44 research outputs found
Crystal Structure and Nontypical Deep-Red Luminescence of Ca<sub>3</sub>Mg[Li<sub>2</sub>Si<sub>2</sub>N<sub>6</sub>]:Eu<sup>2+</sup>
Rare-earth-doped
nitridosilicates exhibit outstanding luminescence
properties and have been intensively studied for solid-state lighting.
Here, we describe the new nitridolithosilicate Ca<sub>3</sub>MgÂ[Li<sub>2</sub>Si<sub>2</sub>N<sub>6</sub>]:Eu<sup>2+</sup> 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. Ca<sub>3</sub>MgÂ[Li<sub>2</sub>Si<sub>2</sub>N<sub>6</sub>]:Eu<sup>2+</sup> 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) Ã…, β = 99.67(3)°, <i>V</i> = 675.9(2) Ã…<sup>3</sup>] and exhibits a layered anionic network
made up of edge- and corner-sharing LiN<sub>4</sub> tetrahedra and
[Si<sub>2</sub>N<sub>6</sub>]<sup>10–</sup> bow-tie units.
The network charge is compensated by Ca<sup>2+</sup> and Mg<sup>2+</sup> ions. Upon irradiation with UV to blue light, red emission at exceptionally
long wavelengths (λ<sub>em</sub> = 734 nm, fwhm ≈2293
cm<sup>–1</sup>) is observed. According to emission in the
near-infrared, application in LEDs for horticultural lighting appears
promising
High-Pressure Polymorph of Phosphorus Nitride Imide HP<sub>4</sub>N<sub>7</sub> Representing a New Framework Topology
A new polymorph of phosphorus nitride
imide HP<sub>4</sub>N<sub>7</sub> has been synthesized under high-pressure/high-temperature
conditions from P<sub>3</sub>N<sub>5</sub> and NH<sub>4</sub>Cl at
6 GPa and temperatures between 800 and 1300 °C. Its crystal structure
was elucidated using single-crystal X-ray diffraction data. β-HP<sub>4</sub>N<sub>7</sub> (space group <i>C</i>2/<i>c</i>, no. 15, <i>Z</i> = 4, <i>a</i> = 12.873(2)
Ã…, <i>b</i> = 4.6587(4) Ã…, <i>c</i> =
8.3222(8) Å, β = 102.351(3)°, <i>R</i><sub>1</sub> = 0.0485, <i>wR</i><sub>2</sub> = 0.1083) crystallizes
in a new framework structure type that is made up of all-side vertex-sharing
PN<sub>4</sub> tetrahedra. The topology of the network is represented
by the point symbol (3<sup>2</sup>.4<sup>2</sup>.5<sup>2</sup>.6<sup>3</sup>.7)Â(3<sup>4</sup>.4<sup>4</sup>.5<sup>4</sup>.6<sup>3</sup>), and it has not been identified in other compounds so far. Structural
differences between the two polymorphs of HP<sub>4</sub>N<sub>7</sub> as well as the topological relationship to the recently discovered
high-pressure polymorph β-HPN<sub>2</sub> are discussed. Additionally,
FTIR and solid-state NMR spectroscopy are used to corroborate the
results of the structure determination
Li<sub>24</sub>Sr<sub>12</sub>[Si<sub>24</sub>N<sub>47</sub>O]F:Eu<sup>2+</sup>î—¸Structure and Luminescence of an Orange Phosphor
The oxonitridosilicate fluoride phosphor
Li<sub>24</sub>Sr<sub>12</sub>[Si<sub>24</sub>N<sub>47</sub>O]ÂF:Eu<sup>2+</sup> was synthesized
from Si<sub>3</sub>N<sub>4</sub>, SrH<sub>2</sub>, LiNH<sub>2</sub>, LiF, and EuF<sub>3</sub> as dopant in a radio frequency furnace.
The crystal structure (space group <i>Pa</i>3Ì… (no.
205), <i>a</i> = 10.72830(10) Ã…, <i>R</i><sub>1</sub> = 0.0401, <i>wR</i><sub>2</sub> = 0.0885, <i>Z</i> = 1) of the host compound Li<sub>24</sub>Sr<sub>12</sub>[Si<sub>24</sub>N<sub>47</sub>O]F was solved and refined on the basis
of single-crystal X-ray diffraction data. Li<sub>24</sub>Sr<sub>12</sub>[Si<sub>24</sub>N<sub>47</sub>O]F is homeotypic with the nitridosilicate
Li<sub>2</sub>SrSi<sub>2</sub>N<sub>4</sub> as both compounds are
characterized by the same tetrahedra network topology, but Li<sub>24</sub>Sr<sub>12</sub>[Si<sub>24</sub>N<sub>47</sub>O]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 Eu<sup>2+</sup>-doped compound
Li<sub>24</sub>Sr<sub>12</sub>[Si<sub>24</sub>N<sub>47</sub>O]ÂF:Eu<sup>2+</sup> shows an orange to red luminescence (λ<sub>max</sub> = 598 nm; fwhm = 81 nm) under excitation with blue light, which
differs from that of Li<sub>2</sub>SrSi<sub>2</sub>N<sub>4</sub>:Eu<sup>2+</sup> (λ<sub>em</sub> = 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 Ca<sub>3</sub>Mg[Li<sub>2</sub>Si<sub>2</sub>N<sub>6</sub>]:Eu<sup>2+</sup>
Rare-earth-doped
nitridosilicates exhibit outstanding luminescence
properties and have been intensively studied for solid-state lighting.
