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Whilst metal borates are well known as optical materials, their potential in solid catalysis has been less investigated. The calcite structured InBO3 was selected as the target borate and was prepared using a solvothermal method. High-resolution transmission electron microscopy and powder X-ray diffraction prove that the material has a nanoparticle morphology with an average size ∼50 nm and high crystallinity. Intrinsic surface oxygen vacancies, which are beneficial to catalysis, were detected using X-ray photoelectron spectroscopy. Lewis acidity and basicity were both observed using NH3-/CO2-temperature-programmed desorption experiments, and the total acid and base amounts were found to be 46.6 and 123.8 μmol g−1, respectively. Catalytic dehydration and dehydrogenation reactions for isopropanol at elevated temperatures were conducted in a fixed bed reactor to evaluate the catalytic performance of InBO3. InBO3 exhibits a high conversion rate (>90.5%) and, most importantly, a high dehydrogenation selectivity (acetone selectivity >92.5%), whilst the optimal acetone yield achieved was 121.3 mmol h−1 g−1cat at 350 °C. This study on InBO3 strongly suggests that metal borates have promising applications in heterogeneous catalysis. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Strecker reactions have an important application in the preparation of α-amino acids and biologically active molecules in homogeneous acid- or base-catalyzed systems. As novel solid acid catalysts, AlxGa1−x-PKU-1 (x = 0–1) solid solutions constructed by two octahedron-based molecular sieves were prepared by using a molten boric acid method and exhibited a continuously-adjustable acidity in heterogeneously-catalyzed Strecker reactions under mild conditions. AlxGa1−x-PKU-1 accommodated a large number of Brønsted acid sites in the framework, and the acidic density and strength were enhanced with the successive incorporation of Al atoms, thus the catalytic activity became superior in the formation of α-aminonitriles. Three-fold coordinated boron atoms, despite the electron-deficient configuration, seemed unable to behave as Lewis acid centers probably due to steric hindrance against the adsorption of large organic substrates. This work illustrates, for the first time, possible application in Strecker reactions using zeolite-like solid solutions, and will potentially provide an effective method and a beneficial reference to control surface acidity for porous materials. [ABSTRACT FROM AUTHOR]

A series of Tb3+ and Eu3+ co-doped Ba6Bi9B79O138 (BBBO) phosphors were prepared by high-temperature solid-state reactions in order to utilize the host-sensitization effect of Bi3+. Energy transfer processes from Bi3+ in the host to both Tb3+ and Eu3+, as well as from Tb3+ to Eu3+ were proved. Under UV-excitation, three emissions can be simultaneously observed in a single-phase phosphor, including a blue band attributed to Bi3+ emission in the host, green lines due to Tb3+ transitions of 5D4→7FJ (J = 6, 5, 4, 3), and red lines due to Eu3+ transitions of 5D0→7FJ (J = 1, 2, 3, 4). Thus, the total emission could be rationally tuned by varying the relative doping contents of Tb3+/Eu3+ and by selecting appropriate exciting irradiation. Thermal stability was investigated by in situ PL spectra at elevated temperatures. It is quite interesting to observe a significant enhancement of red emission in both BBBO:0.05Eu3+ and BBBO:0.05Tb3+,0.005Eu3+ excited at 279 nm. Especially in the latter case, the red emission intensity at 150 °C is roughly 3 times that at room temperature, which is explained by the increasing efficiency of Bi3+→ Eu3+ and Tb3+→ Eu3+ energy transfer upon annealing. [ABSTRACT FROM AUTHOR]

The study of perovskites has been active for a long time. Here, we rationally designed and prepared a double perovskite, LaBaZnSbO6, by selecting Zn2+ and Sb5+ with large size and charge differences, and, indeed, complete B-site ordering can be achieved. Careful study using powder X-ray diffraction data pinpointed its space group to be I2/m, which has rarely been seen in double perovskites. Thereafter, an interesting observation of Sr2+-doping-induced symmetry evolution from I2/m to P21/n was confirmed in the complete solid solutions LaBa1−xSrxZnSbO6, where the tilting system also transferred from a−a−c0 to a−a−c+. The transition boundary is around x = 0.4. It can also be visualized by the variation of ϑ (defined as c/[(a + b)/2]), which is associated with the anisotropic shrinkage of the unit cell lattice and indeed shows a minimum at x = 0.4. Such a successive modulation of both the structural symmetry and the average La/Ba/Sr–O bond distances (revealed by Rietveld refinements) motivated us to study the Eu3+ luminescence in La0.95Eu0.05Ba1−xSrxZnSbO6. Interestingly, the maximum of charge transfer absorption of Eu3+ shows a precise changing tendency with the A–O bond distances along with the Sr2+ doping, clearly revealing the structure–luminescence correlations. [ABSTRACT FROM AUTHOR]

