A-site Vacancies Research Articles

Chemical composition of Na0.5Bi0.5TiO3 solid solutions with Sr0.7Bi0.2TiO3 on a local level

Oxygen vacancies in Na0.5Bi0.5TiO3 are of high demand as they stimulate the ionic conductivity essential to solid fuel cell applications. However, their available concentration, which is directly related to introduced A-site vacancies, is very limited. The objective of the present research is to study how the concentration of A-site vacancies may be increased by forming solid solutions with Sr0.7Bi0.2TiO3. A thorough study of heterogeneity and chemical composition of (1-y)Na0.5Bi0.5TiO3-ySr0.7Bi0.2TiO3 ceramics on a local level is conducted and accompanied by impedance measurements. It is shown that the chemical composition of the matrix grains is stable and follows the designed formula, allowing formation of A-site vacancies in a wide concentration range and, in limited amount, also oxygen vacancies. Inclusions, the content and character of which depend on Sr0.7Bi0.2TiO3 concentration, are detected and discussed. Complex impedance studies suggest that the compositions with y < 0.5 preserve low ionic conductivity up to 600–700 °C, but it significantly increases at higher y.

Catalytic performance and mechanism of A-site vacancy deficient perovskite catalyst over tar cracking during biomass pyrolysis

Tar from biomass pyrolysis is an urgent problem to be solved for the large utilization of biomass pyrolysis technology. This work focuses on the activity evaluation of perovskite catalysts with various strontium content to crack biomass tar at 700 °C. Cracking results show that the catalytic performance of Sr-doped catalysts are better than that of undoped one and the participation of perovskite catalysts increases the yield of gas by promoting the tar conversion. La0.8Sr0.2Ni0.8Fe0.2O3 (LS2) shows the highest gas yield of 606.7 mL/gbio and syngas yield of 475.0 mL/gbio. Characterization results by various advanced technologies show that Sr-doped perovskites have higher redox capacity, oxygen mobility, and better surface area and pore volume in favor of gas adsorption and conversion. A possible tar catalytic cracking mechanism is proposed. Oxygen vacancy-induced redox reaction and metal sites are the main effective mechanism. This work would offer a feasible exploration into the application of perovskite-type catalysts on biomass tar conversion.

Frontispiece: Recent Status and Developments of Vacancies Modulation in the ABO3 Perovskites for Catalytic Applications

The application of ABO3 could promote the development of many chemical engineering processes as catalysts. Meanwhile the creation and improvement of ABO3 with different site vacancies would further enhance its commercial progress due to the possibility of strengthening its physical-chemical properties. In this Review article by J. Chen et al. on page 3 ff., the preparation methods of ABO3 with A-site, B-site, and O-site vacancies; the improved electronic structure as well as an activated lattice oxygen of vacancies-enriched ABO3; and the corresponding reaction mechanism are discussed in detail. In particular, this effective strategy also guides ABO3’ development as electrode materials for energy storage, photovoltaic material for producing energy, and sensor materials for equipment production, etc.

Urea-Assisted Sol-Gel Synthesis of LaMnO3 Perovskite with Accelerated Catalytic Activity for Application in Zn-Air Battery

Precious metal-based materials such as commercial Pt/C are available electrocatalysts for redox reactions in Zn-air batteries. However, their commercial use is still limited by slow kinetics and restricted stability. In this work, we highlight a facial urea-assisted sol-gel method to synthesize A-site vacancy in LaMnO3+δ oxide for boosting its catalytic activity and further explore the effect of the amount of urea on the A-site LaMnO3. The A-site vacancy in LMO was confirmed by XRD, TEM, and XPS, which revealed that the urea-assisted sol-gel method mitigated the A-site vacancy in LaMnO3+δ and increased its surface area, thus ultimately accelerating its redox reaction kinetics. The half-wave potential and current density of the resultant 3.0U-LMO electrocatalyst were 0.74 V and 5.74 mA cm−2, respectively. It is worth noting that the assembled Zn-air battery with the 3.0U-LMO catalyst presented a power output of 130.04 mW cm−2 at 0.51 V and a promising energy efficiency of 58.4% after 150 cycles. This protocol might offer an efficient approach for developing new defect-regulated perovskites for electrocatalysis.