Here, we describe the new nitridolithosilicate Ca<sub>3</sub>MgÂ[Li<sub>2</sub>Si<sub>2</sub>N<sub>6</sub>]:Eu<sup>2+</sup> 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. Ca<sub>3</sub>MgÂ[Li<sub>2</sub>Si<sub>2</sub>N<sub>6</sub>]:Eu<sup>2+</sup> 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) Ã…, β = 99.67(3)°, <i>V</i> = 675.9(2) Ã…<sup>3</sup>] and exhibits a layered anionic network
made up of edge- and corner-sharing LiN<sub>4</sub> tetrahedra and
[Si<sub>2</sub>N<sub>6</sub>]<sup>10–</sup> bow-tie units.
The network charge is compensated by Ca<sup>2+</sup> and Mg<sup>2+</sup> ions. Upon irradiation with UV to blue light, red emission at exceptionally
long wavelengths (λ<sub>em</sub> = 734 nm, fwhm ≈2293
cm<sup>–1</sup>) is observed. According to emission in the
near-infrared, application in LEDs for horticultural lighting appears
promising
LiPr<sub>2</sub>P<sub>4</sub>N<sub>7</sub>O<sub>3</sub>: 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. LiPr<sub>2</sub>P<sub>4</sub>N<sub>7</sub>O<sub>3</sub> exemplifies the benefits of N/O mixed anion positions
as it is the first rare-earth (oxo)Ânitridophosphate 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 PrF<sub>3</sub>, LiPN<sub>2</sub>, Li<sub>2</sub>O, and PON using a hydraulic
1000t press and the multianvil technique. LiPr<sub>2</sub>P<sub>4</sub>N<sub>7</sub>O<sub>3</sub> crystallizes as pale-green single-crystals,
from which its structure was determined (space group <i>P</i>2<sub>1</sub>/<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><sub>1</sub> = 0.020, <i>wR</i><sub>2</sub> = 0.045). The structure
consists of single-layers of vertex-sharing Q<sup>3</sup>-type PÂ(N/O)<sub>4</sub> tetrahedra forming four- and eight-membered rings arranged
in the fashion of the Archimedean <i>fes</i> net. UV–vis
spectroscopy revealed the typical Pr<sup>3+</sup> 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<sub>AE</sub>N<sub>2</sub> (M<sub>AE</sub> = Ca, Sr, Ba) by Controlled Thermal Decomposition of Azides under High Pressure
The alkaline earth diazenides M<sub>AE</sub>N<sub>2</sub> with
M<sub>AE</sub> = 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) Ã…, <i>c</i> = 5.9844(9) Ã…, <i>Z</i> = 2, <i>wR</i><sub>p</sub> = 0.078) was solved
and refined on the basis of powder X-ray diffraction data as well
as that of SrN<sub>2</sub> and BaN<sub>2</sub>. Accordingly, CaN<sub>2</sub> is isotypic with SrN<sub>2</sub> (space group <i>I</i>4/<i>mmm</i> (no. 139), <i>a</i> = 3.8054(2)
Ã…, <i>c</i> = 6.8961(4) Ã…, <i>Z</i> =
2, <i>wR</i><sub>p</sub> = 0.057) and the corresponding
alkaline earth acetylenides
(M<sub>AE</sub>C<sub>2</sub>) crystallizing in a tetragonally distorted
NaCl structure type. In accordance with literature data, BaN<sub>2</sub> adopts a more distorted structure in space group <i>C</i>2<i>/c</i> (no. 15) with <i>a</i> = 7.1608(4)
Ã…, <i>b</i> = 4.3776(3) Ã…, <i>c</i> =
7.2188(4) Å, β = 104.9679(33)°, <i>Z</i> = 4 and <i>wR</i><sub>p</sub> = 0.049). The N–N
bond lengths of 1.202(4) Ã… in CaN<sub>2</sub> (SrN<sub>2</sub> 1.239(4) Ã…, BaN<sub>2</sub> 1.23(2) Ã…) correspond well
with a double-bonded dinitrogen unit confirming a diazenide ion [N<sub>2</sub>]<sup>2–</sup>. Temperature-dependent <i>in situ</i> powder X-ray diffractometry of the three alkaline earth diazenides
resulted in formation of the corresponding subnitrides M<sub>AE<sub>2</sub></sub>N (M<sub>AE</sub> = Ca, Sr, Ba) at higher temperatures.