Luminescence studies on rare earth borates have been active for a long time. Here we performed a careful synthesis to prepare pure samples of La1−xEux[B8O11(OH)5] (0 ≤x≤ 0.135) using the boric acid flux method, and after evaluating its thermal behavior by thermogravimetric analysis and powder X-ray diffraction, an appropriate thermal treatment was applied to prepare anhydrous β-La1−xEuxB5O9 (810 °C, 10 h), thus allowing for the study of their Eu3+-luminescence in a relatively wide range of Eu3+-concentrations. The comparatively strong f–f transition of La1−xEux[B8O11(OH)5] in the excitation spectra suggests its potential as a red-emitting phosphor for white-light emitting diodes, which can be efficiently excited by ultraviolet-light emitting diode chips and show the strongest red emission at 622 nm. In contrast, β-La1−xEuxB5O9 shows a very intense O2−→ Eu3+ CT absorption, and give strong red emissions with red/orange ratios ∼2.4. Both La1−xEux[B8O11(OH)5] and β-La1−xEuxB5O9 show no concentration quenching up to the maximal x = 0.135, which is as-expected due to their large spatial distances between rare earth cations. It is the first time to study the luminescent performance when La[B8O11(OH)5] and β-LaB5O9 were used as hosts. In addition, La0.93Eu0.07[B8O11(OH)5] was selected as a representative to study the luminescence evolution during thermal treatments. [ABSTRACT FROM AUTHOR]

Very recently, a novel barium bismuth borate Ba6Bi9B79O138 (BBBO) was shown to be a new analogue of REBaB9O16 (RE = rare earth), which is a family of excellent phosphor hosts, because it has two types of cationic positions suitable for bi- and tri-valent activators (or sensitizers). In this study, we prepared Eu3+-to-Bi3+ substituted solid solutions Ba6(Bi1−xEux)9B79O138 (0 ≤x≤ 1) and performed a systematic UV-photoluminescence investigation. As anticipated, efficient energy transfer occurs in the whole series of compounds and the majority of the energy absorbed by Bi3+ was transmitted to Eu3+, causing a great enhancement of Eu3+ emission. More interestingly, the Eu3+-to-Bi3+ substitution leads to a centrosymmetric-to-noncentrosymmetric transition of the local coordination symmetry of the trivalent cations due to the size difference between Eu3+ and Bi3+. This is observed in the synergetic changing of the wavelength of Bi3+ 1S0→3P1 absorption (excitation spectra) and the red-to-orange emission ratio (emission spectra) when changing x. BBBO is a new polyborate host with a freshly resolved structure whose interesting preliminary results suggest the further development of BBBO-based phosphors. [ABSTRACT FROM AUTHOR]

Rietveld refinements were performed on CaBaZn2Ga2−xAlxO7 (x = 0, 1, 2) to investigate the site preference of cations in tetrahedral cavities. Tri-valent Ga3+/Al3+ preferred the tetrahedral sites (T1-sites) within the triangular layers. Moreover, a layered type cationic ordering was observed in CaBaZn2Al2O7, where all the T1-sites were occupied by Al3+. This represents the first example of a cationic ordering in “114” compounds. By substituting Al3+ into CaBaZn2Ga2O7, the major change in the structure was the shrinkage of the T1O4 tetrahedrons, especially the shortening of the T1–O2 bond distance along the c-axis, which led to an anisotropic shrinkage of the unit cell. In the literature, such an anisotropic change of unit cell would induce a structure distortion, as well as a decrease in the symmetry from P63mc to Pbn21. All the title compounds crystallized in P63mc, which is unexpected. A comparison with the selected “114” magnetic oxides help us to confirm that the strong antiferromagnetic interactions of magnetic lattices would also be beneficial for symmetry lowering. Overall, the factors affecting cationic ordering on T1-sites and symmetry lowering are discussed for the title compounds, which may also be applicable to other “114” oxides. [ABSTRACT FROM AUTHOR]

