Cesium Oxo-fluoro-aluminates in the CsF-Al2O3 System: Synthesis and Structural Characterization.

In an experiment combining various approaches, a precise examination of a portion of the phase diagram of a CsF-Al2O3 system was carried out up to 40 mol% Al2O3. CsF-Al2O3 solidified mixtures have been investigated using high-field solid-state NMR (133Cs, 27Al, and 19F) spectroscopy and X-ray powder diffraction over a broad range of compositions with synchrotron powder diffraction and Rietveld analysis. A new cesium oxo-fluoro-aluminate, Cs2Al2O3F2, was discovered, prepared, and structurally analyzed by synchrotron diffraction analysis. In addition to Cs2Al2O3F2, we have synthesized the following pure compounds in order to aid in the interpretation of NMR spectra of the solidified samples: CsAlF4, Cs3AlF6, and CsAlO2.

Theoretical and Experimental Studies of Gallate Melilite Electrides from Topotactic Reduction of Interstitial Oxide Ion Conductors.

A nonstoichiometric La1.5Sr0.5Ga3O7.25 melilite oxide ion conductor features active interstitial oxygen defects in its pentagonal rings with high mobility. In this study, electron localization function calculated by density functional theory indicated that the interstitial oxide ions located in the pentagonal rings of gallate melilites may be removed and replaced by electron anions that are confined within the pentagonal rings, which would therefore convert the melilite interstitial oxide ion conductor into a zero-dimensional (0D) electride. The more active interstitial oxide ions, compared to the framework oxide ions, make the La1.5Sr0.5Ga3O7.25 melilite structure more reducible by CaH2 using topotactic reduction, in contrast to the hardly reducible nature of parent LaSrGa3O7. The topotactic reduction enhances the bulk electronic conduction (σ ∼ 0.003 S/cm at 400 °C) by ∼ 1 order of magnitude for La1.5Sr0.5Ga3O7.25. The oxygen loss in the melilite structure was verified and most likely took place on the active interstitial oxide ions. The identified confinement space for electronic anions in melilite interstitial oxide ion conductors presented here provides a strategy to access inorganic electrides from interstitial oxide ion conductor electrolytes.

Stabilization of the Trigonal Langasite Structure in Ca3Ga2-2xZnxGe4+xO14 (0 ≤ x ≤ 1) with Partial Ordering of Three Isoelectronic Cations Characterized by a Multitechnique Approach.

Crystallization of oxide glasses rich in Zn2+, Ga3+, and Ge4+ is of interest for the synthesis of new transparent ceramics. In this context, we report the identification and detailed structural characterization of a new solid solution Ca3Ga2-2xZnxGe4+xO14 (0 ≤ x ≤ 1). These compounds adopt the trigonal langasite structure type, offering three possible crystallographic sites for the coordination of isoelectronic Zn2+, Ga3+, and Ge4+. We used neutron diffraction to determine distributions of Ga3+/Ge4+ and Zn2+/Ge4+ in the simpler end members Ca3Ga2Ge4O14 and Ca3ZnGe5O14, while for the complex intermediate member Ca3GaZn0.5Ge4.5O14, we used an original approach combining quantitative 2D analysis of atomic-resolution STEM-EDS maps with neutron diffraction. This revealed that, across the solid solution, the tetrahedral D sites remain fully occupied by Ge4+, while Zn2+, Ga3+, and the remaining Ge4+ are shared between octahedral B- and tetrahedral C sites in proportions that depend upon their relative ionic radii. The adoption of the trigonal langasite structure by glass-crystallized Ca3ZnGe5O14, a compound that was previously observed only in a distorted monoclinic langasite polymorph, is attributed to substantial disorder between Zn2+ and Ge4+ over the B and C sites. The quantitative 2D refinement of atomic-resolution STEM-EDS maps is applicable to a wide range of materials where multiple cations with poor scattering contrast are distributed over different crystallographic sites in a crystal structure.

First-principles calculations to identify key native point defects in Sr4Al14O25.

This article reports for the first time an in-depth ab initio computational study on intrinsic point defects in Sr4Al14O25 that serves as host lattice for numerous phosphors. Defect Formation Enthalpies (DFEs) and defect concentrations were computed considering the supercell approach for different oxygen atmospheres. The charge transition levels have been determined for several point defects in their thermodynamically stable state and their impact on the electronic structure of the ideal unfaulted material is discussed. Our simulations demonstrated that the formation of most of native point defects is energy intensive under oxygen-rich, -intermediate or -poor synthesis conditions, except for the oxygen vacancies under O-poor atmosphere.