Selective modulation of La-site vacancies in La0.9Ca0.1MnO3 perovskites catalysts for toluene Oxidation: The role of oxygen species on the catalytic mechanism

Regulation of A-site vacancies is crucial to the redox reactions on ABO3 perovskites for VOCs catalytic oxidation. Herein, we reported the high toluene removal performance of La-defects and oxygen vacancies based La0.9Ca0.1MnO3 catalyst prepared by pyrolysis with simple urea modification. The 3-La0.9Ca0.1MnO3 had excellent catalytic activity and stability, and had good water resistance under the condition of 3%-10% H2O. Through analysis of various characterization, we found that the modification of urea made the A-site cation of La0.9Ca0.1MnO3 catalyst lack, which could effectively regulate the concentration of adsorbed oxygen and lattice oxygen in the perovskite catalyst, and enhanced the lattice oxygen participation in the catalytic oxidation of toluene. And the catalyst with abundant oxygen vacancies could promote the rapid degradation and complete oxidation of toluene. This study provided a simple method for the effective regulation of the defect site of perovskite catalyst, which was conducive to the promotion of industrial application.

Synergistic effect enhances energy storage properties of BNT-based relaxor ferroelectric thin films

Lead-free thin film capacitors with high energy density and efficiency are promising candidates for pulse power systems in advanced electronic industries due to their low cost, lightweight, and integration development. In this study, 0.6Bi0.5Na0.5TiO3-0.4Sr0.7Bi0.2TiO3+x mol Mn (BNT-SBT-xMn) thin films are fabricated on Pt/Ti/SiO2/Si substrates via sol-gel method. The BNT-SBT-xMn films with perovskite phase possess uniform grain size, well density, and low roughness. The introduction of Mn ions compensates for valence unbalances of A-site vacancy of BNT-based films and forms defective dipoles. Their synergism hinders the transmission of carriers, reduces the leakage current density, and improves polarization and breakdown behavior. Therefore, the BNT-SBT-2% Mn film achieves a high recoverable energy density of 85.35 J/cm3 together with an enhanced energy efficiency of 73.17% under an electric field of 3114 kV/cm. Meanwhile, the BNT-SBT-2% Mn film displays a good frequency (10 Hz-2k Hz), temperature (20–200 °C), and fatigue (1-105 cycles) stability in electrical energy storage properties, suggesting that it would be a promising candidate for the advanced power systems.

Ligand exchange engineering of FAPbI3 perovskite quantum dots for solar cells

Formamidinium lead triiodide (FAPbI3) perovskite quantum dots (PQDs) show great advantages in photovoltaic applications due to their ideal bandgap energy, high stability and solution processability. The anti-solvent used for the post-treatment of FAPbI3 PQD solid films significantly affects the surface chemistry of the PQDs, and thus the vacancies caused by surface ligand removal inhibit the optoelectronic properties and stability of PQDs. Here, we study the effects of different anti-solvents with different polarities on FAPbI3 PQDs and select a series of organic molecules for surface passivation of PQDs. The results show that methyl acetate could effectively remove surface ligands from the PQD surface without destroying its crystal structure during the post-treatment. The benzamidine hydrochloride (PhFACl) applied as short ligands of PQDs during the post-treatment could fill the A-site and X-site vacancies of PQDs and thus improve the electronic coupling of PQDs. Finally, the PhFACl-based PQD solar cell (PQDSC) achieves a power conversion efficiency of 6.4%, compared to that of 4.63% for the conventional PQDSC. This work provides a reference for insights into the surface passivation of PQDs and the improvement in device performance of PQDSCs.Graphical abstract

Structural investigation of the temperature-stable relaxor dielectric Ba0.8Ca0.2TiO3-Bi(Mg0.5Ti0.5)O3

