13 research outputs found
The Na<sub><i>x</i></sub>MoO<sub>2</sub> Phase Diagram (<sup>1</sup>/<sub>2</sub> ā¤ <i>x</i> < 1): An Electrochemical Devilās Staircase
Layered sodium transition
metal oxides represent a complex class
of materials that exhibit a variety of properties, for example, superconductivity,
and can feature in a range of applications, for example, batteries.
Understanding the structureāfunction relationship is key to
developing better materials. In this context, the phase diagram of
the Na<sub><i>x</i></sub>MoO<sub>2</sub> system has been
studied using electrochemistry combined with in situ synchrotron X-ray
diffraction experiments. The many steps observed in the electrochemical
curve of Na<sub>2/3</sub>MoO<sub>2</sub> during cycling in a sodium
battery suggest numerous reversible structural transitions during
sodium (de)Āintercalation between Na<sub>0.5</sub>MoO<sub>2</sub> and
Na<sub>ā¼1</sub>MoO<sub>2</sub>. In situ X-ray diffraction confirmed
the complexity of the phase diagram within this domain, 13 single
phase domains with minute changes in sodium contents. Almost all display
superstructure or modulation peaks in their X-ray diffraction patterns
suggesting the existence of many Na<sub><i>x</i></sub>MoO<sub>2</sub> specific phases that are believed to be characterized by
sodium/vacancy ordering as well as MoāMo bonds and subsequent
MoāO distances patterning in the structures. Moreover, a room
temperature triclinic distortion was evidenced in the composition
range 0.58 ā¤ <i>x</i> < 0.75, for the first time
in a sodium layered oxide system. Monoclinic and triclinic subcell
parameters were refined for every Na<sub><i>x</i></sub>MoO<sub>2</sub> phase identified. Reversible [MoO<sub>2</sub>] slab glidings
occur during the sodium (de)Āintercalation. This level of structural
detail provides unprecedented insight on the phases present and their
evolution, which may allow each phase to be isolated and examined
in more detail
Defect Structure, Phase Separation, and Electrical Properties of Nonstoichiometric Tetragonal Tungsten Bronze Ba<sub>0.5ā<i>x</i></sub>TaO<sub>3ā<i>x</i></sub>
New
insight into the defect chemistry of the tetragonal tungsten bronze
(TTB) Ba<sub>0.5ā<i>x</i></sub>ĀTaO<sub>3ā<i>x</i></sub> is established here, which is shown to adapt to
a continuous and extensive range of both cationic and anionic defect
stoichiometries. The highly nonstoichiometric TTB Ba<sub>0.5ā<i>x</i></sub>ĀTaO<sub>3ā<i>x</i></sub> (<i>x</i> = 0.25ā0.325) compositions are stabilized via the
interpolation of Ba<sup>2+</sup> cations and (TaO)<sup>3+</sup> groups
into pentagonal tunnels, forming distinct Ba chains and alternate
Ta-O rows in the pentagonal tunnels along the <i>c</i> axis.
The slightly nonstoichiometric Ba<sub>0.5ā<i>x</i></sub>ĀTaO<sub>3ā<i>x</i></sub> (<i>x</i> = 0ā0.1) compositions incorporate framework oxygen and tunnel
cation deficiencies in the TTB structure. These two mechanisms result
in phase separation within the 0.1< <i>x</i> < 0.25
nonstoichiometric range, resulting in two closely related (TaO)<sup>3+</sup>-containing and (TaO)<sup>3+</sup>-free TTB phases. The highly
nonstoichiometric (TaO)<sup>3+</sup>-containing phase exhibits Ba<sup>2+</sup> cationic migration. The incorporation of (TaO)<sup>3+</sup> units into the pentagonal tunnel and the local relaxation of the
octahedral framework around the (TaO)<sup>3+</sup> units are revealed
by diffraction data analysis and are shown to affect the transport
and polarization properties of these compositions
Defect Structure, Phase Separation, and Electrical Properties of Nonstoichiometric Tetragonal Tungsten Bronze Ba<sub>0.5ā<i>x</i></sub>TaO<sub>3ā<i>x</i></sub>
New
insight into the defect chemistry of the tetragonal tungsten bronze
(TTB) Ba<sub>0.5ā<i>x</i></sub>ĀTaO<sub>3ā<i>x</i></sub> is established here, which is shown to adapt to
a continuous and extensive range of both cationic and anionic defect
stoichiometries. The highly nonstoichiometric TTB Ba<sub>0.5ā<i>x</i></sub>ĀTaO<sub>3ā<i>x</i></sub> (<i>x</i> = 0.25ā0.325) compositions are stabilized via the
interpolation of Ba<sup>2+</sup> cations and (TaO)<sup>3+</sup> groups
into pentagonal tunnels, forming distinct Ba chains and alternate
Ta-O rows in the pentagonal tunnels along the <i>c</i> axis.