Extended B-Site Vacancy Content Range and Cation Ordering in Twinned Hexagonal Perovskites Ba8Cr4-xTa4+0.6xO24.

The new 8-layer twinned hexagonal solid solution Ba8Cr4-xTa4+0.6xO24 (x = 0.0-3.0) was isolated through the aliovalent substitution of Ta5+ for Cr3+ in Ba2CrTaO6, showing the widest B-site vacancy content range among the 8-layer twinned hexagonal perovskites. Ba8Cr4-xTa4+0.6xO24 forms a simple 8-layer hexagonal perovskite structure within 0.0 ≤ x < 2.4 and a tripled 8-layer hexagonal perovskite superstructure within 2.4 ≤ x ≤ 3.0. The latter shows expanded a and b axes by 3 times in comparison to the simple 8-layer hexagonal perovskite structure owing to the partial face-sharing octahedral (FSO) B cation ordering along the ab plane. The B-cation and vacancy distributions in the tripled superstructure were characterized by neutron and X-ray powder diffraction and further confirmed by a scanning transmission electron microscopy-high angle annular dark field imaging and intensity profile analysis. The formation of 8-layer twinned hexagonal perovskites Ba8Cr4-xTa4+0.6xO24 in an extended solid solution range can be attributed to the presence of both covalent B-B and B-O-B bonding and B-site vacancies in the FSO sites. This work provides an effective way of combining covalent B-B and B-O-B bonding and vacancy creation as well as the cationic ordering in the FSO sites to reduce electrostatic repulsion, which could further enable the stabilization of new hexagonal perovskite compounds.

Emergence of A-Site Cation Order in the Small Rare-Earth Melilites SrREGa3O7 (RE = Dy-Lu, Y).

SrREGa3O7 melilite ceramics with large rare-earth elements (RE = La to Y) are famous materials especially known for their luminescence properties. Using an innovative approach, the full and congruent crystallization from glass process, SrREGa3O7 transparent polycrystalline ceramics with small rare earth elements (RE = Dy-Lu and Y) have been successfully synthesized and characterized. Interestingly, compared to the classic tetragonal (P4̅21m) melilite structure composed of mixed Sr/RE cationic sites, these compositions can crystallize in a 3 × 1 × 1 orthorhombic (P21212) superstructure. A detailed study of the superstructure, investigated using different techniques (synchrotron and neutron powder diffraction, STEM-HAADF imaging, and EDS mapping), highlights the existence of a Sr/RE cation ordering favored by a large Sr/RE size mismatch and a sufficiently small RE cation. An appropriate control of the synthesis conditions through glass crystallization enables the formation of the desired polymorphs, either ordered or disordered. The influence of this tailored cationic ordering/disordering on the RE luminescent spectroscopic properties have been investigated. A stronger structuration of the RE emission band is observed in the ordered ceramic compared to the disordered ceramic and the glass, whose band shapes are very similar, indicating that the RE environments in the glass and disordered ceramic are close.

Sodium Site Exchange and Migration in a Polar Stuffed-Cristobalite Framework Structure.

The study of ionic dynamics in solids is essential to understanding and developing modern energy technologies. Here we study the ionic dynamics of orthorhombic Na2MgSiO4, an interesting case of a polar stuffed-cristobalite-type structure that contains two inequivalent Na sites within the channels of the magnesium silicate tetrahedral framework. Its preparation by a solid-state reaction method favors the presence of ∼2% of Na vacancies, converting it into a pure Na ionic conductor with an optimized ionic conductivity of ∼10-5 S cm-1 at 200 °C. The macroscopic migration has been characterized through impedance spectroscopy and molecular dynamics simulation, which proves the pure Na ionic character of the compound through hopping between Na1 and Na2 sites, forming three-dimensional migration zigzag-shaped paths. High-resolution solid-state 23Na magic-angle-spinning (MAS) NMR spectroscopy is employed to characterize the local structure and microscopic dynamics of Na-ion transport in Na2MgSiO4. Remarkably, variable-temperature 23Na MAS NMR and two-dimensional exchange spectroscopy evidence for the first time a Na site exchange phenomenon at room temperature, which further triggers Na ionic conduction at elevated temperatures.

