Ba3SnGa10-xInxO20 (0 ≤ x ≤ 2): site-selective doping, band structure engineering and photocatalytic overall water splitting.

Developing new photocatalysts and deciphering the structure-property relationship are always the central topics in photocatalysis. In this study, a new photocatalyst Ba3SnGa10O20 containing two d10 metal cations was prepared by a high temperature solid state reaction, and its crystal structure was investigated by Rietveld refinements of monochromatic X-ray powder diffraction data for the first time. There are 2 Ba, 4 metal cations and 6 O independent atoms in a unit cell. Sn4+ and Ga3+ co-occupy the octahedral cavities named M1 and M2 sites, and the other two metal sites are fully occupied by Ga3+. Rational In3+-to-Ga3+ substitution was performed to reduce the potential of the conduction band minimum and enhance the light absorption ability, which was indeed confirmed using UV-vis diffuse reflectance spectra and Mott-Schottky plots for Ba3SnGa10-xInxO20 (0 ≤ x ≤ 2). Interestingly, In3+ exhibits site selective doping at M1 and M2 sites exclusively. With the light absorption ability enhanced, the photocatalytic overall water splitting activity was also improved, i.e. the photocatalytic H2 generation rate was 1.7(1) µmol h-1 for Ba3SnGa10O20, and the optimal catalyst Ba3SnGa8.5In1.5O20 loaded with 1.0 wt% Pd exhibited the H2 generation rate of 27.5(4) µmol h-1 and the apparent quantum yield at 254 nm was estimated to be 2.28% in pure water.

Ce3+-Doped Li2Ca5Gd(BO3)5 Phosphors with Multiple Luminescent Centers and High Pressure Sensitivity under Near UV Excitation.

The performance of Ce3+-based phosphors under mechanical high pressures becomes attractive due to the potential application as a visual pressure sensor. Li2Ca5Gd(BO3)5 was selected as the host for the Ce3+ doping. Rietveld refinements reveal that rare earth cations occupy M1, M2, and M3 sites, and indeed, the photoluminescent spectra of Li2Ca5Gd1-xCex(BO3)5 (0.005 ≤ x ≤ 0.15) exhibit the characteristic of multiple activators, defined as CeI, CeII, and CeIII, with the maximal emission wavelength at ∼444, 419, and 378 nm, respectively. The optimal internal and external quantum efficiencies are 86.29% for x = 0.005 and 20.26% for x = 0.10, respectively, under the NUV excitation at 363 nm. In-situ high pressure emission spectra under 375 nm excitation exhibit an overall red-shift, and the linear pressure susceptibilities up to 6.7 GPa for CeI and CeII centers are -390 and -279 cm-1/GPa, respectively, which is probably the largest among Ce3+-doped oxides and oxysalts. Due to the above superiorities, Ce3+-doped LCGB possesses a high potential as a visual pressure sensor, and this is a successful study on the structure-property relationship of inorganic materials.

Combined Analyses on Electronic Structure and Molecular Orbitals of d10 Bimetal Oxide In2Ge2O7 and Photocatalytic Performances for Overall Water Splitting and CO2 Reduction.

Semiconducting photocatalytic overall water splitting and CO2 reduction are possible solutions to the emerging worldwide challenges of oil shortage and continual temperature increase, and the key is to develop an efficient photocatalyst. Most photocatalysts contain the d0, d10 or d10ns2 metals, and a guiding principle is desired to help to distinguish outstanding semiconductors. Here, the d10 bimetal oxide In2Ge2O7 was selected as the target. First, density functional theory (DFT) calculations point out that the nonbonding O 2p orbitals dominate the valence band maximum (VBM), and In 5s-O 2s and Ge 4s-O 2s antibonding orbitals are the major components of conduction band minimum (CBM). Moreover, the molecular orbitals were analyzed to consolidate the DFT calculations and make it more understandable for chemists. Due to the very small specific surface area (0.51 m2/g) and wide band gap (4.14 eV), as-prepared In2Ge2O7 did not exhibit any overall water splitting activity; nevertheless, when loading with 1 wt% cocatalyst (i.e., Pt, Pd), the surficial charge recombination can be greatly eliminated and the overall water splitting activity is significantly improved to 33.0(4) and 17.2(7) µmol/h for H2 and O2 generation, respectively. The apparent quantum yield (AQY) at 254 nm is 8.28%. This observation is proof that the inherent electronic structure of In2Ge2O7 is beneficial for the charge migration in bulk. Moreover, this catalyst also exhibits an observable CO2 reduction activity in pure water, which is a competition reaction with water splitting, anyway, the CH4 selectivity can be enhanced by loading Pd. This is a successful attempt to unravel the structure-property relationship by combining the analyses on electronic structure and molecular orbitals and is enlightening to further discover good candidates to photocatalysts.