The slightly nonstoichiometric Ba<sub>0.5ā<i>x</i></sub>ĀTaO<sub>3ā<i>x</i></sub> (<i>x</i> = 0ā0.1) compositions incorporate framework oxygen and tunnel
cation deficiencies in the TTB structure. These two mechanisms result
in phase separation within the 0.1< <i>x</i> < 0.25
nonstoichiometric range, resulting in two closely related (TaO)<sup>3+</sup>-containing and (TaO)<sup>3+</sup>-free TTB phases. The highly
nonstoichiometric (TaO)<sup>3+</sup>-containing phase exhibits Ba<sup>2+</sup> cationic migration. The incorporation of (TaO)<sup>3+</sup> units into the pentagonal tunnel and the local relaxation of the
octahedral framework around the (TaO)<sup>3+</sup> units are revealed
by diffraction data analysis and are shown to affect the transport
and polarization properties of these compositions
Localization of Oxygen Interstitials in CeSrGa<sub>3</sub>O<sub>7+Ī“</sub> Melilite
The
solubility of Ce in the La<sub>1ā<i>x</i></sub>Ce<sub><i>x</i></sub>SrGa<sub>3</sub>O<sub>7+Ī“</sub> and
La<sub>1.54ā<i>x</i></sub>Ce<sub><i>x</i></sub>Sr<sub>0.46</sub>Ga<sub>3</sub>O<sub>7.27+Ī“</sub> melilites
was investigated, along with the thermal redox stability in air of
these melilites and the conductivity variation associated with oxidization
of Ce<sup>3+</sup> into Ce<sup>4+</sup>. Under CO reducing atmosphere,
the La in LaSrGa<sub>3</sub>O<sub>7</sub> may be completely substituted
by Ce to form the La<sub>1ā<i>x</i></sub>Ce<sub><i>x</i></sub>SrGa<sub>3</sub>O<sub>7+Ī“</sub> solid solution,
which is stable in air to ā¼600 Ā°C when <i>x</i> ā„ 0.6. On the other side, the La<sub>1.54ā<i>x</i></sub>Ce<sub><i>x</i></sub>Sr<sub>0.46</sub>Ga<sub>3</sub>O<sub>7.27+Ī“</sub> compositions displayed much lower
Ce solubility (<i>x</i> ā¤ 0.1), irrespective of the
synthesis atmosphere. In the as-made La<sub>1ā<i>x</i></sub>Ce<sub><i>x</i></sub>SrGa<sub>3</sub>O<sub>7+Ī“</sub>, the conductivity increased with the cerium content, due to the
enhanced electronic conduction arising from the 4f electrons in Ce<sup>3+</sup> cations. At 600 Ā°C, CeSrGa<sub>3</sub>O<sub>7+Ī“</sub> showed a conductivity of ā¼10<sup>ā4</sup> S/cm in
air, nearly 4 orders of magnitude higher than that of LaSrGa<sub>3</sub>O<sub>7</sub>. The oxidation of Ce<sup>3+</sup> into Ce<sup>4+</sup> in CeSrGa<sub>3</sub>O<sub>7+Ī“</sub> slightly reduced the
conductivity, and the oxygen excess did not result in apparent increase
of oxide ion conduction in CeSrGa<sub>3</sub>O<sub>7+Ī“</sub>. The Ce doping in air also reduced the interstitial oxide ion conductivity
of La<sub>1.54</sub>Sr<sub>0.46</sub>Ga<sub>3</sub>O<sub>7.27</sub>. Neutron powder diffraction study on CeSrGa<sub>3</sub>O<sub>7.39</sub> composition revealed that the extra oxygen is incorporated in the
four-linked GaO<sub>4</sub> polyhedral environment, leading to distorted
GaO<sub>5</sub> trigonal bipyramid. The stabilization and low mobility
of interstitial oxygen atoms in CeSrGa<sub>3</sub>O<sub>7+Ī“</sub>, in contrast with those in La<sub>1+<i>x</i></sub>Sr<sub>1ā<i>x</i></sub>Ga<sub>3</sub>O<sub>7+0.5<i>x</i></sub>, may be correlated with the cationic size contraction
from the oxidation of Ce<sup>3+</sup> to Ce<sup>4+</sup>. These results
