24 research outputs found

    Evidence of Nanometric-Sized Phosphate Clusters in Bioactive Glasses As Revealed by Solid-State <sup>31</sup>P NMR

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    Bioactive glasses are able to form strong bonds to bone. This property, crucial for medical applications, depends on the glass composition and structure. Dissolution of phosphates in melt-quenched silicate glasses raises the question of chemical homogeneity and possible formation of clusters. A detailed structural characterization of the bioactive glasses is thus highly desirable. In this work, the nature of the distribution of phosphate units in a melt-quenched bioactive glass is elucidated for the first time using <sup>31</sup>P spin-counting solid-state NMR experiments. The structure of a dense bioactive calcium silicate glass with 2.6 mol % of phosphorus oxide is shown to exhibit nanometric-sized chemical and structural heterogeneities. Clear experimental evidence of the presence of phosphate clusters of five and six PO<sub>4</sub> tetrahedral units embedded in the disordered polymeric silicate network is given

    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

    Combined Approach for the Structural Characterization of Alkali Fluoroscandates: Solid-State NMR, Powder Xā€‘ray Diffraction, and Density Functional Theory Calculations

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    The structures of several fluoroscandate compounds are presented here using a characterization approach combining powder X-ray diffraction and solid-state NMR. The structure of K<sub>5</sub>Sc<sub>3</sub>F<sub>14</sub> was fully determined from Rietveld refinement performed on powder X-ray diffraction data. Moreover, the local structures of NaScF<sub>4</sub>, Li<sub>3</sub>ScF<sub>6</sub>, KSc<sub>2</sub>F<sub>7</sub>, and Na<sub>3</sub>ScF<sub>6</sub> compounds were studied in detail from solid-state <sup>19</sup>F and <sup>45</sup>Sc NMR experiments. The <sup>45</sup>Sc chemical shift ranges for six- and seven-coordinated scandium environments were defined. The <sup>19</sup>F chemical shift ranges for bridging and terminal fluorine atoms were also determined. First-principles calculations of the <sup>19</sup>F and <sup>45</sup>Sc NMR parameters were carried out using plane-wave basis sets and periodic boundary conditions (<i>CASTEP</i>), and the results were compared with the experimental data. A good agreement between the calculated shielding constants and experimental chemical shifts was obtained. This demonstrates the good potential of computational methods in spectroscopic assignments of solid-state <sup>45</sup>Sc NMR spectroscopy

    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

    Crystal Structures and Photoluminescence across the La<sub>2</sub>Si<sub>2</sub>O<sub>7</sub>ā€“Ho<sub>2</sub>Si<sub>2</sub>O<sub>7</sub> System

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    It is well-known that when an RE<sub>2</sub>Si<sub>2</sub>O<sub>7</sub> matrix is doped with active lanthanide ions, it displays promising luminescent responses for optical applications. The crystalline structure adopted by the silicate matrix as well as the distribution of the dopants among the available RE crystallographic sites have important effects on the luminescent yields of these compounds. The present study is aimed at analyzing the structural behavior as well as the luminescent properties of Ho<sup>3+</sup>-substituted La<sub>2</sub>Si<sub>2</sub>O<sub>7</sub>. Several compositions across the La<sub>2</sub>Si<sub>2</sub>O<sub>7</sub>ā€“Ho<sub>2</sub>Si<sub>2</sub>O<sub>7</sub> system were synthesized using the solā€“gel method followed by calcination at 1600 Ā°C. The resulting powders were analyzed by means of X-ray and neutron diffraction to determine the phase stabilities across the system. The results indicated a solid solubility region of G-(La,Ho)<sub>2</sub>Si<sub>2</sub>O<sub>7</sub> which extends to the La<sub>0.6</sub>Ho<sub>1.4</sub>Si<sub>2</sub>O<sub>7</sub> composition. Compositions richer in Ho<sup>3+</sup> show a two-phase domain (G+Ī“), while Ī“-(La,Ho)<sub>2</sub>Si<sub>2</sub>O<sub>7</sub> is the stable phase for Ho<sup>3+</sup> contents higher than 90% (La<sub>0.2</sub>Ho<sub>1.8</sub>Si<sub>2</sub>O<sub>7</sub>). Anomalous diffraction data interestingly indicated that the La<sup>3+</sup> for Ho<sup>3+</sup> substitution mechanism in the G-(La,Ho)<sub>2</sub>Si<sub>2</sub>O<sub>7</sub> polymorph is not homogeneous, but a preferential occupation of Ho<sup>3+</sup> for the RE2 site is observed. The Ho<sup>3+</sup>-doped G-La<sub>2</sub>Si<sub>2</sub>O<sub>7</sub> phosphors exhibited a strong green luminescence after excitation at 446 nm. Lifetime measurements indicated that the optimum phosphor was that with a Ho<sup>3+</sup> content of 10%