Color-tunable emissions realized by Tb3+ to Eu3+ energy transfer in ZnGdB5O10 under near-UV excitation.

Photoluminescent (PL) energy transfer (ET) between two typical rare earth activators Tb3+ and Eu3+ is utilized to achieve color-tunable emission and the color range is apparently dependent on the ET efficiency. In the target host ZnGdB5O10 (ZGBO), the relatively low symmetric coordination environment of the rare earth cation not only suppresses the parity-forbidden law of the 4f-4f transitions of Tb3+ in the near-UV region, but also enhances the internal quantum efficiency (IQE), where the optimal IQE is 65.61% for ZGBO:0.8Tb3+. Moreover, its ET to Eu3+ is highly efficient, i.e. 94.71% in ZGBO:0.8Tb3+,0.10Eu3+, which eventually leads to a wide range of color-tunable emissions from green (0.2915, 0.5915) to red (0.6207, 0.3731). The systematic PL spectral study on Tb3+/Eu3+ singly doped and co-doped phosphors suggests that the ET mechanism takes place through the electric dipole-dipole interaction according to the Inokuti-Hirayama (I-H) model. Additionally, the in situ high temperature PL spectra indicate the very high thermal stability of ZnGd0.19Tb0.8Eu0.01B5O10, indicating that it can be a potential candidate for near-UV light emitting diode-pumped phosphors.

Exclusive confinement of Bi3+-activators in the triangular prism enabling efficient and thermally stable green emission in the tridymite-type phosphor CaBaGa4O8:Bi3.

Recently, Bi3+-activated phosphors have been extensively studied for potential applications in phosphor-converted white light-emitting diodes (pc-WLEDs). However, Bi3+ activators usually exhibit low quantum efficiency and poor thermal stability due to the outermost 6s6p-orbitals of Bi3+ being strongly coupled with the host lattice, inhibiting potential applications. Herein, we rationally design a novel phosphor CaBaGa4O8:Bi3+, which adopts a tridymite-type structure and crystallizes in the space group of Imm2. CaBaGa4O8:Bi3+ presents a bright green light emission peaking at 530 nm with a FWHM narrower than 90 nm. Comprehensive structural and spectroscopic analyses unravelled that Bi3+ emitters were site-selectively incorporated into the triangular prism (Ca2+-site) in CaBaGa4O8:Bi3+ since there exist two distinct crystallographic sites that can accommodate the Bi3+ ions. An excellent luminescence thermal stability of 73% of the ambient temperature photoluminescence intensity can be maintained at 423 K for CaBaGa4O8:0.007Bi3+. Impressively, the quantum efficiency (QE) of CaBaGa4O8:0.007Bi3+ was remarkably improved to 47.2% for CaBaGa4O8:0.007Bi3+,0.03Zn2+via incorporating the Zn2+ compensators without sacrificing the luminescence thermal stability. The high thermal stability and QE of CaBaGa4O8:0.007Bi3+,0.03Zn2+ are superior to most of the Bi3+-activated green-emitting oxide phosphors. The perspective applications in pc-WLEDs for CaBaGa4O8:0.007Bi3+,0.03Zn2+ were also studied by fabricating LED devices.

Equivalent chemical substitution in double-double perovskite-type ALaLiTeO6:Mn4+ (A = Ba2+, Sr2+, Ca2+) phosphors enabling wide range crystal field strength regulation and efficient far-red emission.