Scheelite compounds with Eu3+ substitution are well-known red-phosphors. We prepared and performed a detailed structural characterization of A1–1.5xEux□0.5xWO4 and A0.64–0.5yEu0.24Liy□0.12–0.5yWO4 (A = Ca, Sr; □ = vacancy) to confirm the A-site vacancy mechanism for charge balance when bivalent A cations were substituted by Eu3+. All compounds crystallize in I41/a with a disordered arrangement of A2+, Eu3+, □ at the A-site. The title compounds are all good red phosphors with a high R/O ratio (∼10), indicating that Eu3+ is located at a significantly distorted cavity. A1–1.5xEux□0.5xWO4 shows a saturation phenomenon at a high doping level, x = 0.20. With the incorporation of Li+, the emission intensity was generally enhanced compared to the Li+-free samples, moreover, an increase of the Li+ content reduces the content of vacancies, resulting in further increase of the luminescence intensity. [ABSTRACT FROM AUTHOR]

Rietveld refinements were performed on CaBaZn2Ga2−xAlxO7 (x = 0, 1, 2) to investigate the site preference of cations in tetrahedral cavities. Tri-valent Ga3+/Al3+ preferred the tetrahedral sites (T1-sites) within the triangular layers. Moreover, a layered type cationic ordering was observed in CaBaZn2Al2O7, where all the T1-sites were occupied by Al3+. This represents the first example of a cationic ordering in “114” compounds. By substituting Al3+ into CaBaZn2Ga2O7, the major change in the structure was the shrinkage of the T1O4 tetrahedrons, especially the shortening of the T1–O2 bond distance along the c-axis, which led to an anisotropic shrinkage of the unit cell. In the literature, such an anisotropic change of unit cell would induce a structure distortion, as well as a decrease in the symmetry from P63mc to Pbn21. All the title compounds crystallized in P63mc, which is unexpected. A comparison with the selected “114” magnetic oxides help us to confirm that the strong antiferromagnetic interactions of magnetic lattices would also be beneficial for symmetry lowering. Overall, the factors affecting cationic ordering on T1-sites and symmetry lowering are discussed for the title compounds, which may also be applicable to other “114” oxides. [ABSTRACT FROM AUTHOR]

Sol–gel method was applied to prepare homogenous and highly crystalline phosphors with the formulas α-GdB5O9:xTb3+ (0 ≤x≤ 1), α-Gd1−xCexB5O9 (0 ≤x≤ 0.40), α-GdB5O9:xCe3+, 0.30Tb3+ (0 ≤x≤ 0.15) and α-GdB5O9:0.20Ce3+, xTb3+ (0 ≤x≤ 0.10). The success of the syntheses was proved by the linear shrinkage or expansion of the cell volumes against the substitution contents. In α-GdB5O9:xTb3+, an efficient energy transfer from Gd3+ to Tb3+ was observed and there was no luminescence quenching. The exceptionally high efficiency of the f–f excitations of Tb3+ implies that these phosphors may be good green-emitting UV-LED phosphors. For α-Gd1−xCexB5O9, Ce3+ absorbs the majority of the energy and transfers it to Gd3+. Therefore, the co-doping of Ce3+ and Tb3+ leads to a significant enhancement in the green emission of Tb3+. Our current results together with the study on α-GdB5O9:xEu3+ in the literature indicate that α-GdB5O9 is a good phosphor host with advantages including controllable preparation, diverse cationic doping, the absence of concentration quenching, and effective energy transfer. [ABSTRACT FROM AUTHOR]

There are very few luminescence studies for rare earth borates with hydroxyl or crystalline water molecules, which were believed to have a low luminescence efficiency because the vibrations of –OH or H2O may lead to quenching of the emission. We were motivated to study the luminescence properties of Gd1−xEux[B6O9(OH)3] (x = 0.10–1) and their dehydrated products, α-Gd1−xEuxB5O9. Efficient energy transfer from Gd3+ to Eu3+ was found in all of the studied polyborates. By TG-DSC and powder XRD experiments, we observed the dehydration of Eu[B6O9(OH)3], the re-crystallization to α-EuB5O9, and further decomposition to α-EuB3O6. During those processes, the Eu3+ luminescence spectra show interesting variations, meaning it is a good medium to understand the coordination environment evolution of Eu3+. It is observed that the symmetry of the Eu3+ coordination environment is the lowest in the amorphous state. Interestingly, this amorphous phase possesses a high efficiency of f–f transitions and a large R/O value (4.0), which implies its potential as a good red-emitting UV-LED phosphor. Anhydrous α-EuB5O9 shows the highest luminescence efficiency when excited by Eu3+ CT transition. For the first time, complete solid solutions α-Gd1−xEuxB5O9 were synthesized directly by the sol–gel method, and their luminescence properties were also studied. [ABSTRACT FROM AUTHOR]