Aberration corrected scanning transmission electron microscopy (STEM) and electron diffraction have been used to disclose local structure and nano-chemistry in a Ca modified BaTiO3-Bi(Mg0.5Ti0.5)O3 relaxor dielectric ceramic which exhibits high and near-invariant relative permittivity over a wide temperature range. High-resolution, synchrotron X-ray diffraction indicated a globally cubic structure (Pm3̅m), but direct atomic-scale imaging by STEM revealed local tetragonal distortions. Nanopolar clusters were identified from B-site atomic displacement vectors measured relative to oxygen ion positions along < 100 > and < 110 > zone axes of integrated differential phase contrast (iDPC) STEM images, highlighting cluster sizes of 2–5 nm. Chemical analysis by STEM-energy dispersive X-ray spectroscopy and full pattern refinements of X-ray powder diffraction data each implied high levels of Bi vacancies within the matrix. The possibility that these A-site vacancies modulate the nanopolar structure and promote flattening of the permittivity-temperature response in this class of dielectric is discussed.

Annealing effects on epitaxial (K,Na)NbO3 thin films grown on Si substrates

Epitaxial (K,Na)NbO3 (KNN) thin films were deposited on (001)SrRuO3/Pt/ZrO2/Si substrates by RF magnetron sputtering, and post-annealing effects on the as-deposited epitaxial KNN thin films were investigated. According to the findings, by annealing, the crystal system of these thin films changes from a tetragonal to an orthorhombic system because of the release of internal stress. The asymmetry of polarization–electric field hysteresis loops along the electric field changes from a positive to a negative side by annealing. This means that stable spontaneous polarization P s changes from the upward to downward direction with an increase in the number of A-site vacancies. In addition, the displacement–electric field curves of epitaxial KNN/Si unimorph cantilevers exhibit asymmetric behaviors. A relatively high converse piezoelectric coefficient ∣e 31,f ∣ = 6.4 C m−2 is obtained for 5 h annealed epitaxial KNN thin films.

Mineral Chemistry of Pyrochlore Supergroup Minerals from the Boziguoer Nb-Ta-Zr-Rb-REE Deposit, NW China: Implications for Nb Enrichment by Alkaline Magma Differentiation

Alkaline rocks are generally enriched in rare metals (e.g., Nb, Ta, and Zr) and rare earth elements (REE), but the key factors controlling Nb-Ta-REE enrichment remain unclear. The Boziguoer Nb (Ta-Zr-Rb-REE) deposit in Southwest Tianshan (northern margin of Tarim Basin) is China’s largest, with reserves of 0.32 Mt Nb2O5 and 0.02 Mt Ta2O5. It is an alkaline felsic complex 4.45 km in length and 0.5–1.3 km in width, composed of alkalic granite and syenite, which can be subdivided into syenite I and syenite II. The main minerals in each lithofacies are the same (albite, K-feldspar, quartz, arfvedsonite and aegirine). The Nb in the deposit is mainly hosted in pyrochlore supergroup minerals, ubiquitous in alkalic granite and syenite of the Boziguoer deposit. The wide variation in cations (Ca, Na, REE, U, Th) in the A-site further classifies the Boziguoer pyrochlore supergroup minerals as fluornatropyrochlore, fluorcalciopyrochlore and fluorkenopyrochlore. All Boziguoer pyrochlore supergroup minerals are Nb-rich and Ta-poor at the B-site and dominated by F at the Y-site. These cation occurrence illustrate a new mechanism of substitution in the Boziguoer pyrochlore supergroup minerals (2Ca2+ +Ti4+ +4Ta5+ = REE3+ +A-V + 5Nb5+, where A-V is the A-site vacancy). This substitution mechanism is different from that in the pyrochlore supergroup minerals from other rocks such as carbonatite and nepheline syenite, which are dominated by the replacement of Ba (Rb, Sr) with Ca+ Na + A-V. In addition, the substitution of REE (mainly La, Ce) for Ca in the Boziguoer pyrochlore supergroup minerals is likely a result of either REE enrichment or a change in the REE partition coefficient during the evolution of the alkaline magma. Both the pyrochlore supergroup minerals and their host rocks display negative large ion lithophile element (LILE; K, Rb, Sr, and Ba) anomalies, positive high-field-strength element (HFSE) anomalies and light rare earth element (LREE) enrichment with negative Eu anomalies. This is consistent with the crystallization of the pyrochlore supergroup minerals from the magma rather than from hydrothermal fluids, suggesting a magmatic origin. These findings indicate that the mechanisms of pyrochlore supergroup minerals crystallization in alkaline magma may be significantly different from those in carbonatite and nepheline syenite, and that magmatic differentiation processes may have played a role in the enrichment of the Boziguoer deposit by Nb.

