, which are alternatingly stacked with layers of <i>vierer</i> and <i>dreier</i> rings. Several split positions indicate two different local structure variants. Infrared spectroscopy confirms the absence of N–H bonds. Powder X-ray diffraction data show that bulk samples contain only a small amount of La₁₁Si₁₃N_27.636O_1.046:Ce³⁺. However, once the exact composition was determined from structure analysis, it was possible to optimize the synthesis using fluorides as starting materials. Thereby, bulk samples of the homeotypic compound La₁₁Si₁₃N_27.376O_0.936F were obtained and investigated

Li₁₄Ln₅[Si₁₁N₁₉O₅]O₂F₂ with Ln = Ce, NdRepresentatives of a Family of Potential Lithium Ion Conductors

The isotypic layered oxonitridosilicates Li₁₄Ln₅[Si₁₁N₁₉O₅]­O₂F₂ (Ln = Ce, Nd) have been synthesized using Li as fluxing agent and crystallize in the orthorhombic space group <i>Pmmn</i> (<i>Z</i> = 2, Li₁₄Ce₅[Si₁₁N₁₉O₅]­O₂F₂: <i>a</i> = 17.178(3), <i>b</i> = 7.6500(15), <i>c</i> = 10.116(2) Å, <i>R</i>1 = 0.0409, <i>wR</i>2 = 0.0896; Li₁₄Nd₅[Si₁₁N₁₉O₅]­O₂F₂: <i>a</i> = 17.126(2), <i>b</i> = 7.6155(15), <i>c</i> = 10.123(2) Å, <i>R</i>1 = 0.0419, <i>wR</i>2 = 0.0929). The silicate layers consist of <i>dreier</i> and <i>sechser</i> rings interconnected via common corners, yielding an unprecedented silicate substructure. A topostructural analysis indicates possible 1D ion migration pathways between five crystallographic independent Li positions. The specific Li-ionic conductivity and its temperature dependence were determined by impedance spectroscopy as well as DC polarization/depolarization measurements. The ionic conductivity is on the order of 5 × 10^–5 S/cm at 300 °C, while the activation energy is 0.69 eV. Further adjustments of the defect chemistry (e.g., through doping) can make these compounds interesting candidates for novel oxonitridosilicate based ion conductors