Polymorphs of Rb3ScF6: X-ray and Neutron Diffraction, Solid-State NMR, and Density Functional Theory Calculations Study.

The crystal structures of three polymorphs of Rb3ScF6 have been determined through a combination of synchrotron, laboratory X-ray, and neutron powder diffraction, electron diffraction, and multinuclear high-field solid-state NMR studies. The room temperature (RT; α) and medium-temperature (ß) structures are tetragonal, with space groups I41/a (Z = 80) and I4/m (Z = 10) and lattice parameters a = 20.2561(4) Å, c = 36.5160(0) Å and a = 14.4093(2) Å, c = 9.2015(1) Å at RT and 187 °C, respectively. The high-temperature (γ) structure is cubic space group Fm3̅m (Z = 4) with a = 9.1944(1) Å at 250 °C. The temperatures of the phase transitions were measured at 141 and 201 °C. The three α, ß, and γ Rb3ScF6 phases are isostructural with the α, ß, and δ forms of the potassium cryolite. Detailed structural characterizations were performed by density functional theory as well as NMR. In the case of the ß polymorph, the dynamic rotations of the ScF6 octahedra of both Sc crystallographic sites have been detailed.

Development of Melilite-Type Oxide Ion Conductors.

Lowering the operating temperature of solid oxide fuel cells (SOFCs) requires high performance oxide ion conductor electrolytes. Recently tetrahedra-based structures have been attracting considerable attention for oxide ion conductor development, among which the layered tetrahedral network melilite structure appears particularly interesting owing to its remarkable capability to accommodate and transport interstitial oxide ions, compared with isolated tetrahedral anion structures. Stabilization and migration mechanisms of interstitial oxide ions in melilites have been systematically investigated using local structural relaxation from both electrostatic Coulomb interaction and chemical bonding aspects based on atomic and electronic structures respectively using experimental and theoretical approaches. These reveal cationic size and chemical bonding effects on stabilization and migration mechanisms of interstitial oxide ions. Lately, full crystallization from glass, an innovative synthesis method, was employed to produce new metastable melilite oxide ion conductors which are inaccessible using classic solid state reaction owing to cationic size effect. Finally, the thermal and chemical stability at low temperature and the high oxide ion conductivity of the best melilite oxide ion conductors based on LaSrGa3 O7 are likely to provide real possibilities of applications of melilite-type electrolytes in SOFCs and other related devices.

X-ray Diffraction, NMR Studies, and DFT Calculations of the Room and High Temperature Structures of Rubidium Cryolite, Rb3AlF6.

A crystallographic approach incorporating multinuclear high field solid state NMR (SSNMR), X-ray structure determinations, TEM observation, and density functional theory (DFT) was used to characterize two polymorphs of rubidium cryolite, Rb3AlF6. The room temperature phase was found to be ordered and crystallizes in the Fddd (no. 70) space group with a = 37.26491(1) Å, b = 12.45405(4) Å, and c = 17.68341(6) Å. Comparison of NMR measurements and computational results revealed the dynamic rotations of the AlF6 octahedra. Using in situ variable temperature MAS NMR measurements, the chemical exchange between rubidium sites was observed. The ß-phase, i.e., high temperature polymorph, adopts the ideal cubic double-perovskite structure, space group Fm3m, with a = 8.9930(2) Å at 600 °C. Additionally, a series of polymorphs of K3AlF6 has been further characterized by high field high temperature SSNMR and DFT computation.

Ba8CoNb6-xTaxO24 Eight-Layer Shifted Hexagonal Perovskite Ceramics with Spontaneous Ta5+ Ordering and Near-Zero τf.