Effect of Relaxations on the Conductivity of La1/2+1/2x Li1/2-1/2x Ti1-x Al x O3 Fast Ion Conductors.

Perovskite-type solid-state electrolytes, Li3xLa2/3–xTiO3 (LLTO), are considered among the most promising candidates for the development of all-solid-state batteries based on lithium metal. Their high bulk ionic conductivity can be modulated by substituting part of the atoms hosted in the A- or B-site of the LLTO structure. In this work, we investigate the crystal structure and the long-range charge migration processes characterizing a family of perovskites with the general formula La1/2+1/2xLi1/2–1/2xTi1–xAlxO3 (0 ≤ x ≤ 0.6), in which the charge balance and the nominal A-site vacancies (nA = 0) are preserved. X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) investigations reveal the presence of a very complex nanostructure constituted by a mixture of two different ordered nanoregions of tetragonal P4/mmm and rhombohedral R3̅c symmetries. Broadband electrical spectroscopy studies confirm the presence of different crystalline domains and demonstrate that the structural fluctuations of the BO6 octahedra require to be intra- and intercell coupled, to enable the long-range diffusion of the lithium cation, in a similar way to the segmental mode that takes place in polymer-ion conductors. These hypotheses are corroborated by density functional theory (DFT) calculations and molecular dynamic simulations.

Iron and aluminum substitution mechanism in the perovskite phase in the system MgSiO3-FeAlO3-MgO

AbstractFe,Al-bearing MgSiO3 perovskite (bridgmanite) is considered to be the most abundant mineral in Earth’s lower mantle, hosting ferric iron in its structure as charge-coupled (Fe2O3 and FeAlO3) and vacancy components (MgFeO2.5 and Fe2/3SiO3). We examined concentrations of ferric iron and aluminum in the perovskite phase as a function of temperature (1700–2300 K) in the MgSiO3-FeAlO3-MgO system at 27 GPa using a multi-anvil high-pressure apparatus. We found a LiNbO3-structured phase in the quenched run product, which was the perovskite phase under high pressures and high temperatures. The perovskite phase coexists with corundum and a phase with (Mg,Fe3+,☐)(Al,Fe3+)2O4 composition (☐ = vacancy). The FeAlO3 component in the perovskite phase decreases from 69 to 65 mol% with increasing temperature. The Fe2O3 component in the perovskite phase remains unchanged at ~1 mol% with temperature. The A-site vacancy component of Fe2/3SiO3 in the perovskite phase exists as 1–2 mol% at 1700–2000 K, whereas 1 mol% of the oxygen vacancy component of MgFeO2.5 appears at higher temperatures, although the analytical errors prevent definite conclusions. The A-site vacancy component might be more important than the oxygen vacancy component for the defect chemistry of bridgmanite in slabs and for average mantle conditions when the FeAlO3 charge-coupled component is dominant.

Zero-Valent-Dominant Pyrochlores: Endmember Formula Calculation and Petrogenetic Significance