Mn4+-activated phosphors have shown wide prospective applications in phosphor-converted white light-emitting diodes (pc-WLEDs) and pc-LEDs used in illumination and indoor plant cultivation, respectively. Recently, double perovskites A2B'B''O6 with a tunable crystal structure and versatile octahedral sites have been extensively studied as good host matrixes for Mn4+-emitters to realize tunable far-red emissions. Herein, a series of double-double perovskite-type ALaLiTeO6:Mn4+ (A = Ba, Ba0.5Sr0.5, Sr, Sr0.5Ca0.5, Ca) phosphors were synthesized and structurally characterized, and the correlations between their structure and luminescence were also studied systematically. With a decrease of the A-cation size, an increased distortion in the average structure and a structure symmetry lowering (I2/m → P21/n) were observed for ALaLiTeO6:Mn4+. In contrast, on the local scale, the degree of (Li/Te)O6-octahedral distortion is positively correlated with the ΔIR value, which is the ionic radius difference between A2+ and La3+. The local structural changes were found to be irrelevant to the significant improvements in photoluminescence properties. In combination with careful spectroscopic analysis, we deciphered that a decreased A-cation is in fact helpful for the enhancements in crystal field strength (Dq/B = 2.12-2.82) and Mn-O covalent bonding, thereby resulting in an improved quantum efficiency, a suppressed nonradiative transition, and a redshift in photoluminescence spectra. Amongst the ALaLiTeO6:Mn4+ phosphor series, CaLaLiTeO6:Mn4+ exhibits the highest external quantum efficiency of 70.1% and internal quantum efficiency of 96.4% and superior thermal stability (93.3%@423 K), making CaLaLiTeO6:Mn4+ very promising as far-red phosphors for pc-LEDs. The findings of this work will serve as a new guide for rational design of high-performance Mn4+-activated double-double perovskite-type far-red phosphors.

Bi3+ photoluminescence in Y1-xBixCa3(GaO)3(BO3)4 and energy transfer to Eu3+ and Tb3+ in co-doped phosphors.

Bi3+ possesses outer shell lone pair electrons, and, thus the so-involved photoluminescence (PL) is sensitive to the surrounding coordination. Besides, the similarity in the structural chemistry between Bi3+ and rare earth (RE) ions inspires us to investigate the Bi3+-PL performance in RE3+-containing hosts. Herein, Y1-xBixCa3(GaO)3(BO3)4 (0.01 ≤ x ≤ 0.15) compounds were prepared by a high temperature solid state reaction method. The successful cationic doping and phase purity were confirmed by powder X-ray diffraction analysis. This series of phosphors exhibit the very strong absorption of Bi3+ 1S0 → 3P1 at 272 nm along with the intense blue emission with a maximum at 399 nm and a full width at half maximum (FWHM) of 59 nm. They retained 78.86% of the emission intensity at 150 °C, with reference to that at room temperature. Moreover, Bi3+ could also behave as a sensitizer to enhance the emission efficiency of RE3+ and thus to realize color-tunable phosphors. The energy transfer was proved in the co-doped phosphors Y0.95-yBi0.05EuyCa3(GaO)3(BO3)4 (0.05 ≤ y ≤ 0.6) and Y0.95-zBi0.05TbzCa3(GaO)3(BO3)4 (0.05 ≤ z ≤ 0.6), and color-tunable emissions from blue to red, or from blue to green were realized in these two series of phosphors.

Enhancing the oxide-ionic conductivity of Ba3Mo1+xNb1-2xGexO8.5 at intermediate temperatures: the effect of site-selective Ge4+-substitution.

