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.

A computationally-guided non-equilibrium synthesis approach to materials discovery in the SrO-Al2O3-SiO2 phase field.

Glass-crystallisation synthesis is coupled to probe structure prediction for the guided discovery of new metastable oxides in the SrO-Al2O3-SiO2 phase field, yielding a new ternary ribbon-silicate, Sr2Si3O8. In principle, this methodology can be applied to a wide range of oxide chemistries by selecting an appropriate non-equilibrium synthesis route.

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.

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.

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.

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.

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.

Non-Isothermal Decomposition as Efficient and Simple Synthesis Method of NiO/C Nanoparticles for Asymmetric Supercapacitors.

A series of NiO/C nanocomposites with NiO concentrations ranging from 10 to 90 wt% was synthesized using a simple and efficient two-step method based on non-isothermal decomposition of Nickel(II) bis(acetylacetonate). X-ray diffraction (XRD) measurements of these NiO/C nanocomposites demonstrate the presence of ß-NiO. NiO/C nanocomposites are composed of spherical particles distributed over the carbon support surface. The average diameter of nickel oxide spheres increases with the NiO content and are estimated as 36, 50 and 205 nm for nanocomposites with 10, 50 and 80 wt% NiO concentrations, respectively. In turn, each NiO sphere contains several nickel oxide nanoparticles, whose average sizes are 7-8 nm. According to the tests performed using a three-electrode cell, specific capacitance (SC) of NiO/C nanocomposites increases from 200 to 400 F/g as the NiO content achieves a maximum of 60 wt% concentration, after which the SC decreases. The study of the NiO/C composite showing the highest SC in three- and two-electrode cells reveals that its SC remains almost unchanged while increasing the current density, and the sample demonstrates excellent cycling stability properties. Finally, NiO/C (60% NiO) composites are shown to be promising materials for charging quartz clocks with a power rating of 1.5 V (30 min).

Hexagonal Sr1-x/2Al2-xSixO4:Eu2+,Dy3+ transparent ceramics with tuneable persistent luminescence properties.

Co-doped hexagonal Sr1-x/2Al2-xSixO4:Eu2+,Dy3+ (0.1 ≤ x ≤ 0.5) transparent ceramics have been elaborated by full glass crystallization. The compositions with low SiO2 content (x ≤ 0.4) require fast quenching conditions to form glass, i.e. specific elaboration processes such as aerodynamic levitation coupled to laser heating, whereas the x = 0.5 glass composition can be prepared on a large scale by the classic melt-quenching method in commercial furnaces. After a single thermal treatment, the resulting SrAl2O4-based transparent ceramics show varying photoluminescence emission properties when x increases. These variations are also observable in persistent luminescence, resulting in an afterglow colour-tuning ranging from green to light blue. Afterglow excitation spectra highlight the possible activation in the visible range of the obtained persistent luminescence. Indeed, persistent luminescence of hexagonal Sr0.75Al1.5Si0.5O4:Eu2+,Dy3+ large transparent ceramics has been successfully charged using a typical smartphone low power white light source. Moreover, thermoluminescence glow curves of samples containing different Dy3+ doping concentrations are studied to gain insights regarding the traps' origin and depth. Coupling thermoluminescence results together with luminescence thermal quenching and band gap calculations appear useful to understand the charge trapping and detrapping evolution with the material composition. Varying the Si-content in hexagonal Sr1-x/2Al2-xSixO4:Eu2+,Dy3+ compounds appears as a promising strategy to obtain transparent materials with tuneable green to light blue persistent luminescence.

Modulated structure determination and ion transport mechanism of oxide-ion conductor CeNbO4+δ.

CeNbO4+δ, a family of oxygen hyperstoichiometry materials with varying oxygen content (CeNbO4, CeNbO4.08, CeNbO4.25, CeNbO4.33) that shows mixed electronic and oxide ionic conduction, has been known for four decades. However, the oxide ionic transport mechanism has remained unclear due to the unknown atomic structures of CeNbO4.08 and CeNbO4.33. Here, we report the complex (3 + 1)D incommensurately modulated structure of CeNbO4.08, and the supercell structure of CeNbO4.33 from single nanocrystals by using a three-dimensional electron diffraction technique. Two oxide ion migration events are identified in CeNbO4.08 and CeNbO4.25 by molecular dynamics simulations, which was a synergic-cooperation knock-on mechanism involving continuous breaking and reformation of Nb2O9 units. However, the excess oxygen in CeNbO4.33 hardly migrates because of the high concentration and the ordered distribution of the excess oxide ions. The relationship between the structure and oxide ion migration for the whole series of CeNbO4+δ compounds elucidated here provides a direction for the performance optimization of these compounds.

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.

Attempting to Verify the Existence of ZnY2O4 Using Electron Backscatter Diffraction.

Attempts to synthesize ZnY2O4 are made via a solid-state reaction in a high-temperature X-ray powder diffraction chamber as well as analyzing Y2O3 sinter ceramics pressure infiltrated by ZnO in a scanning electron microscope using energy-dispersive X-ray spectroscopy and electron backscatter diffraction (EBSD). The microstructure of the sinter ceramic is composed of ZnO grains dispersed in an Y2O3 matrix. Superimposed EBSD patterns of Y2O3 are misindexed as ZnY2O4 during the EBSD scan. The literature concerning ZnY2O4 is critically discussed.

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.

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.

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.

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.

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.

Cooperative mechanisms of oxygen vacancy stabilization and migration in the isolated tetrahedral anion Scheelite structure.

Tetrahedral units can transport oxide anions via interstitial or vacancy defects owing to their great deformation and rotation flexibility. Compared with interstitial defects, vacancy-mediated oxide-ion conduction in tetrahedra-based structures is more difficult and occurs rarely. The isolated tetrahedral anion Scheelite structure has showed the advantage of conducting oxygen interstitials but oxygen vacancies can hardly be introduced into Scheelite to promote the oxide ion migration. Here we demonstrate that oxygen vacancies can be stabilized in the BiVO4 Scheelite structure through Sr2+ for Bi3+ substitution, leading to corner-sharing V2O7 tetrahedral dimers, and migrate via a cooperative mechanism involving V2O7-dimer breaking and reforming assisted by synergic rotation and deformation of neighboring VO4 tetrahedra. This finding reveals the ability of Scheelite structure to transport oxide ion through vacancies or interstitials, emphasizing the possibility to develop oxide-ion conductors with parallel vacancy and interstitial doping strategies within the same tetrahedra-based structure type.