19 research outputs found

    Ordering of (Cr,V) Layers in Nanolamellar (Cr<sub>0.5</sub>V<sub>0.5</sub>)<sub><i>n</i>+1</sub>AlC<sub><i>n</i></sub> Compounds

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    <div><p>Nanolamellar MAX phase compounds (Cr<sub>0.5</sub>V<sub>0.5</sub>)<sub><i>n</i>+1</sub>AlC<sub><i>n</i></sub> are formed with <i>n</i>ā€‰=ā€‰1, 2 and 3, and their 300ā€…K structure is studied in detail by high-resolution neutron diffraction. While the <i>n</i>ā€‰=ā€‰1 compound is found to have complete disordering of vanadium and chromium in the metallic layers, the <i>n</i>ā€‰=ā€‰2 and 3 compounds show strong tendency for these elements' ordering, with the layer in the 2a(0,0,0) site of (Cr<sub>0.5</sub>V<sub>0.5</sub>)<sub>3</sub>AlC<sub>2</sub> fully occupied by vanadium. The thermal expansion dependency of temperature is also studied by neutron diffraction for 2ā€‰<ā€‰<i>T</i>ā€‰<ā€‰550ā€…K, revealing a negligible thermal expansion below 100ā€…K for all of the compounds.</p></div

    Structural Properties and Reversible Deuterium Loading of MgD<sub>2</sub>ā€“TiD<sub>2</sub> Nanocomposites

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    Structural properties and reversible deuterium uptake of MgD<sub>2</sub>ā€“TiD<sub>2</sub> nanocomposites have been studied by joint X-ray and neutron diffraction analyses to shed light on the extremely fast hydrogenation kinetics of these materials. (1 ā€“ <i>x</i>)Ā­MgD<sub>2</sub>ā€“<i>x</i>TiD<sub>2</sub> nanocomposites with compositions ranging between <i>x</i> = 0 and 0.5 have been prepared by reactive ball milling of Mg and Ti powders under deuterium pressure. They consist of mixtures of MgD<sub>2</sub> (Ī²-and Ī³-polymorphs) and Īµ-TiD<sub>2</sub> phases homogenously distributed at the nanoscale with crystallite sizes below 15 nm. Minor phase miscibility is detected with Mg solubility in the TiD<sub>2</sub> phase up to 8 at.% and Ti solubility in the Ī²-MgD<sub>2</sub> up to 7 at.% Ti. At moderate temperatures and pressures (<i>T</i> < 600 K, <i>P</i><sub>D2</sub> < 1 MPa) reversible deuterium loading in MgD<sub>2</sub>ā€“TiD<sub>2</sub> nanocomposites only occurs through the Ī²-MgD<sub>2</sub> to Mg transformation. Mg/MgD<sub>2</sub> thermodynamics is not modified as Ī³-MgD<sub>2</sub> and Ti solubility in Ī²-MgD<sub>2</sub> are metastable and do not operate during reversible deuterium loading. However, the TiD<sub>2</sub> phase allows for outstanding D-sorption kinetics in the Mg/MgD<sub>2</sub> system. This paper demonstrates that TiD<sub>2</sub> inclusions limit the grain growth of Mg and MgD<sub>2</sub> phases allowing for short D-diffusion paths. Furthermore, we provide evidence that the TiD<sub>2</sub> phase also favors H-mobility through the existence of coherent coupling between TiD<sub>2</sub> and Mg/MgD<sub>2</sub> phases and the presence of sub-stoichiometric MgD<sub>2ā€‘Ī·</sub> and TiD<sub>2ā€‘Ī·</sub> phases

    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

    Mixed Metallic Ba(Co,Fe)X<sub>0.2</sub>O<sub>3āˆ’Ī“</sub> (X = F, Cl) Hexagonal Perovskites: Drastic Effect of Fe-Incorporation on Structural and Electronic Features