Significant oxide-ionic conductivity has been recently reported for a family of cation-deficient hexagonal perovskite derivatives Ba3M2O8.5 (M = Mo/W6+ and Nb5+/V5+). Herein, strong 4-fold coordination geometry preferring Ge4+ ions are doped into Ba3Mo1+xNb1-2xGexO8.5 to manipulate the oxygen distribution within palmierite-like layers for the enhancement of oxide-ionic conductivity. Rietveld refinement of the neutron diffraction data of Ba3Mo1.2Nb0.6Ge0.2O8.5 reveals that Ge4+-ions are selectively incorporated into the palmierite-like layers, owing to their strong 4-fold coordination environment preference. Such a site-selective doping behavior leads to an increase in the occupation proportion of the O3 site and a concomitant decrease in the occupancy factor for O2. Ionic conduction measurements show that the bulk conductivity of Ba3Mo1.2Nb0.6Ge0.2O8.5 is about twice higher than that of the parent compound at intermediate temperatures (300-500 °C). Furthermore, bond-valence site energy (BVSE) landscape analysis reveals that the oxygen ionic conduction of Ba3Mo1+xNb1-2xGexO8.5 is dominated by the two-dimensional pathways along the palmierite-like layers, despite the three-dimensional (3D) oxygen diffusion pathways being present in the hybrid structure, which strongly confirms that the enhancement in ionic conductivity at intermediate temperatures is attributed to the site-selective Ge4+-substitution-induced redistribution of oxygen ions within the palmierite-like layers.

Structural Diversity and Incompatibility Induced Complex Phase Formation Behavior in the Stuffed Tridymites Ca1-xSrxGa2O4.

Stuffed tridymites AM2O4 composed of a condensed MO4-tetrahedra-based framework have been widely investigated due to their structural diversity and rich physical properties. Herein, the strategy of stuffing mixed Ca2+ and Sr2+ cations into the [Ga2O4]2- framework in (Ca1-xSrx)Ga2O4 (CSGO, 0 ≤ x ≤ 1) is utilized to manipulate the phase formation behavior with different structure types at particular annealing temperatures. Five derivatives, including α- and ß-CaGa2O4, ß- and γ-SrGa2O4, and new CSGO-type structures, were observed. The distinctive feature of the CSGO-structure is the coexistence of UUDDUD- and UDUDUD-type six-membered rings, where U (up) and D (down) denote the orientations of GaO4-tetrahedra with respect to the plane grids, in a ratio of 2:1. Single-phase α-Ca1-xSrxGa2O4 (x < 0.2) and γ-Ca1-xSrxGa2O4 (x > 0.67) could be obtained at low temperatures. Biphasic regions, including α-Ca1-xSrxGa2O4/CSGO (0.2 ≤ x ≤ 0.67), γ-Ca1-xSrxGa2O4/CSGO (0.67 < x ≤ 0.8), and ß-Ca1-xSrxGa2O4/CSGO (0.8 < x < 1), were observed at the intermediate temperature region and evolve irreversibly into the CSGO single-phase region upon elevating the temperature. Moreover, the structure-property relationship of the new CSGO-phase was further studied by doping coordination-sensitive Bi3+ activators to advance the development and applications of stuffed tridymites.

Complex crystal structure and photoluminescence of Bi3+-doped and Bi3+/Eu3+ co-doped Ca7Mg2Ga6O18.

Recently, broad-band white-light-emitting using Eu2+, Ce3+, or Bi3+ in multi-sites of a single-phase phosphor have drawn extensive attention due to their potential to realize high-quality indoor lighting. This work reports a novel oxide, Ca7Mg2Ga6O18 (CMG), possessing a complex crystal structure in the space group F432. Rietveld refinements of high-resolution synchrotron X-ray diffraction data were performed to determine the accurate atomic positions and occupancy factors, giving a reasonable composition of Ca7Mg1.91(4)Ga6.09(4)O18. Due to the multiple sites of Ca2+, which are suitable for the doping of Bi3+ activators, CMG is a promising host to achieve broad-band white-light emission. Detailed structural and spectroscopic analyses revealed that the Bi3+-activator shows multiple and site-selective occupancy, which is the origin of the red-shifts in both broad-band excitation and emission spectra upon increasing the Bi3+ content. A series of Bi3+ and Eu3+ codoped phosphors with tunable blue-pink-red emission were prepared due to the Bi3+-to-Eu3+ energy transfer. Due to the distinctive thermal behaviors of Bi3+ and Eu3+ emissions, CMG:Bi3+,Eu3+ is a candidate for optical thermometry.

