24 research outputs found
Evidence of Nanometric-Sized Phosphate Clusters in Bioactive Glasses As Revealed by Solid-State <sup>31</sup>P NMR
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>
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
Considerable Improvement of Long-Persistent Luminescence in Germanium and Tin Substituted ZnGa<sub>2</sub>O<sub>4</sub>
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>2ā2<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>2ā2<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
Combined Approach for the Structural Characterization of Alkali Fluoroscandates: Solid-State NMR, Powder Xāray Diffraction, and Density Functional Theory Calculations
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
Crystal Structures and Photoluminescence across the La<sub>2</sub>Si<sub>2</sub>O<sub>7</sub>āHo<sub>2</sub>Si<sub>2</sub>O<sub>7</sub> System
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>āHo<sub>2</sub>Si<sub>2</sub>O<sub>7</sub> system were synthesized using the solāgel
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>
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 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 29ā000
GHz and Ļ<sub>f</sub> ā 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>
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>2ā2<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>2ā2<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>āHo<sub>2</sub>Si<sub>2</sub>O<sub>7</sub> System
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>āHo<sub>2</sub>Si<sub>2</sub>O<sub>7</sub> system were synthesized using the solāgel
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%