Evidence of Nanometric-Sized Phosphate Clusters in Bioactive Glasses As Revealed by Solid-State ³¹P 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 ³¹P 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 PO₄ tetrahedral units embedded in the disordered polymeric silicate network is given

Defect Structure, Phase Separation, and Electrical Properties of Nonstoichiometric Tetragonal Tungsten Bronze Ba_{0.5–<i>x</i>}TaO_3–<i>x</i>

New insight into the defect chemistry of the tetragonal tungsten bronze (TTB) Ba_{0.5–<i>x</i>}­TaO_3–<i>x</i> 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_{0.5–<i>x</i>}­TaO_3–<i>x</i> (<i>x</i> = 0.25–0.325) compositions are stabilized via the interpolation of Ba²⁺ cations and (TaO)³⁺ 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_{0.5–<i>x</i>}­TaO_3–<i>x</i> (<i>x</i> = 0–0.1) 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)³⁺-containing and (TaO)³⁺-free TTB phases. The highly nonstoichiometric (TaO)³⁺-containing phase exhibits Ba²⁺ cationic migration. The incorporation of (TaO)³⁺ units into the pentagonal tunnel and the local relaxation of the octahedral framework around the (TaO)³⁺ 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_{0.5–<i>x</i>}TaO_3–<i>x</i>

New insight into the defect chemistry of the tetragonal tungsten bronze (TTB) Ba_{0.5–<i>x</i>}­TaO_3–<i>x</i> 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_{0.5–<i>x</i>}­TaO_3–<i>x</i> (<i>x</i> = 0.25–0.325) compositions are stabilized via the interpolation of Ba²⁺ cations and (TaO)³⁺ 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_{0.5–<i>x</i>}­TaO_3–<i>x</i> (<i>x</i> = 0–0.1) 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)³⁺-containing and (TaO)³⁺-free TTB phases. The highly nonstoichiometric (TaO)³⁺-containing phase exhibits Ba²⁺ cationic migration. The incorporation of (TaO)³⁺ units into the pentagonal tunnel and the local relaxation of the octahedral framework around the (TaO)³⁺ 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 CeSrGa₃O_7+δ Melilite

The solubility of Ce in the La_1–<i>x</i>Ce_<i>x</i>SrGa₃O_7+δ and La_{1.54–<i>x</i>}Ce_<i>x</i>Sr_0.46Ga₃O_7.27+δ melilites was investigated, along with the thermal redox stability in air of these melilites and the conductivity variation associated with oxidization of Ce³⁺ into Ce⁴⁺. Under CO reducing atmosphere, the La in LaSrGa₃O₇ may be completely substituted by Ce to form the La_1–<i>x</i>Ce_<i>x</i>SrGa₃O_7+δ solid solution, which is stable in air to ∼600 °C when <i>x</i> ≥ 0.6. On the other side, the La_{1.54–<i>x</i>}Ce_<i>x</i>Sr_0.46Ga₃O_7.27+δ compositions displayed much lower Ce solubility (<i>x</i> ≤ 0.1), irrespective of the synthesis atmosphere. In the as-made La_1–<i>x</i>Ce_<i>x</i>SrGa₃O_7+δ, the conductivity increased with the cerium content, due to the enhanced electronic conduction arising from the 4f electrons in Ce³⁺ cations. At 600 °C, CeSrGa₃O_7+δ showed a conductivity of ∼10^–4 S/cm in air, nearly 4 orders of magnitude higher than that of LaSrGa₃O₇. The oxidation of Ce³⁺ into Ce⁴⁺ in CeSrGa₃O_7+δ slightly reduced the conductivity, and the oxygen excess did not result in apparent increase of oxide ion conduction in CeSrGa₃O_7+δ. The Ce doping in air also reduced the interstitial oxide ion conductivity of La_1.54Sr_0.46Ga₃O_7.27. Neutron powder diffraction study on CeSrGa₃O_7.39 composition revealed that the extra oxygen is incorporated in the four-linked GaO₄ polyhedral environment, leading to distorted GaO₅ trigonal bipyramid. The stabilization and low mobility of interstitial oxygen atoms in CeSrGa₃O_7+δ, in contrast with those in La_1+<i>x</i>Sr_1–<i>x</i>Ga₃O_{7+0.5<i>x</i>}, may be correlated with the cationic size contraction from the oxidation of Ce³⁺ to Ce⁴⁺. 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 X‑ray 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 K₅Sc₃F₁₄ was fully determined from Rietveld refinement performed on powder X-ray diffraction data. Moreover, the local structures of NaScF₄, Li₃ScF₆, KSc₂F₇, and Na₃ScF₆ compounds were studied in detail from solid-state ¹⁹F and ⁴⁵Sc NMR experiments. The ⁴⁵Sc chemical shift ranges for six- and seven-coordinated scandium environments were defined. The ¹⁹F chemical shift ranges for bridging and terminal fluorine atoms were also determined. First-principles calculations of the ¹⁹F and ⁴⁵Sc 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 ⁴⁵Sc NMR spectroscopy