Efficient Bi3+ to Eu3+ energy transfer and color tunable emissions in K7CaY2(B5O10)3-based phosphors.

Rare-earth borates are well known good photoluminescent materials due to the easy manipulation of activator concentration. K7CaY2(B5O10)3 belongs to a recently discovered borate family with outstanding nonlinear optical performances. A systematic study on Bi3+ and Eu3+ doped phosphors was performed to explore their potential in photoluminescence. K7Ca(Y1-xBix)2(B5O10)3 (0.01 ≤x≤ 0.06), K7Ca(Y1-yEuy)2(B5O10)3 (0.10 ≤y≤ 1) and K7Ca(Y0.99-zBi0.01Euz)2(B5O10)3 (0.05 ≤z≤ 0.90) were prepared by high temperature solid state reactions. Rietveld refinements reveal an 8% anti-site occupancy of Ca2+ and Eu3+ in K7CaEu2(B5O10)3, and two sets of Bi3+ emission and excitation spectra are also observed once Bi3+ is introduced. For instance, the two strongest 1S0→3P1 excitations at 270 nm and 281 nm correspond to two 3P1→1S0 emissions at 383 and 334 nm, respectively. The Eu3+ emission shows a maximal intensity at y = 0.50 under charge transfer excitation, while there is no concentration quenching effect under f-f excitation. The Bi3+-to-Eu3+ energy transfer is firmly supported by the steady photoluminescence spectra and the decreased lifetime of Bi3+ upon increasing the Eu3+ content in K7Ca(Y0.99-zBi0.01Euz)2(B5O10)3. This energy transfer mechanism occurs through the electric dipole-dipole interaction. Under excitation at 281 nm, the emission is tunable from deep blue (Bi3+) to pink and finally to red (Eu3+) all with high quantum yields (>80%).

Rationalize the Significantly Enhanced Photocatalytic Efficiency of In3+-doped α'-Ga2S3 by Bond Theory and Local Structural Distortion.

Mechanistic understanding on the electronic structure of α'-Ga2S3 unravel that the electrons in nonbonding 3pz orbitals of two-coordinated S2- anions are photoexcited to the adjacent σ-type antibonding orbitals (Ga-4s and S-3p) and migrate thereafter to the surface along the a-axis. By introduction of the In-S antibonding on the one hand and modifying the local dipole moment on the other hand, the light absorption ability and charge separation efficiency can be both enhanced by In3+-to-Ga3+ substitution, and the photocatalytic H2 evolution rate can be significantly promoted. Local geometric distortion is common in solid solutions, but its effect on charge migration behavior has yet been considered in semiconducting photocatalysis. Our case study on In3+-doped Ga2S3 is a good reminder of such the importance.

d10 or d0? Theoretical and experimental comparison between rutile GeO2 and TiO2 for photocatalytic water splitting.

Rutile GeO2 with d10 metal in octahedral coordination possesses both a high charge separation rate and carrier mobility because the partial charge density is dominated by Ge-O anti-bonding for CBM. GeO2 is capable of photocatalytic water splitting, even visible light water splitting through combination with the sensitizer melon.

Ambient pressure synthesis of Eu3+ doped δ-BiB3O6 by seed-assisted high temperature solid state reactions.

δ-BiB3O6 shows a comparable nonlinear optical performance with the famous α-BiB3O6. It can be prepared either by a high temperature-high pressure method or by a sol-gel method associated with RE3+-doping (RE = La, Ce, Pr, and Nd). A direct solid state reaction at 700 °C stabilized 20 atom% La3+ doped δ-BiB3O6 compared to the previously reported record (15 atom%) in the literature. La3+ doping would not only expand the thermodynamically stable region, which is critical for the synthesis, but also hasten the crystallization process. Moreover, Eu3+, a smaller RE cation, was also doped for the first time at a high doping level (7 atom%) simply by a seed-assisted solid state reaction, which apparently provides additional functionality and will prompt further efforts on single crystal growth of RE3+-doped δ-BiB3O6 under ambient pressure. The combined powder X-ray diffraction and transmission electron microscopy analysis underpinned the phase purity and doping homogeneity of cation doped bismuth borates. UV-excited red luminescence was observed for δ-Bi0.93Eu0.07B3O6.