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    Starting from the parent 10Hā€“Ba<sub>5</sub>Co<sub>5</sub>X<sub>1ā€“<i>x</i></sub>O<sub>13āˆ’Ī“</sub> (trimeric strings of face-sharing CoO<sub>6</sub> octahedra with terminal CoO<sub>4</sub> tetrahedra, stacking sequence (chhchā€²)<sub>2</sub>) and 6Hā€“Ba<sub>6</sub>Co<sub>6</sub>X<sub>1ā€“<i>x</i></sub>O<sub>16ā€‘Ī“</sub> (similar with tetrameric strings, stacking sequence chhhchā€²) hexagonal perovskites forms (X = F, Cl; <i>c</i>, <i>h</i> = [BaO<sub>3</sub>] layers ; <i>h</i>ā€² = [BaOX<sub>1ā€“<i>y</i></sub>] layers), we show here that the Fe incorporation leads to large domains of solid solutions for both X = F and Cl but exclusively stabilizes the 10H-form independently of the synthesis method. In this form, the lowest concentration of h-layers is stabilized by a sensitive metal reduction with increasing the Fe ratio. In a more general context of competition between several hexagonal perovskite polymorphs available for most of the transition metals, this redox change is most probably the key factor driving 1D (face-sharing chains) to 3D (corner-sharing) connectivities. Strikingly, ND data evidence the location of oxygen deficiencies in the tetrahedral (Co/Fe) coordination. This effect is exaggerated at high temperature, while (Co/Fe)Ā­O<sub>4ā€‘Ī“</sub> coordinations are completed by the displacement of X<sup>ā€“</sup> anions toward the (Co/Fe) sphere of coordination following a ā€œ<i>push-and-pull</i>ā€ mechanism within <i>h</i>ā€²ā€“[BaOX<sub>1ā€“<i>y</i></sub>] layers. The Fe-incorporation is also accompanied by increasing conduction gaps with predominant 1D variable range hopping. The full series show antiferromagnetic behavior with increasing <i>T</i><sub><i>N</i></sub> as [Fe] increases. For Fe-rich compounds <i>T</i><sub><i>N</i></sub> is estimated about 600 K, as rarely observed for hexagonal perovskite compounds. Finally, magnetic structures of all iron-doped compounds show a site-to-site AFM ordering, different of the magnetic structure of Co-only parent compounds. Here, DFT calculations predict low-spin octahedral Co configurations, but high-spin Fe species in the same sites

    Incorporation of Jahnā€“Teller Cu<sup>2+</sup> Ions into Magnetoelectric Multiferroic MnWO<sub>4</sub>: Structural, Magnetic, and Dielectric Permittivity Properties of Mn<sub>1ā€“<i>x</i></sub>Cu<sub><i>x</i></sub>WO<sub>4</sub> (<i>x</i> ā‰¤ 0.25)

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    Polycrystalline samples of Mn<sub>1ā€“<i>x</i></sub>Cu<sub><i>x</i></sub>WO<sub>4</sub> (<i>x</i> ā‰¤ 0.5) have been prepared by a solid-state synthesis as well as from a citrate synthesis at moderate temperature (850 Ā°C). The goal is to study changes in the structural, magnetic, and dielectric properties of magnetoelectric type-II multiferroic MnWO<sub>4</sub> caused by replacing Jahnā€“Teller-inactive Mn<sup>2+</sup> (d<sup>5</sup>, <i>S</i> = 5/2) ions with Jahnā€“Teller-active Cu<sup>2+</sup> (d<sup>9</sup>, <i>S</i> = 1/2) ions. Combination of techniques including scanning electron microscopy, powder X-ray and neutron diffraction, and Raman spectroscopy demonstrates that the polycrystalline samples with low copper content 0 ā‰¤ <i>x</i> ā‰¤ 0.25 are solid solution that forms in the monoclinic <i>P</i>2/c space group. Rietveld analyses indicate that Cu atoms substitutes for Mn atoms at the Mn crystallographic site of the MnWO<sub>4</sub> structure and suggest random distributions of Jahnā€“Teller-distorted CuO<sub>6</sub> octahedra in the solid solution. Magnetic susceptibility reveals that only 5% of Cu substitution suppresses the nonpolar collinear AF1 antiferromagnetic structure observed in pure MnWO<sub>4</sub>. Type-II multiferroicity survives a weak Cu substitution rate (<i>x</i> < 0.15). Multiferroic transition temperature and NeĢel temperature increase as the amount of Cu increases. New trends in some of the magnetic properties and in dielectric behaviors are observed for <i>x</i> = 0.20 and 0.25. Careful analysis of the magnetic susceptibility reveals that the incorporation of Cu into MnWO<sub>4</sub> strengthens the overall antiferromagnetic interaction and reduces the magnetic frustration

    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

    Considerable Improvement of Long-Persistent Luminescence in Germanium and Tin Substituted ZnGa<sub>2</sub>O<sub>4</sub>