Highly positive temperature coefficients of the resonant frequency (τf) of eight-layer hexagonal perovskites are hardly tunable, hindering their application as microwave dielectric resonators. Here, we show that a near-zero τf (∼0.48 ppm °C-1) can be achieved on eight-layer shifted hexagonal perovskite Ba8CoNb4Ta2O24, along with a permittivity εr of ∼30.6 and a Qf of ∼36400 GHz, through substitution of Ta for Nb, satisfying the resonator application requirement. The decrease in the τf of Ba8CoNb6-xTaxO24 takes place mainly through the decrease in the εr or temperature coefficient of permittivity, owing to the less covalent bonding and lower polarizability of Ta5+ compared to those of Nb5+. Synchrotron and neutron powder diffraction data, scanning transmission electron microscopy-high-angle annular dark field imaging, and atomic-scale X-ray energy dispersive spectroscopy elemental mapping reveal that Ta5+ cations in Ba8CoNb4Ta2O24 are naturally distributed in a partially ordered manner, showing a strong site preference on the Nb layers close to the central Co layer over the most out-of-center distorted Nb layers next to empty octahedral layers. This spontaneous Ta ordering in the niobate host is driven by different covalent bonding nature and second-order Jahn-Teller distortion extents of Ta5+ and Nb5+. The results demonstrate an effective way of substituting more ionic Ta5+ for Nb5+, which decreases the τf to near-zero values for eight-layer hexagonal perovskite niobate dielectrics.

Trigonal-Planar Low-Spin Co2+ in a Layered Mixed-Polyhedral Network from Topotactic Reduction.

Topotactic reduction of the perovskite oxide TbBaCo2O5.5 with CaH2 leads to a new crystalline phase TbBaCo2O4.5, adopting a 2 × 2 × 1 superstructure compared to TbBaCo2O5.5. The structure consists of a corner-shared network of square pyramidal CoO5 and trigonal planar CoO3 units. Magnetic susceptibility and variable temperature neutron diffraction data reveal that TbBaCo2O4.5 adopts a G-type antiferromagnetically ordered structure (TN ∼ 322 K). The ordered moments are consistent with the presence of low-spin Co2+ (S = 1/2) in trigonal-planar coordination and high-spin Co2+ centers in square pyramidal coordination. TbBaCo2O4.5 shows lower conductivity than TbBaCo2O5.5, which is consistent with the p-type conduction behavior. The unique anion vacancy arrangements in TbBaCo2O4.5 further complement the role of A-cations in controlling the oxygen vacancy distribution in LnBaCo2O5+δ series and demonstrate more opportunity to tune the structural and physical properties based on cationic and anionic lattice coupling.

Highly Transparent Fluorotellurite Glass-Ceramics: Structural Investigations and Luminescence Properties.

Crystallization from glass can lead to the stabilization of metastable crystalline phases, which offers an interesting way to unveil novel compounds and control the optical properties of resulting glass-ceramics. Here, we report on a crystallization study of the ZrF4-TeO2 glass system and show that under specific synthesis conditions, a previously unreported Te0.47Zr0.53OxFy zirconium oxyfluorotellurite antiglass phase can be selectively crystallized at the nanometric scale within the 65TeO2-35ZrF4 amorphous matrix. This leads to highly transparent glass-ceramics in both the visible and near-infrared ranges. Under longer heat treatment, the stable cubic ZrTe3O8 phase crystallizes in addition to the previous unreported antiglass phase. The structure, microstructure, and optical properties of 65TeO2-35ZrF4Tm3+-doped glass-ceramics, were investigated in detail by means of X-ray diffraction, scanning and transmission electron microscopies, and 19F, 91Zr, and 125Te NMR, Raman, and photoluminescence spectroscopies. The crystal chemistry study of several single crystals samples by X-ray diffraction evidence that the novel phase, derived from α-UO3 type, corresponds in terms of long-range ordering inside this basic hexagonal/trigonal disordered phase (antiglass) to a complex series of modulated microphases rather than a stoichiometric compound with various superstructures analogous to those observed in the UO3-U3O8 subsystem. These results highlight the peculiar disorder-order phenomenon occurring in tellurite materials.

Persistent energy transfer in ZGO:Cr3+,Yb3+: a new strategy to design nano glass-ceramics featuring deep red and near infrared persistent luminescence.