There are limited photoluminescence (PL) studies for rare earth borates with crystalline water molecules, which are usually supposed to have low PL efficiency because the vibrations of H2O or –OH may lead to emission quenching. We investigated the PL properties of Sm1−xEux[B9O13(OH)4]·H2O (x = 0–1.00) and their dehydrated products α-Sm1−xEuxB5O9. There is no quenching effect in those studied polyborates because the large borate ionic groups isolate the Eu3+ activators very well. Sm3+ and Eu3+ are basically separated luminescent activators. Comparatively, Sm3+ shows a very small emission intensity, which can be almost ignored, therefore our interest is focused on the Eu3+ luminescence. By TG-DSC and powder XRD experiments, we defined three weight-loss steps for Eu[B9O13(OH)4]·H2O and a re-crystallization process to α-EuB5O9, during which luminescent spectra of Eu3+ are recorded. It shows an interesting variety and therefore is a good medium to understand the coordination environment evolution of Eu3+, even for the intermediate amorphous phase. In fact, the coordination symmetry of Eu3+ in the amorphous state is the lowest. The high efficiency of the f–f transitions and large R/O value (3.8) imply this amorphous phase is potentially a good red-emitting UV-LED phosphor. Anhydrous α-EuB5O9 shows the highest luminescent efficiency excited by Eu3+ CT transition. In addition, α-Sm1−xEuxB5O9 was synthesized by a sol–gel method directly for the first time, and α-EuB5O9 shows superior PL properties due to its better crystallinity. A lot of hydrated polyborates with crystalline water molecules remain unexplored and our study shows their potential as good phosphors. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

There are very few luminescence studies for rare earth borates with hydroxyl or crystalline water molecules, which were believed to have a low luminescence efficiency because the vibrations of –OH or H2O may lead to quenching of the emission. We were motivated to study the luminescence properties of Gd1−xEux[B6O9(OH)3] (x = 0.10–1) and their dehydrated products, α-Gd1−xEuxB5O9. Efficient energy transfer from Gd3+ to Eu3+ was found in all of the studied polyborates. By TG-DSC and powder XRD experiments, we observed the dehydration of Eu[B6O9(OH)3], the re-crystallization to α-EuB5O9, and further decomposition to α-EuB3O6. During those processes, the Eu3+ luminescence spectra show interesting variations, meaning it is a good medium to understand the coordination environment evolution of Eu3+. It is observed that the symmetry of the Eu3+ coordination environment is the lowest in the amorphous state. Interestingly, this amorphous phase possesses a high efficiency of f–f transitions and a large R/O value (4.0), which implies its potential as a good red-emitting UV-LED phosphor. Anhydrous α-EuB5O9 shows the highest luminescence efficiency when excited by Eu3+ CT transition. For the first time, complete solid solutions α-Gd1−xEuxB5O9 were synthesized directly by the sol–gel method, and their luminescence properties were also studied. [ABSTRACT FROM AUTHOR]

Whilst metal borates are well known as optical materials, their potential in solid catalysis has been less investigated. The calcite structured InBO 3 was selected as the target borate and was prepared using a solvothermal method. High-resolution transmission electron microscopy and powder X-ray diffraction prove that the material has a nanoparticle morphology with an average size ∼50 nm and high crystallinity. Intrinsic surface oxygen vacancies, which are beneficial to catalysis, were detected using X-ray photoelectron spectroscopy. Lewis acidity and basicity were both observed using NH 3 -/CO 2 -temperature-programmed desorption experiments, and the total acid and base amounts were found to be 46.6 and 123.8 μmol g -1 , respectively. Catalytic dehydration and dehydrogenation reactions for isopropanol at elevated temperatures were conducted in a fixed bed reactor to evaluate the catalytic performance of InBO 3 . InBO 3 exhibits a high conversion rate (>90.5%) and, most importantly, a high dehydrogenation selectivity (acetone selectivity >92.5%), whilst the optimal acetone yield achieved was 121.3 mmol h -1 g -1 cat at 350 °C. This study on InBO 3 strongly suggests that metal borates have promising applications in heterogeneous catalysis.