Unprecedented lattice volume expansion on doping stereochemically active Pb2+ into uniaxially strained structure of CaBa1-xPbxZn2Ga2O7.

Lone pair cations like Pb2+ are extensively utilized to modify and tune physical properties, such as nonlinear optical property and ferroelectricity, of some specific structures owing to their preference to adopt a local distorted coordination environment. Here we report that the incorporation of Pb2+ into the polar "114"-type structure of CaBaZn2Ga2O7 leads to an unexpected cell volume expansion of CaBa1-xPbxZn2Ga2O7 (0 ≤ x ≤ 1), which is a unique structural phenomenon in solid state chemistry. Structure refinements against neutron diffraction and total scattering data and theoretical calculations demonstrate that the unusual evolution of the unit cell for CaBa1-xPbxZn2Ga2O7 is due to the combination of the high stereochemical activity of Pb2+ with the extremely strained [Zn2Ga2O7]4- framework along the c-axis. The unprecedented cell volume expansion of the CaBa1-xPbxZn2Ga2O7 solid solution in fact is a macroscopic performance of the release of uniaxial strain along c-axis when Ba2+ is replaced with smaller Pb2+.

Chemical-substitution-induced successive symmetry descent and structure-property correlation for "114" oxides CaBa1-xSrxZn2Al2O7.

The crystal structures, photoluminescence properties, and transport properties of a series of new "114" oxides CaBa1-xSrxZn2Al2O7 (x = 0-1) were investigated in detail. Careful Rietveld refinements performed on solid solution samples revealed that the structural symmetry of CaBa1-xSrxZn2Al2O7 evolves from hexagonal P63mc (x < 0.2) to trigonal P31c (0.2 ≤ x ≤ 0.6) and then to orthorhombic Pna21 (x > 0.6) with an increase of the Sr2+-content, which is cooperative with the rotation of T1O4 tetrahedra around the c-axis. Eu3+ was used as a local structural probe to gain an insight into the structure, which further corroborated the correctness of the observed structural symmetry descending sequence in CaBa1-xSrxZn2Al2O7. More importantly, the reduction of structural symmetry is also associated with a tendency from layered ordering to complete charge ordering transition for Zn2+/Al3+ cations, which was revealed to have a significant influence on the transport properties. These findings are expected to offer a route to manipulate the physical properties of "114" oxides containing magnetic cations.

Site-selective doping effect, phase separation, and structure evolution in 1:1:1 triple-cation B-site ordered perovskites Ca4-x Sr x GaNbO8.

Oxygen-deficient perovskites are a family of important materials that may exhibit oxide ionic conductivities. We attempted to introduce oxygen-vacancy disordering in perovskite Ca4GaNbO8 (Ca4-type) by substituting Ca2+ with larger Sr2+. Sr2+-to-Ca2+ substitution did not lead to oxygen-vacancy ordering-disordering transition but an interesting Ca4-to-Sr4 type structure transition. Rietveld refinements revealed that the two-type structures exhibit similar oxygen-vacancy ordering and identical 1:1:1 triple-cation B-site ordering. Close inspection of the two-type structures revealed the subtle structure difference lies in the orientations of GaO4 tetrahedra, which is the origin of the formation of the narrow two-phase region (0.3 ≤ x < 0.65) in Ca4-x Sr x GaNbO8. More importantly, the A- and B-site cavities with large differences in size for both structures resulted in a site-selective doping behaviour for Sr2+ in Ca4-x Sr x GaNbO8. These structural changes found in Ca4-x Sr x GaNbO8 will provide a broad route approaching new oxygen-deficient phases with oxide ionic conductivities.

Ca2 PbGa8 O15 : Rational Design, Synthesis, and Structure Determination of a Purely Tetrahedra-Based Intergrowth Oxide.