Considerable Improvement of Long-Persistent Luminescence in Germanium and Tin Substituted ZnGa₂O₄

The red long-lasting luminescence properties of the ZnGa₂O₄:Cr³⁺ spinel material are shown to be much improved when germanium or tin is substituted to the nominal composition. The resulting Zn_1+<i>x</i>Ga_{2–2<i>x</i>}(Ge/Sn)_<i>x</i>O₄ (0 ≤ <i>x</i> ≤ 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 ⁷¹Ga solid state NMR studies. In contrast to ZnGa₂O₄:Cr³⁺ 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_1+<i>x</i>Ga_{2–2<i>x</i>}(Ge/Sn)_<i>x</i>O₄. 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 La₂Si₂O₇–Ho₂Si₂O₇ System

It is well-known that when an RE₂Si₂O₇ 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 Ho³⁺-substituted La₂Si₂O₇. Several compositions across the La₂Si₂O₇–Ho₂Si₂O₇ system were synthesized using the sol–gel method followed by calcination at 1600 °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)₂Si₂O₇ which extends to the La_0.6Ho_1.4Si₂O₇ composition. Compositions richer in Ho³⁺ show a two-phase domain (G+δ), while δ-(La,Ho)₂Si₂O₇ is the stable phase for Ho³⁺ contents higher than 90% (La_0.2Ho_1.8Si₂O₇). Anomalous diffraction data interestingly indicated that the La³⁺ for Ho³⁺ substitution mechanism in the G-(La,Ho)₂Si₂O₇ polymorph is not homogeneous, but a preferential occupation of Ho³⁺ for the RE2 site is observed. The Ho³⁺-doped G-La₂Si₂O₇ phosphors exhibited a strong green luminescence after excitation at 446 nm. Lifetime measurements indicated that the optimum phosphor was that with a Ho³⁺ content of 10%

New 8-Layer Twinned Hexagonal Perovskite Microwave Dielectric Ceramics Ba₈Ga_4–<i>x</i>Ta_{4+0.6<i>x</i>}O₂₄

An 8-layer B-site deficient twinned hexagonal perovskite Ba₈Ga_4–<i>x</i>Ta_{4+0.6<i>x</i>}O₂₄ has been synthesized and its structure and microwave dielectric properties characterized. This hexagonal perovskite consists of eight close-packed BaO₃ layers stacked by a sequence of (ccch)₂, where c and h refer to cubic and hexagonal BaO₃ layers, respectively. The Ba₈Ga_4–<i>x</i>Ta_{4+0.6<i>x</i>}O₂₄ ceramic materials exhibit composition-independent dielectric permittivity ε_r ≈ 29, improved <i>Q</i><i>f</i> value with the B-site vacancy content increase, and tunable temperature coefficient of resonant frequency τ_f from negative to positive. An optimum microwave dielectric performance was achieved for Ba₈Ga_0.8Ta_5.92O₂₄: <i>Q</i><i>f</i> ≈ 29 000 GHz and τ_f ≈ 11 ppm/°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 ZnGa₂O₄

The red long-lasting luminescence properties of the ZnGa₂O₄:Cr³⁺ spinel material are shown to be much improved when germanium or tin is substituted to the nominal composition. The resulting Zn_1+<i>x</i>Ga_{2–2<i>x</i>}(Ge/Sn)_<i>x</i>O₄ (0 ≤ <i>x</i> ≤ 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 ⁷¹Ga solid state NMR studies. In contrast to ZnGa₂O₄:Cr³⁺ 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_1+<i>x</i>Ga_{2–2<i>x</i>}(Ge/Sn)_<i>x</i>O₄. 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 La₂Si₂O₇–Ho₂Si₂O₇ System

It is well-known that when an RE₂Si₂O₇ 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 Ho³⁺-substituted La₂Si₂O₇. Several compositions across the La₂Si₂O₇–Ho₂Si₂O₇ system were synthesized using the sol–gel method followed by calcination at 1600 °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)₂Si₂O₇ which extends to the La_0.6Ho_1.4Si₂O₇ composition. Compositions richer in Ho³⁺ show a two-phase domain (G+δ), while δ-(La,Ho)₂Si₂O₇ is the stable phase for Ho³⁺ contents higher than 90% (La_0.2Ho_1.8Si₂O₇). Anomalous diffraction data interestingly indicated that the La³⁺ for Ho³⁺ substitution mechanism in the G-(La,Ho)₂Si₂O₇ polymorph is not homogeneous, but a preferential occupation of Ho³⁺ for the RE2 site is observed. The Ho³⁺-doped G-La₂Si₂O₇ phosphors exhibited a strong green luminescence after excitation at 446 nm. Lifetime measurements indicated that the optimum phosphor was that with a Ho³⁺ content of 10%