ZnGa2O4:Cr3+, owing to its persistent luminescence properties in the deep red range, is an exceptional material in view of foreseen in vivo imaging applications. In the present work, we report the elaboration process and detailed investigations of the optical properties of nano glass-ceramics composed of spinel ZnGa2O4:Cr3+,Yb3+ nanocrystals embedded in a transparent, silica rich, glass matrix. The as-prepared materials show good incorporation of the dopants in the crystallites leading in both Cr3+ and Yb3+ emissions. These emissions occur while exciting in the Cr3+ bands, indicating an energy transfer process from Cr3+ to Yb3+. Furthermore, excitation in the Yb3+ band in the near-infrared (NIR) range suggests an interesting up-conversion process, which promotes the Cr3+ emission. Persistent luminescence of both Cr3+ and Yb3+ doping ions can be activated by charging the Cr3+ excitation bands, leading to persistent luminescence of zinc gallate nanocrystals in both first and second biological windows. The influence of Yb3+ co-doping on persistent luminescence properties has been investigated by persistent luminescence decay profiles and thermoluminescence studies. Indeed, thermoluminescence glow curves of Yb3+ exhibit similar shape to those of Cr3+ but appear broader and shifted towards higher temperatures. This temperature shift may be explained by the temperature dependence of the involved energy transfer process.

8H-10H Stacking Periodicity Control in Twinned Hexagonal Perovskite Dielectrics.

Isovalent substitution of Zr4+ for smaller Ti4+ was performed in the 8-layer twinned hexagonal perovskite (referred to as 8H) tantalate Ba8Ti3Ta4O24, which stabilizes a 10-layer twinned hexagonal perovskite (referred to as 10H). The formation of the 10H phase occurs at low substitution concentration ( x = 0.1) in Ba8Zr xTi3- xTa4O24 at 1300 °C and reverts back to the 8H phase upon heating at elevated temperatures. Such a 10H-to-8H phase transformation is suppressed at higher Zr-substitution contents ( x > 0.1). The approach combining simulated annealing and Rietveld refinement with compositional constrain indicates that the 10H Ba8Zr0.4Ti2.6Ta4O24 ( x = 0.4) composition adopts a simply P63/ mmc disordered structure with Zr cations preferably located in corner-sharing octahedral (CSO) sites compared to face-sharing octahedral (FSO) sites. This 8H-10H phase competition, dependent on the substitution of Zr4+ for Ti4+ and firing temperature, is discussed in terms of the FSO B-B repulsion controlled by the cationic size, as well as the stacking periodicity which affects the thermodynamic stability. Both 8H- and 10H-phase pellets of Ba8Zr xTi3- xTa4O24 exhibit comparable and poorer microwave dielectric properties than the parent 8H Ba8Ti3Ta4O24, which is characterized by cationic disorder and FSO B-B repulsion. The 8H and 10H Ba8Zr xTi3- xTa4O24 ceramics display electrical insulator behavior but with electrically heterogeneous microstructure on the bulk grains. This study demonstrates the opportunity to control the stacking periodicity for the twinned hexagonal perovskites via tuning the B-cationic size and the firing temperature.

Oxo- and Oxofluoroaluminates in the RbF-Al2O3 System: Synthesis and Structural Characterization.

Precise research on the RbF-Al2O3 system was carried out by means of combining X-ray powder diffraction, high-field solid-state NMR spectroscopy, and thermal analysis methods. α-Rb3AlF6, RbAlO2, Rb2Al22O34, and new phase, Rb2Al2O3F2, were identified in the system. The structure of this new rubidium oxofluoroaluminate was determined. It is built up from single layers of oxygen-connected AlO3F tetrahedra, those layers beeing separated by fluorine atoms. This type of structure exhibits a decent ionic conductivity at ambient temperature, 1.74 × 10-6 S cm-1. The similar structural arrangement of O3Al-O-AlO3 and FO2Al-O-AlO2F tetrahedra of the conduction planes in Rb2Al22O34 and Rb2Al2O3F2 were confirmed by 27Al NMR measurements. A thermal analysis of the RbF-Al2O3 system revealed that it can be defined as a pseudobinary subsystem of the more general quaternary RbF-AlF3-Al2O3-Rb2O phase diagram. From a phase analysis of individual phase fields, the mutual metastable behavior of all founded phases can be considered. It was observed that fluoro- and oxoaluminates exist together. Rb2Al2O3F2 is more stable under high temperature. Rubidium fluoro- and oxoaluminates are metastable precursors of the thermodynamically more stable structure of rubidium oxofluoroaluminate.

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 (CASTEP), 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.

8-Layer Shifted Hexagonal Perovskite Ba8MnNb6O24: Long-Range Ordering of High-Spin d5 Mn2+ Layers and Electronic Structure.