Significant oxide-ionic conductivity has been recently reported for a family of cation-deficient hexagonal perovskite derivatives Ba 3 M 2 O 8.5 (M = Mo/W 6+ and Nb 5+ /V 5+ ). Herein, strong 4-fold coordination geometry preferring Ge 4+ ions are doped into Ba 3 Mo 1+ x Nb 1-2 x Ge x O 8.5 to manipulate the oxygen distribution within palmierite-like layers for the enhancement of oxide-ionic conductivity. Rietveld refinement of the neutron diffraction data of Ba 3 Mo 1.2 Nb 0.6 Ge 0.2 O 8.5 reveals that Ge 4+ -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 Ba 3 Mo 1.2 Nb 0.6 Ge 0.2 O 8.5 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 Ba 3 Mo 1+ x Nb 1-2 x Ge x O 8.5 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 Ge 4+ -substitution-induced redistribution of oxygen ions within the palmierite-like layers.

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.

δ-BiB 3 O 6 shows a comparable nonlinear optical performance with the famous α-BiB 3 O 6 . It can be prepared either by a high temperature-high pressure method or by a sol-gel method associated with RE 3+ -doping (RE = La, Ce, Pr, and Nd). A direct solid state reaction at 700 °C stabilized 20 atom% La 3+ doped δ-BiB 3 O 6 compared to the previously reported record (15 atom%) in the literature. La 3+ doping would not only expand the thermodynamically stable region, which is critical for the synthesis, but also hasten the crystallization process. Moreover, Eu 3+ , 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 RE 3+ -doped δ-BiB 3 O 6 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-Bi 0.93 Eu 0.07 B 3 O 6 .

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.

Oxygen-deficient perovskites are a family of important materials that may exhibit oxide ionic conductivities. We attempted to introduce oxygen-vacancy disordering in perovskite Ca 4 GaNbO 8 (Ca 4 -type) by substituting Ca 2+ with larger Sr 2+ . Sr 2+ -to-Ca 2+ substitution did not lead to oxygen-vacancy ordering-disordering transition but an interesting Ca 4 -to-Sr 4 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 GaO 4 tetrahedra, which is the origin of the formation of the narrow two-phase region (0.3 ≤ x < 0.65) in Ca 4- x Sr x GaNbO 8 . More importantly, the A- and B-site cavities with large differences in size for both structures resulted in a site-selective doping behaviour for Sr 2+ in Ca 4- x Sr x GaNbO 8 . These structural changes found in Ca 4- x Sr x GaNbO 8 will provide a broad route approaching new oxygen-deficient phases with oxide ionic conductivities., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Semiconductor-based photocatalytic H 2 generation is a promising technique and the development of efficient photocatalysts has attracted great attention. Columbite-ZnNb 2 O 6 is a wide-bandgap semiconductor capable of photocatalytic water splitting. Here we employed a two-step hydrothermal method to first dissolve Nb 2 O 5 with a highly basic aqueous solution and further react it with Zn 2+ to form nanosized ZnNb 2 O 6 . The reaction time plays an important role on its morphology and photocatalytic performance in water reduction. The sample synthesized through 7 days of reaction was the optimal one with an appropriate crystallinity and a large specific surface area, however the severe surficial defects prohibited its photocatalytic activity in pure water. The H 2 generation at a rate of 23.6(5) μmol h -1 g -1 emerged when 20 vol% methanol was used as the hole-sacrificial agent. Most remarkably, once metal or metal oxide cocatalysts, including Pt, Au, NiO, RuO 2 , Ag 2 O, and Pd/PdO, were loaded appropriately, the photocatalytic H 2 generation rate ultimately achieved 3200(100) or 680(20) μmol h -1 g -1 with or without using methanol, respectively. Apparent quantum yields (AQYs) at 295 nm were investigated by changing the experimental parameters, and the optimal AQYs are 4.54% and 9.25% in water and methanol solution, respectively. Further post-modifications like bandgap engineering may be performed on this highly efficient nano-ZnNb 2 O 6 ., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