ABSTRACT The existing classification of pyrochlore group minerals is essentially based on the dominant valence rule. However, coupled heterovalent-homovalent substitutions at the A-, B-, and Y-sites commonly result in charge-imbalanced endmember formulae. The application of the site total charge (STC) method permits the determination of a charge-balanced endmember. Species names are assigned by using the dominant constituent rule. According to the current IMA nomenclature scheme, some previously established pyrochlore species, such as kalipyrochlore, strontiopyrochlore, bariopyrochlore, plumbopyrochlore, ceriopyrochlore, yttropyrochlore, bismutopyrochlore, and uranpyrochlore, are all grouped as zero-valent-dominant pyrochlores, resulting in the loss of petrogenetic information. In this work, the zero-valent-dominant pyrochlores of the pyrochlore group (sensu stricto) are classified into R+-, R2+-, R3+-, and R4+-pyrochlores where the respective cations (R) are the dominant valencies at the A- and Y-sites (for R+-pyrochlores) after vacancies (□) and H2O. The endmember charge arrangements are determined by the STC method to obtain charge-balanced endmember formulae for all possible zero-valent pyrochlore species. It is recommended that suitable adjectival modifiers be used along with the species name to emphasize the abundance of certain cations, which may or may not be reflected in the endmember formula. This approach would facilitate the usage of pyrochlore group minerals for all practical petrological and exploration purposes. It is considered that pyrochlores with significant A-site vacancies do not necessarily reflect formation in a supergene environment, as such pyrochlores can also form in hydrothermal parageneses.

Mineralogical and Geochemical Evidence of Dissolution-Reprecipitation Controlled Hydrothermal Rare Earth Element Mineralization in the Amba Dongar Carbonatite Complex, Gujarat, Western India

Abstract The Amba Dongar carbonatite complex in western India comprises an inner ring of carbonatite breccia surrounded by a sövite ring dike. The various carbonatite units in the body include calcite carbonatite, alvikite, dolomite carbonatite, and ankerite carbonatite. The carbonate phases (calcite and ankerite) occur as phenocrysts, groundmass phases, fresh primary grains, and partially altered grains and/or pseudomorphs when hydrothermally overprinted. Rare earth element (REE) enrichment in the groundmass/altered calcite grains compared to the magmatic ones is ascribed to the presence of micron-sized REE phases. Fluorapatite and pyrochlore constitute important accessory phases that are altered to variable extents. Higher concentrations of Sr, Si, and REEs in fluorapatite are suggestive of a magmatic origin. Fresh pyrochlore preserves its magmatic composition, characterized by low A-site vacancy and high F in the Y-site, which on alteration becomes poorer in Na, Ca, and F and displays an increase in vacancy. The C-O isotope compositions of the carbonates also corroborate the extensive low-temperature hydrothermal alteration of the carbonatites. The REE mineralization is the result of interaction of the carbonatite with a sulfur-bearing, F-rich hydrothermal fluid that exsolved from late-stage carbonatitic magmas. The hydrothermal fluids caused dissolution of the primary carbonates and simultaneous precipitation of REEs and other high field strength element (HFSE)-bearing minerals. Complex spatial associations of the magmatic minerals with the REE fluorocarbonates, [synchysite-(Ce), parisite-(Ce), bastnäsite-(Ce)] and florencite-(Ce) point to the formation of these REE phases as a consequence of postmagmatic hydrothermal dissolution of the REEs from fluorapatite, pyrochlore, and carbonates. Ubiquitous association of fluorite and barite with REE minerals indicates transport of REEs as sulfate complexes in F-rich fluids. Precipitation of REE fluorocarbonates/florencite resulted from fluid-carbonate interaction, concomitant increase in pH, and decrease in temperature. Additionally, REE precipitation was aided and abetted by the removal of sulfur from the fluid by the precipitation of barite, which destabilized the REE sulfate complexes.

Investigating the Perovskite Ag1-3xLaxNbO3 as a High-Rate Negative Electrode for Li-Ion Batteries.