Intergrowth oxides, like Aurivillius, Ruddlesden-Popper phase, comprise functional layers and exhibit interesting physical properties. The hitherto known intergrowth structures mainly were composed of closed-packing of oxygen ions, and it is very challenging to develop new types of intergrowth structures. We proposed the possible match between the tridymite and grossite, both of which are purely tetrahedra-based structures. We synthesized Ca2 PbGa8 O15 ((Ca0.5 Pb0.5 Ga2 O4 )2 (CaGa4 O7 )) and its structure was solved by ab-initio method. Pb2+ is vitally important to stabilize this first example of tetrahedra-based intergrowth oxide. The appropriate size difference between Pb2+ and Ca2+ causes the layered type cationic ordering, and reduced the thermodynamic potential, in addition, the high hybridization between Pb 6s6p and O 2p orbitals further consolidate the covalency of the tetrahedra-base framework.

Strong f-f Excitation and Bright Red Emission in Cd4 Gd1-x Eux O(BO3 )3 (0≤x≤1): Near-UV LED Pumped Red Phosphor with Low Thermal Quenching.

Searching efficient red phosphors under near-UV or blue light excitation is practically important to improve the current white light-emitting diodes (WLEDs). Eu2+ - and Mn4+ -based red phosphors have been extensively studied. Here we proposed that Eu3+ is also a promising activator when it resides on a noncentrosymmetric coordination site. We proved that Cd4 GdO(BO3 )3 is a good host, which has a significantly distorted coordination for Eu3+ . A careful crystallographic study was performed on the solid solutions of Cd4 Gd1-x Eux O(BO3 )3 (0≤x≤1) by Rietveld refinements. The as-doped Eu3+ cations locate at the Gd3+ site and are well separated by CdO8 , CdO6 and BO3 groups; thus, only a slight concentration quenching was observed at ≈80 atom % Eu3+ . Most importantly, the parity-forbidden law of 4f-4f transitions for Eu3+ are severely depressed, thus the absorptions at ≈393 and ≈465 nm are remarkable. Cd4 Gd0.2 Eu0.8 O(BO3 )3 can be pumped by a 395 nm LED chip to give a bright red emission, and when mixed with other commercial blue and green phosphors, it can emit the proper white light (0.3657, 0.3613) with a suitable Ra ≈87 and correlated colour temperature ≈4326 K. In-situ photoluminescence study indicated the low thermal quenching of these borate phosphors, especially under 465 nm excitation. Our case proves the practicability to develop near-UV excited red phosphors in rare-earth-containing borates.

Substitution-Induced Structure Evolution and Zn2+/Ga3+ Ordering in "114" Oxides MAZn2Ga2O7 ( M = Ca2+, Sr2+; A = Sr2+, Ba2+).

The "114" oxides LnBa(Co/Fe)4O7+δ represent a new family of materials that exhibits intriguing physical properties, including geometrically frustrated magnetism, oxygen storage, and magnetoelectric couplings. Various chemical substitutions have been conducted to modify their crystal and magnetic structures as well as physical properties. However, the principles beneath the substitution-induced structural evolution and charge/cationic ordering have not yet been understood. Thus, in this contribution, two complete solid solutions of MAZn2Ga2O7 ( M = Ca2+, Sr2+; A = Sr2+, Ba2+) were designed, synthesized, and characterized by Rietveld refinements based on high-resolution X-ray diffraction (XRD) and neutron diffraction (ND) data. The structure symmetry of MAZn2Ga2O7 is determined by the cationic size mismatch between M and A cations that can be defined by the tolerance factor t, i.e., symmetry transitions from P63 mc ( t > 0.87) to P31 c (0.87 > t > 0.75) and to Pna21 ( t < 0.75) were observed for MAZn2Ga2O7, associated with the rotation of T1O4 tetrahedra in the triangular layers. The Zn2+/Ga3+ ordering at T sites is also a consequence of the increase or decrease of the average sizes of M and A cations. A small concentration of interstitial oxygen ions can be obtained in Sr2Zn2- xGa2+ xO7+ x/2 ( x = 0.1, 0.2); however, no oxygen ionic conduction was observed at high temperatures, indicating the migration ability of the interstitial oxygen was very limited.