The study of perovskites has been active for a long time. Here, we rationally designed and prepared a double perovskite, LaBaZnSbO6, by selecting Zn(2+) and Sb(5+) with large size and charge differences, and, indeed, complete B-site ordering can be achieved. Careful study using powder X-ray diffraction data pinpointed its space group to be I2/m, which has rarely been seen in double perovskites. Thereafter, an interesting observation of Sr(2+)-doping-induced symmetry evolution from I2/m to P21/n was confirmed in the complete solid solutions LaBa1-xSrxZnSbO6, where the tilting system also transferred from a(-)a(-)c(0) to a(-)a(-)c(+). The transition boundary is around x = 0.4. It can also be visualized by the variation of θ (defined as c/[(a + b)/2]), which is associated with the anisotropic shrinkage of the unit cell lattice and indeed shows a minimum at x = 0.4. Such a successive modulation of both the structural symmetry and the average La/Ba/Sr-O bond distances (revealed by Rietveld refinements) motivated us to study the Eu(3+) luminescence in La0.95Eu0.05Ba1-xSrxZnSbO6. Interestingly, the maximum of charge transfer absorption of Eu(3+) shows a precise changing tendency with the A-O bond distances along with the Sr(2+) doping, clearly revealing the structure-luminescence correlations.

Scheelite compounds with Eu(3+) substitution are well-known red-phosphors. We prepared and performed a detailed structural characterization of A(1-1.5x)Eu(x)□(0.5x)WO4 and A(0.64-0.5y)Eu(0.24)Li(y)□(0.12-0.5y)WO4 (A = Ca, Sr; □ = vacancy) to confirm the A-site vacancy mechanism for charge balance when bivalent A cations were substituted by Eu(3+). All compounds crystallize in I4₁/a with a disordered arrangement of A(2+), Eu(3+), □ at the A-site. The title compounds are all good red phosphors with a high R/O ratio (∼10), indicating that Eu(3+) is located at a significantly distorted cavity. A(1-1.5x)Eu(x)□(0.5x)WO4 shows a saturation phenomenon at a high doping level, x = 0.20. With the incorporation of Li(+), the emission intensity was generally enhanced compared to the Li(+)-free samples, moreover, an increase of the Li(+) content reduces the content of vacancies, resulting in further increase of the luminescence intensity.

Rietveld refinements were performed on CaBaZn2Ga(2-x)Al(x)O7 (x = 0, 1, 2) to investigate the site preference of cations in tetrahedral cavities. Tri-valent Ga(3+)/Al(3+) preferred the tetrahedral sites (T1-sites) within the triangular layers. Moreover, a layered type cationic ordering was observed in CaBaZn2Al2O7, where all the T1-sites were occupied by Al(3+). This represents the first example of a cationic ordering in "114" compounds. By substituting Al(3+) into CaBaZn2Ga2O7, the major change in the structure was the shrinkage of the T1O4 tetrahedrons, especially the shortening of the T1-O2 bond distance along the c-axis, which led to an anisotropic shrinkage of the unit cell. In the literature, such an anisotropic change of unit cell would induce a structure distortion, as well as a decrease in the symmetry from P6₃mc to Pbn2₁. All the title compounds crystallized in P6₃mc, which is unexpected. A comparison with the selected "114" magnetic oxides help us to confirm that the strong antiferromagnetic interactions of magnetic lattices would also be beneficial for symmetry lowering. Overall, the factors affecting cationic ordering on T1-sites and symmetry lowering are discussed for the title compounds, which may also be applicable to other "114" oxides.

Sol-gel method was applied to prepare homogenous and highly crystalline phosphors with the formulas α-GdB5O9:xTb(3+) (0 ≤ x ≤ 1), α-Gd1-xCexB5O9 (0 ≤ x ≤ 0.40), α-GdB5O9:xCe(3+), 0.30Tb(3+) (0 ≤ x ≤ 0.15) and α-GdB5O9:0.20Ce(3+), xTb(3+) (0 ≤ x ≤ 0.10). The success of the syntheses was proved by the linear shrinkage or expansion of the cell volumes against the substitution contents. In α-GdB5O9:xTb(3+), an efficient energy transfer from Gd(3+) to Tb(3+) was observed and there was no luminescence quenching. The exceptionally high efficiency of the f-f excitations of Tb(3+) implies that these phosphors may be good green-emitting UV-LED phosphors. For α-Gd1-xCexB5O9, Ce(3+) absorbs the majority of the energy and transfers it to Gd(3+). Therefore, the co-doping of Ce(3+) and Tb(3+) leads to a significant enhancement in the green emission of Tb(3+). Our current results together with the study on α-GdB5O9:xEu(3+) in the literature indicate that α-GdB5O9 is a good phosphor host with advantages including controllable preparation, diverse cationic doping, the absence of concentration quenching, and effective energy transfer.