The broader development of the electric car for tomorrow's mobility requires the emergence of new fast-charging negative electrode materials to replace graphite in Li-ion batteries. In this area, the design of new compounds using innovative approaches could be the key to discovering new negative electrode materials that allow for faster charging and discharging processes. Here, we present a partially substituted AgNbO3 perovskite material by introducing lanthanum in the A-site. By creating two vacancies for every lanthanum introduced in the structure, the resulting general formula becomes Ag1-3xLax□2xNbO3 (with x ≤ 0.20 and where □ is a A-site vacancy), allowing the insertion of lithium ions. The highly substituted Ag0.40La0.20□0.40NbO3 oxide shows a specific capacity of 40 mAh.g-1 at a low sweep rate (0.1mVs-1). Interestingly, Ag0.70La0.10□0.20NbO3 retains 64% of its capacity at a very high sweep rate (50mVs-1) and about 95% after 800 cycles. Ex situ 7Li MAS NMR experiments confirmed the insertion of lithium ions in these materials. A kinetic analysis of Ag1-3xLax□2xNbO3 underlines the ability to store charge without solid-state ion-diffusion limitations. Furthermore, in situ XRD indicates no structural modification of the compound when accommodating lithium ions, which can be considered as zero-strain material. This finding explains the interesting capacity retention observed after 800 cycles. This paper thus demonstrates an alternative approach to traditional insertion materials and identifies a different way to explore not-so common electrode materials for fast energy storage application.

Methylammonium Lead Bromide Perovskite Nano-Crystals Grown in a Poly[styrene-co-(2-(dimethylamino)ethyl Methacrylate)] Matrix Immobilized on Exfoliated Graphene Nano-Sheets.

Development of graphene/perovskite heterostructures mediated by polymeric materials may constitute a robust strategy to resolve the environmental instability of metal halide perovskites and provide barrierless charge transport. Herein, a straightforward approach for the growth of perovskite nano-crystals and their electronic communication with graphene is presented. Methylammonium lead bromide (CH3NH3PbBr3) nano-crystals were grown in a poly[styrene-co-(2-(dimethylamino)ethyl methacrylate)], P[St-co-DMAEMA], bi-functional random co-polymer matrix and non-covalently immobilized on graphene. P[St-co-DMAEMA] was selected as a bi-modal polymer capable to stabilize the perovskite nano-crystals via electrostatic interactions between the tri-alkylamine amine sites of the co-polymer and the A-site vacancies of the perovskite and simultaneously enable Van der Waals attractive interactions between the aromatic arene sites of the co-polymer and the surface of graphene. The newly synthesized CH3NH3PbBr3/co-polymer and graphene/CH3NH3PbBr3/co-polymer ensembles were formed by physical mixing of the components in organic media at room temperature. Complementary characterization by dynamic light scattering, microscopy, and energy-dispersive X-ray spectroscopy revealed the formation of uniform spherical perovskite nano-crystals immobilized on the graphene nano-sheets. Complementary photophysical characterization by UV-Vis absorption, steady-state, and time-resolved fluorescence spectroscopy unveiled the photophysical properties of the CH3NH3PbBr3/co-polymer colloid perovskite solution and verified the electronic communication within the graphene/CH3NH3PbBr3/co-polymer ensembles at the ground and excited states.

A Selective Targeting Anchor Strategy Affords Efficient and Stable Ideal-Bandgap Perovskite Solar Cells.

Mixed lead-tin perovskite solar cells (LTPSCs) with an ideal bandgap are demonstrated as a promising candidate to reach higher power conversion efficiency (PCE) than their Pb-counterparts. Herein, a Br-free mixed lead-tin perovskite material, FA0.8 MA0.2 Pb0.8 Sn0.2 I3 , with a bandgap of 1.33eV, as a perovskite absorber, is selected. Through density functional theory calculations and optoelectronic techniques, it is demonstrated that both Pb- and Sn-related A-site vacancies are pushed into deeper energetic depth, causing severe nonradiative recombination. Hence, a selective targeting anchor strategy that employs phenethylammonium iodide and ethylenediamine diiodide as co-modifiers to selectively anchor with Pb- and Sn-related active sites and passivate bimetallic traps, respectively, is established. Furthermore, the selectivity of the molecular oriented anchor passivation is demonstrated through energetic depth specificity of Pb- and Sn-related traps. As a result, a substantially enhanced open-circuit voltage (VOC ) from 0.79 to 0.90 V for the LTPSCs is achieved, yielding a champion PCE of 22.51%, which is the highest PCE among the reported ideal-bandgap PSCs. The VOC loss is reduced to 0.43V.