provide a new comprehensive understanding of the accommodation and
conduction mechanism of the oxygen interstitials in the melilite structure
Crystal Structure and Luminescent Properties of Eu<sup>3+</sup>-Doped AāLa<sub>2</sub>Si<sub>2</sub>O<sub>7</sub> Tetragonal Phase Stabilized by Spray Pyrolysis Synthesis
Pure A-La<sub>2</sub>Si<sub>2</sub>O<sub>7</sub> powder has been
synthesized through a spray pyrolysis method followed by calcination
at 1100 Ā°C for 15 h. The crystallographic structure, refined
from the synchrotron powder diffraction pattern of the sample, showed
tetragonal symmetry with space group <i>P</i>4<sub>1</sub>, <i>a</i> = 6.83565(1) Ć
, and <i>c</i> =
24.84133(1) Ć
. The <sup>29</sup>Si and <sup>139</sup>La NMR spectra
have been described here for the first time in the literature and
could be simulated with four Si and four La resonances, respectively,
in good agreement with the presence of four Si and four La crystallographic
sites in the unit cell. The same synthesis method was successful for
the synthesis of Eu<sup>3+</sup>-doped A-La<sub>2</sub>Si<sub>2</sub>O<sub>7</sub> (%Eu = 3ā 40). The analysis of the unit cell
volumes indicated that Eu<sup>3+</sup> replaces La<sup>3+</sup> in
the unit cell for all Eu<sup>3+</sup> substitution levels investigated.
However, anomalous diffraction data indicated that the La/Eu substitution
mechanism was not homogeneous, but Eu much prefers to occupy the RE3
sites. The Eu-doped A-La<sub>2</sub>Si<sub>2</sub>O<sub>7</sub> phosphors
thus synthesized exhibited a strong orange-red luminescence after
excitation at 393 nm. Lifetime measurements indicated that the optimum
phosphor was that with an Eu<sup>3+</sup> content of 20%, which showed
a lifetime of 2.3 ms. The quantum yield of the latter was found to
be 12% at 393 nm excitation. These experimental observations together
with the high purity of the phase obtained by the proposed spray pyrolysis
method make this material an excellent phosphor for optoelectronic
applications
Crystal Structure and Luminescent Properties of Eu<sup>3+</sup>-Doped AāLa<sub>2</sub>Si<sub>2</sub>O<sub>7</sub> Tetragonal Phase Stabilized by Spray Pyrolysis Synthesis
Pure A-La<sub>2</sub>Si<sub>2</sub>O<sub>7</sub> powder has been
synthesized through a spray pyrolysis method followed by calcination
at 1100 Ā°C for 15 h. The crystallographic structure, refined
from the synchrotron powder diffraction pattern of the sample, showed
tetragonal symmetry with space group <i>P</i>4<sub>1</sub>, <i>a</i> = 6.83565(1) Ć
, and <i>c</i> =
24.84133(1) Ć
. The <sup>29</sup>Si and <sup>139</sup>La NMR spectra
have been described here for the first time in the literature and
could be simulated with four Si and four La resonances, respectively,
in good agreement with the presence of four Si and four La crystallographic
sites in the unit cell. The same synthesis method was successful for
the synthesis of Eu<sup>3+</sup>-doped A-La<sub>2</sub>Si<sub>2</sub>O<sub>7</sub> (%Eu = 3ā 40). The analysis of the unit cell
volumes indicated that Eu<sup>3+</sup> replaces La<sup>3+</sup> in
the unit cell for all Eu<sup>3+</sup> substitution levels investigated.