In literature, it is very difficult to obtain sulfides with Cr(3+) in tetrahedral coordination. Here, a thiourea-oxalic acid co-molten solvothermal method was applied to synthesize chalcopyrite CuFe(1-x)Cr(x)S2 (x ≤ 0.4) solid solutions. We propose that oxalic acid plays an important role in the crystallization of CuFe(1-x)Cr(x)S2 and can considerably restrain the formation of other undesirable impurities. The successful incorporation of Cr(3+) was confirmed by powder XRD, SEM and EDX mapping (2D elemental distribution). The UV-Vis reflectance spectra of CuFe(1-x)Cr(x)S2 suggest that the bandgap energies decrease from 0.80 to 0.61 eV along with an increase in the Cr(3+) concentration. All the CuFe(1-x)Cr(x)S2 (0 ≤ x ≤ 0.4) samples show considerably higher photocatalytic activities than P25 toward the reduction of nitrate ions in aqueous solution. We speculate that the thiourea-oxalic acid co-molten method may not only be effective to synthesize Fe(3+)-Cr(3+) sulfides, but can also be helpful to incorporate Cr(3+) to other sulfide systems with MS4 tetrahedra.

A new strategy via coupling a polyol route with an oxidation process has been developed to successfully synthesize p-n junction CuO/BiVO4 heterogeneous nanostructures. The experimental results reveal that the as-prepared p-n junction CuO/BiVO4 heterogeneous nanostructures exhibit much higher visible-light-driven photocatalytic activity for the degradation of model dye rhodamine B (RhB) than the pure BiVO4 nanocrystals. The photocatalytic degradation rate (C/C0) of the RhB for p-n junction CuO/BiVO4 heterogeneous nanostructures is about two times higher than that of pure BiVO4 nanocrystals. The enhanced photocatalytic efficiency is attributed to a large number of p-n junctions in CuO/BiVO4 heterogeneous nanostructures, which effectively reduces the recombination of electrons and holes by charge transfer from n-type BiVO4 to the attached p-type CuO nanoparticles. This work not only provides an efficient route to enhance the visible-light-driven photocatalytic activity of BiVO4, but also offers a new strategy for fabricating p-n junction heterogeneous nanostructure photocatalysts, which are expected to show considerable potential application in solar-driven wastewater treatment and water splitting.

There are limited photoluminescence (PL) studies for rare earth borates with crystalline water molecules, which are usually supposed to have low PL efficiency because the vibrations of H2O or -OH may lead to emission quenching. We investigated the PL properties of Sm(1-x)Eu(x)[B9O13(OH)4]·H2O (x = 0-1.00) and their dehydrated products α-Sm(1-x)Eu(x)B5O9. There is no quenching effect in those studied polyborates because the large borate ionic groups isolate the Eu(3+) activators very well. Sm(3+) and Eu(3+) are basically separated luminescent activators. Comparatively, Sm(3+) shows a very small emission intensity, which can be almost ignored, therefore our interest is focused on the Eu(3+) luminescence. By TG-DSC and powder XRD experiments, we defined three weight-loss steps for Eu[B9O13(OH)4]·H2O and a re-crystallization process to α-EuB5O9, during which luminescent spectra of Eu(3+) are recorded. It shows an interesting variety and therefore is a good medium to understand the coordination environment evolution of Eu(3+), even for the intermediate amorphous phase. In fact, the coordination symmetry of Eu(3+) in the amorphous state is the lowest. The high efficiency of the f-f transitions and large R/O value (3.8) imply this amorphous phase is potentially a good red-emitting UV-LED phosphor. Anhydrous α-EuB5O9 shows the highest luminescent efficiency excited by Eu(3+) CT transition. In addition, α-Sm(1-x)Eu(x)B5O9 was synthesized by a sol-gel method directly for the first time, and α-EuB5O9 shows superior PL properties due to its better crystallinity. A lot of hydrated polyborates with crystalline water molecules remain unexplored and our study shows their potential as good phosphors.