A new 8-layer shifted hexagonal perovskite Ba8MnNb6O24 has been synthesized in air, featuring unusual long-range B-cation ordering with single octahedral high-spin d5 Mn2+ layers separated by ∼1.9 nm within the corner-sharing octahedral d0 Nb5+ host, analogous to Ba8(Zn/Co)Nb6O24. The large size and charge differences between high-spin Mn2+ and Nb5+, as well as the out-of-center distortion of NbO6 octahedra associated with the bonding covalence and second-order Jahn-Teller effect of Nb5+, drive long-range cationic ordering, thus stabilizing Ba8MnNb6O24. The Ba8MnNb6O24 pellet exhibits a high dielectric permittivity, εr ∼ 38, and a large temperature coefficient of resonant frequency, τf ∼ 20 ppm/K, but a dielectric loss ( Qf ∼ 987 GHz) and conductivity (∼10-8-10-3 S/cm within 473-1173 K) much higher than those of Ba8ZnNb6O24. Electronic structures from density functional theory calculations reveal that Ba8MnNb6O24 is a Mott insulator in contrast with the charge-transfer insulator nature of Ba8ZnNb6O24, and they confirm that the off-center distortion of Nb5+ contributes to stabilization of the 8-layer ordered shifted structure. The contrast between conductivity and dielectric loss of Ba8MnNb6O24 and Ba8ZnNb6O24 is understood based on the electronic structure that depends on high-spin d5 Mn2+ and d10 Zn2+ cations. The hopping of 3d valence electrons in high-spin Mn2+ to Nb5+ 4d conduction bands over a small gap (∼2.0 eV) makes Ba8MnNb6O24 more conductive than Ba8ZnNb6O24, where the electrons are conducted via the hopping of a lattice O 2p valence electron to the Nb5+ 4d conduction bands over a larger gap (∼3.9 eV). The high microwave dielectric loss of BMN may be mainly ascribed to the half-filled Mn 3d orbits, which is understood based on the softened infrared modes that increase the lattice vibration anharmonicity as well as the resonant spin excitation of unpaired d electrons.

Local Disorder and Tunable Luminescence in Sr1-x/2Al2-xSixO4 (0.2 ≤ x ≤ 0.5) Transparent Ceramics.

Eu-doped Sr1-x/2Al2-xSixO4 (x = 0.2, 0.4, and 0.5) transparent ceramics have been synthesized by full and congruent crystallization from glasses prepared by aerodynamic levitation and laser-heating method. Structural refinements from synchrotron and neutron powder diffraction data show that the ceramics adopt a 1 × 1 × 2 superstructure compared to the SrAl2O4 hexagonal polymorph. While the observed superstructure reflections indicate a long-range ordering of the Sr vacancies in the structure, 29Si and 27Al solid-state NMR measurements associated with DFT computations reveal a significant degree of disorder in the fully polymerized tetrahedral network. This is evidenced through the presence of Si-O-Si bonds, as well as Si(OAl)4 units at remote distances of the Sr vacancies and Al(OAl)4 units in the close vicinity of Sr vacancies departing from local charge compensation in the network. The transparent ceramics can be doped by europium to induce light emission arising from the volume under UV excitation. Luminescence measurements then reveal the coexistence of Eu2+ and Eu3+ in the samples, thereby allowing tuning the emission color depending on the excitation wavelength and suggesting possible applications such as solid state lighting.

Original Synthetic Route To Obtain a SrAl2O4 Phosphor by the Molten Salt Method: Insights into the Reaction Mechanism and Enhancement of the Persistent Luminescence.

SrAl2O4:Eu(2+), Dy(3+) has been extensively studied for industrial applications in the luminescent materials field, because of its excellent persistent luminescence properties and chemical stability. Traditionally, this strontium aluminate material is synthesized in bulk form and/or fine powder by the classic solid-state method. Here, we report an original synthetic route, a molten salt assisted process, to obtain highly crystalline SrAl2O4 powder with nanometer-scale crystals. The main advantages of salt addition are the increase of the reaction rate and the significant reduction of the synthesis temperature because of much higher mobility of reactants in the liquid medium than in the solid-state method. In particular, the formation mechanism of SrAl2O4, the role of the salt, and the phase's evolution have been explored as a function of temperature and time. Phosphorescent powders based on SrAl2O4:Eu(2+), Dy(3+) with high crystallinity are obtained after 1 h treatment at 900 °C. This work could promote further interest in adopting the molten salt strategy to process high-crystallinity materials with enhanced luminescence to design technologically relevant phosphors.