However, anomalous diffraction data indicated that the La/Eu substitution
mechanism was not homogeneous, but Eu much prefers to occupy the RE3
sites. The Eu-doped A-La<sub>2</sub>Si<sub>2</sub>O<sub>7</sub> phosphors
thus synthesized exhibited a strong orange-red luminescence after
excitation at 393 nm. Lifetime measurements indicated that the optimum
phosphor was that with an Eu<sup>3+</sup> content of 20%, which showed
a lifetime of 2.3 ms. The quantum yield of the latter was found to
be 12% at 393 nm excitation. These experimental observations together
with the high purity of the phase obtained by the proposed spray pyrolysis
method make this material an excellent phosphor for optoelectronic
applications
High-Resolution Structural Characterization of Two Layered Aluminophosphates by Synchrotron Powder Diffraction and NMR Crystallographies
The
syntheses and structure resolution process of two highly complex
powdered aluminophosphates with an original 5:7 Al/P ratio are presented:
[Al<sub>5</sub>(OH)Ā(PO<sub>4</sub>)<sub>3</sub>(PO<sub>3</sub>OH)<sub>4</sub>] [NH<sub>3</sub>(CH<sub>2</sub>)<sub>2</sub>NH<sub>3</sub>]<sub>2</sub> [2H<sub>2</sub>O], compound <b>1</b>, and [Al<sub>5</sub>(PO<sub>4</sub>)<sub>5</sub>(PO<sub>3</sub>OH)<sub>2</sub>] [NH<sub>3</sub>(CH<sub>2</sub>)<sub>3</sub>NH<sub>3</sub>]<sub>2</sub> [H<sub>2</sub>O], compound <b>2</b>. We have previously
reported the structure of the periodic part of <b>1</b> by coupling
synchrotron powder diffraction and solid-state nuclear magnetic resonance
(NMR) crystallographies. With a similar strategy, that is, input of
large parts of the building blocks determined by analysis of the <sup>27</sup>Alā<sup>31</sup>P correlation pattern of the two-dimensional
(2D) NMR spectrum in the structure search process, we first determine
the periodic structure of <b>2</b>, using the powder synchrotron
diffraction data as cost function. Both <b>1</b> and <b>2</b> are layered materials, in which the inorganic layers contain five
P and seven Al inequivalent atoms, with aluminum atoms that are found
in three different coordination states, AlO<sub>4</sub>, AlO<sub>5</sub>, and AlO<sub>6</sub>, and the interlayer space contains the amines
and water molecules. In <b>1</b>, the inorganic layers are stacked
on each other with a 4<sub>2</sub> element of symmetry along the <i>c</i>-axis, while they are stacked with a 180Ā° rotation
angle in <b>2</b>. By analysis of a set of high-resolution 1D
and 2D NMR spectra (<sup>31</sup>P, <sup>27</sup>Al, <sup>1</sup>H, <sup>15</sup>N, <sup>13</sup>C, <sup>27</sup>Alā<sup>31</sup>P, <sup>1</sup>Hā<sup>31</sup>P, and <sup>1</sup>Hā<sup>14</sup>N), the structure analysis of <b>1</b> and <b>2</b> is
extended beyond the strict periodicity, to which diffraction is restricted,
and provides localization of the hydroxyl groups and water molecules
in the frameworks and an attempt to correlate the presence of these
latter species to the structural features of the two samples is presented.