FTIR spectroscopy revealed a band at about 1380 cm<sup>–1</sup> assigned to the N–N stretching vibration of the diazenide
unit. Electronic structure calculations support the metallic character
of alkaline earth diazenides
Li<sub>14</sub>Ln<sub>5</sub>[Si<sub>11</sub>N<sub>19</sub>O<sub>5</sub>]O<sub>2</sub>F<sub>2</sub> with Ln = Ce, Ndî—¸Representatives of a Family of Potential Lithium Ion Conductors
The isotypic layered oxonitridosilicates Li<sub>14</sub>Ln<sub>5</sub>[Si<sub>11</sub>N<sub>19</sub>O<sub>5</sub>]ÂO<sub>2</sub>F<sub>2</sub> (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, Li<sub>14</sub>Ce<sub>5</sub>[Si<sub>11</sub>N<sub>19</sub>O<sub>5</sub>]ÂO<sub>2</sub>F<sub>2</sub>: <i>a</i> = 17.178(3), <i>b</i> = 7.6500(15), <i>c</i> = 10.116(2) Ã…, <i>R</i>1 = 0.0409, <i>wR</i>2 = 0.0896; Li<sub>14</sub>Nd<sub>5</sub>[Si<sub>11</sub>N<sub>19</sub>O<sub>5</sub>]ÂO<sub>2</sub>F<sub>2</sub>: <i>a</i> = 17.126(2), <i>b</i> = 7.6155(15), <i>c</i> = 10.123(2) Ã…, <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<sup>–5</sup> 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
La<sub>6</sub>Ba<sub>3</sub>[Si<sub>17</sub>N<sub>29</sub>O<sub>2</sub>]Clî—¸An Oxonitridosilicate Chloride with Exceptional Structural Motifs
The oxonitridosilicate
chloride La<sub>6</sub>Ba<sub>3</sub>[Si<sub>17</sub>N<sub>29</sub>O<sub>2</sub>]Cl was synthesized by a high-temperature reaction in
a radiofrequency furnace starting from LaCl<sub>3</sub>, BaH<sub>2</sub>, and the ammonolysis product of Si<sub>2</sub>Cl<sub>6</sub>. 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>6<sub>3</sub>/<i>m</i> (no. 176), <i>a</i> = 9.8117(14), <i>c</i> =
19.286(6) Ã…, <i>Z</i> = 2] contains an unprecedented
interrupted three-dimensional network of vertex-sharing SiN<sub>4</sub> and SiN<sub>3</sub>O tetrahedra. The SiN<sub>4</sub> 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 SiN<sub>4</sub> tetrahedra that are part of <i>dreier</i> rings and via six Q<sup>3</sup>-type SiN<sub>3</sub>O 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 La<sub>11</sub>Si<sub>13</sub>N<sub>27.636</sub>O<sub>1.046</sub>:Ce<sup>3+</sup> were obtained by exploratory
high-temperature synthesis starting from La, LaÂ(NH<sub>2</sub>)<sub>3</sub>, SiÂ(NH)<sub>2</sub>, BaH<sub>2</sub>, and CeF<sub>3</sub>. Owing to the small size of the crystals, microfocused synchrotron
radiation was used for structure investigations (space group <i>Cmc</i>2<sub>1</sub> (No. 36), <i>a</i> = 9.5074(4)
Ã…, <i>b</i> = 32.0626(9) Ã…, <i>c</i> = 18.5076(8) Ã…, Z = 8, R1Â(all) = 0.0267). The crystal structure
consists of an unprecedented interrupted three-dimensional network
of vertex-sharing SiN<sub>4–<i>x</i></sub>O<sub><i>x</i></sub> 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 La<sub>11</sub>Si<sub>13</sub>N<sub>27.636</sub>O<sub>1.046</sub>:Ce<sup>3+</sup>. 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 La<sub>11</sub>Si<sub>13</sub>N<sub>27.376</sub>O<sub>0.936</sub>F were obtained and investigated
Li<sub>14</sub>Ln<sub>5</sub>[Si<sub>11</sub>N<sub>19</sub>O<sub>5</sub>]O<sub>2</sub>F<sub>2</sub> with Ln = Ce, Ndî—¸Representatives of a Family of Potential Lithium Ion Conductors
The isotypic layered oxonitridosilicates Li<sub>14</sub>Ln<sub>5</sub>[Si<sub>11</sub>N<sub>19</sub>O<sub>5</sub>]ÂO<sub>2</sub>F<sub>2</sub> (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, Li<sub>14</sub>Ce<sub>5</sub>[Si<sub>11</sub>N<sub>19</sub>O<sub>5</sub>]ÂO<sub>2</sub>F<sub>2</sub>: <i>a</i> = 17.178(3), <i>b</i> = 7.6500(15), <i>c</i> = 10.116(2) Ã…, <i>R</i>1 = 0.0409, <i>wR</i>2 = 0.0896; Li<sub>14</sub>Nd<sub>5</sub>[Si<sub>11</sub>N<sub>19</sub>O<sub>5</sub>]ÂO<sub>2</sub>F<sub>2</sub>: <i>a</i> = 17.126(2), <i>b</i> = 7.6155(15), <i>c</i> = 10.123(2) Ã…, <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<sup>–5</sup> 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