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    The red long-lasting luminescence properties of the ZnGa<sub>2</sub>O<sub>4</sub>:Cr<sup>3+</sup> spinel material are shown to be much improved when germanium or tin is substituted to the nominal composition. The resulting Zn<sub>1+<i>x</i></sub>Ga<sub>2ā€“2<i>x</i></sub>(Ge/Sn)<sub><i>x</i></sub>O<sub>4</sub> (0 ā‰¤ <i>x</i> ā‰¤ 0.5) spinel solid solutions synthesized here by a classic solid state method have been structurally characterized by X-ray and neutron powder diffraction refinements coupled to <sup>71</sup>Ga solid state NMR studies. In contrast to ZnGa<sub>2</sub>O<sub>4</sub>:Cr<sup>3+</sup> for which long lasting luminescence properties have been reported to arise from tetrahedral positively charged defects resulting from the spinel inversion, our results show that a different mechanism occurs complementary for Zn<sub>1+<i>x</i></sub>Ga<sub>2ā€“2<i>x</i></sub>(Ge/Sn)<sub><i>x</i></sub>O<sub>4</sub>. Here, the great enhancement of the brightness and decay time of the long lasting luminescence properties is directly driven by the substitution mechanism which creates distorted octahedral sites surrounded by octahedral Ge and Sn positive substitutional defects which likely act as new efficient traps

    Considerable Improvement of Long-Persistent Luminescence in Germanium and Tin Substituted ZnGa<sub>2</sub>O<sub>4</sub>

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    The red long-lasting luminescence properties of the ZnGa<sub>2</sub>O<sub>4</sub>:Cr<sup>3+</sup> spinel material are shown to be much improved when germanium or tin is substituted to the nominal composition. The resulting Zn<sub>1+<i>x</i></sub>Ga<sub>2ā€“2<i>x</i></sub>(Ge/Sn)<sub><i>x</i></sub>O<sub>4</sub> (0 ā‰¤ <i>x</i> ā‰¤ 0.5) spinel solid solutions synthesized here by a classic solid state method have been structurally characterized by X-ray and neutron powder diffraction refinements coupled to <sup>71</sup>Ga solid state NMR studies. In contrast to ZnGa<sub>2</sub>O<sub>4</sub>:Cr<sup>3+</sup> for which long lasting luminescence properties have been reported to arise from tetrahedral positively charged defects resulting from the spinel inversion, our results show that a different mechanism occurs complementary for Zn<sub>1+<i>x</i></sub>Ga<sub>2ā€“2<i>x</i></sub>(Ge/Sn)<sub><i>x</i></sub>O<sub>4</sub>. Here, the great enhancement of the brightness and decay time of the long lasting luminescence properties is directly driven by the substitution mechanism which creates distorted octahedral sites surrounded by octahedral Ge and Sn positive substitutional defects which likely act as new efficient traps

    Considerable Improvement of Long-Persistent Luminescence in Germanium and Tin Substituted ZnGa<sub>2</sub>O<sub>4</sub>

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    The red long-lasting luminescence properties of the ZnGa<sub>2</sub>O<sub>4</sub>:Cr<sup>3+</sup> spinel material are shown to be much improved when germanium or tin is substituted to the nominal composition. The resulting Zn<sub>1+<i>x</i></sub>Ga<sub>2ā€“2<i>x</i></sub>(Ge/Sn)<sub><i>x</i></sub>O<sub>4</sub> (0 ā‰¤ <i>x</i> ā‰¤ 0.5) spinel solid solutions synthesized here by a classic solid state method have been structurally characterized by X-ray and neutron powder diffraction refinements coupled to <sup>71</sup>Ga solid state NMR studies. In contrast to ZnGa<sub>2</sub>O<sub>4</sub>:Cr<sup>3+</sup> for which long lasting luminescence properties have been reported to arise from tetrahedral positively charged defects resulting from the spinel inversion, our results show that a different mechanism occurs complementary for Zn<sub>1+<i>x</i></sub>Ga<sub>2ā€“2<i>x</i></sub>(Ge/Sn)<sub><i>x</i></sub>O<sub>4</sub>. Here, the great enhancement of the brightness and decay time of the long lasting luminescence properties is directly driven by the substitution mechanism which creates distorted octahedral sites surrounded by octahedral Ge and Sn positive substitutional defects which likely act as new efficient traps
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