Finally, the dehydration/rehydration processes occurring in these
solids are analyzed. The methodology of the structure determination
for these dehydrated forms uses the same principles, combining X-ray
powder diffraction and solid-state NMR data
High-Resolution Structural Characterization of Two Layered Aluminophosphates by Synchrotron Powder Diffraction and NMR Crystallographies
The
syntheses and structure resolution process of two highly complex
powdered aluminophosphates with an original 5:7 Al/P ratio are presented:
[Al<sub>5</sub>(OH)Ā(PO<sub>4</sub>)<sub>3</sub>(PO<sub>3</sub>OH)<sub>4</sub>] [NH<sub>3</sub>(CH<sub>2</sub>)<sub>2</sub>NH<sub>3</sub>]<sub>2</sub> [2H<sub>2</sub>O], compound <b>1</b>, and [Al<sub>5</sub>(PO<sub>4</sub>)<sub>5</sub>(PO<sub>3</sub>OH)<sub>2</sub>] [NH<sub>3</sub>(CH<sub>2</sub>)<sub>3</sub>NH<sub>3</sub>]<sub>2</sub> [H<sub>2</sub>O], compound <b>2</b>. We have previously
reported the structure of the periodic part of <b>1</b> by coupling
synchrotron powder diffraction and solid-state nuclear magnetic resonance
(NMR) crystallographies. With a similar strategy, that is, input of
large parts of the building blocks determined by analysis of the <sup>27</sup>Alā<sup>31</sup>P correlation pattern of the two-dimensional
(2D) NMR spectrum in the structure search process, we first determine
the periodic structure of <b>2</b>, using the powder synchrotron
diffraction data as cost function. Both <b>1</b> and <b>2</b> are layered materials, in which the inorganic layers contain five
P and seven Al inequivalent atoms, with aluminum atoms that are found
in three different coordination states, AlO<sub>4</sub>, AlO<sub>5</sub>, and AlO<sub>6</sub>, and the interlayer space contains the amines
and water molecules. In <b>1</b>, the inorganic layers are stacked
on each other with a 4<sub>2</sub> element of symmetry along the <i>c</i>-axis, while they are stacked with a 180Ā° rotation
angle in <b>2</b>. By analysis of a set of high-resolution 1D
and 2D NMR spectra (<sup>31</sup>P, <sup>27</sup>Al, <sup>1</sup>H, <sup>15</sup>N, <sup>13</sup>C, <sup>27</sup>Alā<sup>31</sup>P, <sup>1</sup>Hā<sup>31</sup>P, and <sup>1</sup>Hā<sup>14</sup>N), the structure analysis of <b>1</b> and <b>2</b> is
extended beyond the strict periodicity, to which diffraction is restricted,
and provides localization of the hydroxyl groups and water molecules
in the frameworks and an attempt to correlate the presence of these
latter species to the structural features of the two samples is presented.