Controlling the A-site deficiency and oxygen vacancies by donor-doping in pre-reductive-sintered thermoelectric SrTiO3 ceramics

Oxygen vacancies are intrinsic defects in ABO3 perovskite structures, and the reductive sintering of SrTiO3 ceramics combined with donor-doping can create mobile carriers and/or vacancies at A-site to improve their thermoelectric performance. By scanning and transmission electron microscopy analyses, A-site deficiency was detected in undoped and donor-doped SrTiO3 ceramics, reaching 19% of A-site vacancies by co-doping La, Ce and Nb respectively into A- and B-sites. Titania with mixed Ti4+/Ti3+ cations were commonly detected as intergranular phases of Ti4+1−mTi3+mO2−m/2, which amounts to 17 vol.% in the undoped SrTiO3, thus revealing their inheritance from the pre-reductive nano-powders via liquid-phase sintering. Oxygen and A-site vacancies are both detectable by cathodoluminescence analysis from either the nano-powders or sintered phases, which reveal that mixed-donor-doping creates more vacancies at A-site, together with more oxygen vacancies to form Schottky pairs. The combination of pre-reductive sintering and mixed-donor-doping creates abundant vacancies as electronic defects in SrTiO3, while carrier transport were not hindered through the grain boundaries which enables zT value of an oxide to exceed 0.38 at 1000 K.

Mechanical properties, thermal conductivity and defect formation energies of samarium immobilization in Gd2Zr2O7: First-principles study and irradiation experiment

A density functional theory (DFT) study was employed to investigate the mechanical property, thermal conductivity, Debye temperature, electronic structure and defect chemistry of (Gd1–xSmx)2Zr2O7. All the (Gd1–xSmx)2Zr2O7 compounds exhibit an excellent structural and mechanical stability (Gd0.25Sm0.75)2Zr2O7 has the lowest Young's modulus of 213.7 GPa, the largest Possion's ratio of 0.292, the lowest Debye temperature of 491.8 K and the lowest thermal conductivity. The calculated thermal conductivities of (Gd1–xSmx)2Zr2O7 are 1.17–1.21 W/(m·K) by the Clark's model and 1.32–1.36 W/(m·K) by the Cahall's model, respectively. The formation energies of O vacancies at 48f site are negative, which increase with the Sm content, however, the formation energies of O vacancies at 8b site are almost invariable. In addition, Sm partly occupying the Gd-site reduces distinctly the formation energies of defects such as A-site vacancies, cation antisite defects, anion Frenkel pairs of oxygen at 8b site and cation interstitials, which suggests that Sm-doped Gd2Zr2O7, especially equimolar GdSmZr2O7, has a better irradiation tolerance. After the 16 MeV Ta-ion irradiation at a fluence of 1 × 1014 or 2 × 1014 ions/cm2, the crystal structure of GdSmZr2O7 transforms from pyrochlore to a defect fluorite without obvious amorphous phase.

Enhancing the luminescence performance of an LED-pumped Mn4+-activated highly efficient double perovskite phosphor with A-site defects via local lattice tuning.

Altering the local symmetry of an activator by lattice tuning is considered an effective strategy to optimize the luminescence performance of phosphors. Herein, the novel Mn4+-activated double perovskite phosphor La1.67MgTaO6 (LMTO) with A-site defects was successfully prepared. Benefiting from the random occupation of the nearest A-site by cations and vacancies, the distorted [MnO6] octahedra lack the inversion center. The LMTO:0.4 mol%Mn4+ phosphor has a significant zero-phonon line intensity with a high internal quantum efficiency (IQE) value of 62.97% after the Laporte selection rule is broken. Furthermore, local lattice tuning was performed by increasing the randomness of the A-site and the distortion index of the [MnO6] octahedron by co-doping with Ca2+ or Sr2+ ions. Importantly, the IQE value of the samples was enhanced from 62.97% to 72.65% and the activation energy increased from 0.497 eV to 0.548 eV, which can be well applied in the fields of plant cultivation and warm white light-emitting diodes. These studies provide valid fundamental insights for the selection of excellent luminescent matrices to obtain efficient Mn4+-activated phosphors via local lattice tuning.