    New 8-Layer Twinned Hexagonal Perovskite Microwave Dielectric Ceramics Ba<sub>8</sub>Ga<sub>4ā€“<i>x</i></sub>Ta<sub>4+0.6<i>x</i></sub>O<sub>24</sub>

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    An 8-layer B-site deficient twinned hexagonal perovskite Ba<sub>8</sub>Ga<sub>4ā€“<i>x</i></sub>Ta<sub>4+0.6<i>x</i></sub>O<sub>24</sub> has been synthesized and its structure and microwave dielectric properties characterized. This hexagonal perovskite consists of eight close-packed BaO<sub>3</sub> layers stacked by a sequence of (ccch)<sub>2</sub>, where c and h refer to cubic and hexagonal BaO<sub>3</sub> layers, respectively. The Ba<sub>8</sub>Ga<sub>4ā€“<i>x</i></sub>Ta<sub>4+0.6<i>x</i></sub>O<sub>24</sub> ceramic materials exhibit composition-independent dielectric permittivity Īµ<sub>r</sub> ā‰ˆ 29, improved <i>Q</i><i>f</i> value with the B-site vacancy content increase, and tunable temperature coefficient of resonant frequency Ļ„<sub>f</sub> from negative to positive. An optimum microwave dielectric performance was achieved for Ba<sub>8</sub>Ga<sub>0.8</sub>Ta<sub>5.92</sub>O<sub>24</sub>: <i>Q</i><i>f</i> ā‰ˆ 29ā€‰000 GHz and Ļ„<sub>f</sub> ā‰ˆ 11 ppm/Ā°C. The factors controlling the microwave dielectric properties are discussed in comparison with 8-layer twinned analogues and related 10-layer twinned hexagonal perovskites based on their structural and property data

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

    No full text
    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

    Crystal Structures and Photoluminescence across the La<sub>2</sub>Si<sub>2</sub>O<sub>7</sub>ā€“Ho<sub>2</sub>Si<sub>2</sub>O<sub>7</sub> System

    No full text
    It is well-known that when an RE<sub>2</sub>Si<sub>2</sub>O<sub>7</sub> matrix is doped with active lanthanide ions, it displays promising luminescent responses for optical applications. The crystalline structure adopted by the silicate matrix as well as the distribution of the dopants among the available RE crystallographic sites have important effects on the luminescent yields of these compounds. The present study is aimed at analyzing the structural behavior as well as the luminescent properties of Ho<sup>3+</sup>-substituted La<sub>2</sub>Si<sub>2</sub>O<sub>7</sub>. Several compositions across the La<sub>2</sub>Si<sub>2</sub>O<sub>7</sub>ā€“Ho<sub>2</sub>Si<sub>2</sub>O<sub>7</sub> system were synthesized using the solā€“gel method followed by calcination at 1600 Ā°C. The resulting powders were analyzed by means of X-ray and neutron diffraction to determine the phase stabilities across the system. The results indicated a solid solubility region of G-(La,Ho)<sub>2</sub>Si<sub>2</sub>O<sub>7</sub> which extends to the La<sub>0.6</sub>Ho<sub>1.4</sub>Si<sub>2</sub>O<sub>7</sub> composition. Compositions richer in Ho<sup>3+</sup> show a two-phase domain (G+Ī“), while Ī“-(La,Ho)<sub>2</sub>Si<sub>2</sub>O<sub>7</sub> is the stable phase for Ho<sup>3+</sup> contents higher than 90% (La<sub>0.2</sub>Ho<sub>1.8</sub>Si<sub>2</sub>O<sub>7</sub>). Anomalous diffraction data interestingly indicated that the La<sup>3+</sup> for Ho<sup>3+</sup> substitution mechanism in the G-(La,Ho)<sub>2</sub>Si<sub>2</sub>O<sub>7</sub> polymorph is not homogeneous, but a preferential occupation of Ho<sup>3+</sup> for the RE2 site is observed. The Ho<sup>3+</sup>-doped G-La<sub>2</sub>Si<sub>2</sub>O<sub>7</sub> phosphors exhibited a strong green luminescence after excitation at 446 nm. Lifetime measurements indicated that the optimum phosphor was that with a Ho<sup>3+</sup> content of 10%
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