Finally, the dehydration/rehydration processes occurring in these
solids are analyzed. The methodology of the structure determination
for these dehydrated forms uses the same principles, combining X-ray
powder diffraction and solid-state NMR data
High-Resolution Structural Characterization of Two Layered Aluminophosphates by Synchrotron Powder Diffraction and NMR Crystallographies
The
syntheses and structure resolution process of two highly complex
powdered aluminophosphates with an original 5:7 Al/P ratio are presented:
[Al<sub>5</sub>(OH)Ā(PO<sub>4</sub>)<sub>3</sub>(PO<sub>3</sub>OH)<sub>4</sub>] [NH<sub>3</sub>(CH<sub>2</sub>)<sub>2</sub>NH<sub>3</sub>]<sub>2</sub> [2H<sub>2</sub>O], compound <b>1</b>, and [Al<sub>5</sub>(PO<sub>4</sub>)<sub>5</sub>(PO<sub>3</sub>OH)<sub>2</sub>] [NH<sub>3</sub>(CH<sub>2</sub>)<sub>3</sub>NH<sub>3</sub>]<sub>2</sub> [H<sub>2</sub>O], compound <b>2</b>. We have previously
reported the structure of the periodic part of <b>1</b> by coupling
synchrotron powder diffraction and solid-state nuclear magnetic resonance
(NMR) crystallographies. With a similar strategy, that is, input of
large parts of the building blocks determined by analysis of the <sup>27</sup>Alā<sup>31</sup>P correlation pattern of the two-dimensional
(2D) NMR spectrum in the structure search process, we first determine
the periodic structure of <b>2</b>, using the powder synchrotron
diffraction data as cost function. Both <b>1</b> and <b>2</b> are layered materials, in which the inorganic layers contain five
P and seven Al inequivalent atoms, with aluminum atoms that are found
in three different coordination states, AlO<sub>4</sub>, AlO<sub>5</sub>, and AlO<sub>6</sub>, and the interlayer space contains the amines
and water molecules. In <b>1</b>, the inorganic layers are stacked
on each other with a 4<sub>2</sub> element of symmetry along the <i>c</i>-axis, while they are stacked with a 180Ā° rotation
angle in <b>2</b>. By analysis of a set of high-resolution 1D
and 2D NMR spectra (<sup>31</sup>P, <sup>27</sup>Al, <sup>1</sup>H, <sup>15</sup>N, <sup>13</sup>C, <sup>27</sup>Alā<sup>31</sup>P, <sup>1</sup>Hā<sup>31</sup>P, and <sup>1</sup>Hā<sup>14</sup>N), the structure analysis of <b>1</b> and <b>2</b> is
extended beyond the strict periodicity, to which diffraction is restricted,
and provides localization of the hydroxyl groups and water molecules
in the frameworks and an attempt to correlate the presence of these
latter species to the structural features of the two samples is presented.
Finally, the dehydration/rehydration processes occurring in these
solids are analyzed. The methodology of the structure determination
for these dehydrated forms uses the same principles, combining X-ray
powder diffraction and solid-state NMR data
Crystal Structure and Luminescent Properties of Eu<sup>3+</sup>-Doped AāLa<sub>2</sub>Si<sub>2</sub>O<sub>7</sub> Tetragonal Phase Stabilized by Spray Pyrolysis Synthesis
Pure A-La<sub>2</sub>Si<sub>2</sub>O<sub>7</sub> powder has been
synthesized through a spray pyrolysis method followed by calcination
at 1100 Ā°C for 15 h. The crystallographic structure, refined
from the synchrotron powder diffraction pattern of the sample, showed
tetragonal symmetry with space group <i>P</i>4<sub>1</sub>, <i>a</i> = 6.83565(1) Ć
, and <i>c</i> =
24.84133(1) Ć
. The <sup>29</sup>Si and <sup>139</sup>La NMR spectra
have been described here for the first time in the literature and
could be simulated with four Si and four La resonances, respectively,
in good agreement with the presence of four Si and four La crystallographic
sites in the unit cell. The same synthesis method was successful for
the synthesis of Eu<sup>3+</sup>-doped A-La<sub>2</sub>Si<sub>2</sub>O<sub>7</sub> (%Eu = 3ā 40). The analysis of the unit cell
volumes indicated that Eu<sup>3+</sup> replaces La<sup>3+</sup> in
the unit cell for all Eu<sup>3+</sup> substitution levels investigated.
However, anomalous diffraction data indicated that the La/Eu substitution
mechanism was not homogeneous, but Eu much prefers to occupy the RE3
sites. The Eu-doped A-La<sub>2</sub>Si<sub>2</sub>O<sub>7</sub> phosphors
thus synthesized exhibited a strong orange-red luminescence after
excitation at 393 nm. Lifetime measurements indicated that the optimum
phosphor was that with an Eu<sup>3+</sup> content of 20%, which showed
a lifetime of 2.3 ms. The quantum yield of the latter was found to
be 12% at 393 nm excitation. These experimental observations together
with the high purity of the phase obtained by the proposed spray pyrolysis
method make this material an excellent phosphor for optoelectronic
applications