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 Devilā€™s Staircase

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    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 structureā€“function 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>āˆ¼1</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 Moā€“Mo bonds and subsequent Moā€“O 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>

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    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> = 0.25ā€“0.325) 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> = 0ā€“0.1) 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>

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    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> = 0.25ā€“0.325) 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> = 0ā€“0.1) 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

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    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 āˆ¼600 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 āˆ¼10<sup>ā€“4</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 Aā€‘La<sub>2</sub>Si<sub>2</sub>O<sub>7</sub> Tetragonal Phase Stabilized by Spray Pyrolysis Synthesis

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    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) ƅ, and <i>c</i> = 24.84133(1) ƅ. 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 Aā€‘La<sub>2</sub>Si<sub>2</sub>O<sub>7</sub> Tetragonal Phase Stabilized by Spray Pyrolysis Synthesis

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    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) ƅ, and <i>c</i> = 24.84133(1) ƅ. 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

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    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

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    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

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    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 Aā€‘La<sub>2</sub>Si<sub>2</sub>O<sub>7</sub> Tetragonal Phase Stabilized by Spray Pyrolysis Synthesis

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    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) ƅ, and <i>c</i> = 24.84133(1) ƅ. 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
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