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 (Ca_1-xSr_x)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 β-CaGa₂O₄, β- 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-Ca_1-xSr_xGa₂O₄ (x < 0.2) and γ-Ca_1-xSr_xGa₂O₄ (x > 0.67) could be obtained at low temperatures. Biphasic regions, including α-Ca_1-xSr_xGa₂O₄/CSGO (0.2 ≤ x ≤ 0.67), γ-Ca_1-xSr_xGa₂O₄/CSGO (0.67 < x ≤ 0.8), and β-Ca_1-xSr_xGa₂O₄/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

The multiple and site-selective occupancies of Bi³⁺ activators in Ca₇Mg₂Ga₆O₁₈:xBi³⁺ lead to a continuous red-shift of the broad emission band upon increasing the Bi³⁺-content.

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 CaBa_1-xPb_xZn₂Ga₂O₇ (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 CaBa_1-xPb_xZn₂Ga₂O₇ 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 CaBa_1−xPb_xZn₂Ga₂O₇ solid solution in fact is a macroscopic performance of the release of uniaxial strain along c-axis when Ba²⁺ is replaced with smaller Pb²⁺.

Lone pair cations can impart interesting features in some structures, such as noncentrosymmetry. Here the authors show unexpected cell volume expansion in a polar “114”-type oxide upon replacing Ba²⁺ with a smaller Pb²⁺, and attribute it to high stereochemical activity of Pb²⁺ with the strained framework.

Highly improved photocatalytic degradation of rhodamine B over Bi2Ga4-x Fe x O9 solid solutions under visible light irradiation

In this work, Bi2Ga4-x Fe x O9 (0 ≤ x ≤ 1.2) solid solutions were prepared via the traditional high-temperature solid-state reaction. The Le Bail fitting on the powder X-ray diffraction patterns shows that these solid solutions were successfully synthesized. Scanning electron microscopy showed that the Bi2Ga3.2Fe0.8O9 sample was composed of sub-micron particle crystallites. Energy dispersive spectroscopy analysis and X-ray photoelectron spectroscopy were used to identify that the Fe element is trivalent when doping into the crystal structure. Ultraviolet-visible diffused reflectance spectra suggested that the bandgap of Bi2Ga3.2Fe0.8O9 is narrower than that of the undoped Bi2Ga4O9 sample. Three strategies, including Fe3+ doping, addition of H2O2, and loading of the cocatalyst, were utilized to improve the photocatalytic degradation activity. The optimum photocatalytic performance was obtained over 2.5 wt% Cu/Bi2Ga3.2Fe0.8O9 sample in 20 ppm RhB aqueous solution (containing 1.5 mL H2O2) under visible light irradiation. Its photodegradation rate is 8.0 times that of Bi2Ga4O9 containing 0.5 mL H2O2. The 2.5 wt% Cu/Bi2Ga3.2Fe0.8O9 photocatalyst remained stable and active even after four cycles. Also, its photocatalytic conversion efficiency for RhB was nearly 100%, which was achieved in 3 hours. The photocatalytic mechanism indicated that ·OH and h+ played an important role in the photocatalytic degradation reaction.