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Abstract The present work concerns calcium pyroselenites CaSe 2 O 5 with two independent crystal structures, their difference physical properties and phase transformations. α-CaSe 2 O 5 in orthorhombic form shows 2D calcium oxide anionic layers whereas new compound β-CaSe 2 O 5 crystallizes in monoclinic space group C 2 /c. It's a new structure type with 1D calcium oxide thick chains which are further connected by Se 2 O 5 dimers. Rietveld refinement indicates that the weight fraction for α/β-CaSe 2 O 5 mixture phase was found to be 84.5% and 15.5% for α- and β-CaSe 2 O 5 , respectively. Optical diffuse reflectance spectrum coupled with Kubelka–Munk fit gives a result that α-CaSe 2 O 5 adopts direct optical transition model while β-CaSe 2 O 5 is an indirect semiconductor. The experimental result matches well with the prediction of theory calculation. The optical bandgap values were fitted to be 4.74 and 4.12 eV for α- and β-CaSe 2 O 5 , respectively. Thermal analysis coupled with variable temperature powder XRD indicates the phase transformation between these two phases. Graphical abstract β-CaSe 2 O 5 features a new structure which was reported for the first time. Compare with hyper-symmetric of α-CaSe 2 O 5 , structural differences lead into the changes for their physical performances. Optical properties analysis indicates that α-CaSe 2 O 5 is a direct bandgap semiconductor whereas β-CaSe 2 O 5 features an indirect optical transition. Thermal analysis coupled with variable temperature powder XRD indicates the phase transformation of two phases. fx1 Highlights • β-CaSe 2 O 5 was reported for the first time. • Pure α-CaSe 2 O 5 was obtained under a rapid microwave-assist reaction conditions. • α-CaSe 2 O 5 features a direct optical transition model whereas β-CaSe 2 O 5 is an indirect semiconductor. • Thermal analysis coupled with variable temperature powder XRD indicates the phase transformation of two phases. [ABSTRACT FROM AUTHOR]

We reported the synthesis and crystal structures of alkali metal and alkali-earth metal phosphite, namely, CsV 2 (H 3 O)(HPO 3 ) 4 ( 1 ), and Ba 3 V 2 (HPO 3 ) 6 ( 2 ). Both compounds were prepared by hydrothermal reactions and feature unique new structures. They both exhibit 3D complicated frameworks based on VO 6 octahedra which are connected by HPO 3 tetrahedra via corner-sharing. Alkali or alkali earth metal cations are filled in the different channels of the frameworks. Topological analysis shows that the framework of CsV 2 (H 3 O) (HPO 3 ) 4 ( 1 ) is a new 3,3,3,4,5-connected network with the Schläfli symbol of {4.6 2 } 2 {4 2 .6 6 .8 2 }{6 3 }{6 5 .8}. The investigations of X-ray photoelectron spectroscopy (XPS) and magnetic measurement on CsV 2 (H 3 O)(HPO 3 ) 4 suggest a +3 oxidation state of the vanadium ions in compound 1 . Photocatalytic performance was evaluated by photocatalytic H 2 evolution and degradation of methylene blue, which shows that both compounds exhibit activity under visible-light irradiation. IR spectrum, UV–vis-NIR spectrum and thermogravimetric analysis (TGA) of compounds were also investigated. [ABSTRACT FROM AUTHOR]

Our exploration in vanadate selenites by using boric acid flux method has successfully lead into a new phase, namely, γ-V 2 Se 2 O 9. Single crystal X-ray diffraction indicates its a new structure which crystallizes in monoclinic space group P 2 1 /c with a lattice parameter of a ​= ​7.4857 (12) Å, b ​= ​7.4388 (8) Å, c ​= ​13.042 (2) Å, β ​= ​106.619 (9)°. Its crystal structure features novel 2D electroneutral [V 2 Se 2 O 9 ] layers composed of 1D [V 2 SeO 9 ]4- chains that are further bridged by Se (1)O 3 triangular pyramids. [V 2 SeO 9 ]4- chain consist corner-sharing of VO 6 and tridentate Se (2)O 3. Unlike previously reported α - and β -V 2 Se 2 O 9 with isolated [V 4 O 18 ]16− tetramers, corner-sharing of VO 6 octahedra lead into a 1D [V 2 O 9 ]8- chain in γ-V 2 Se 2 O 9. X-ray powder diffraction and thermal behavior analysis were performed to study its phase purity and thermal stability. Optical diffuse reflectance spectra associated with Tauc's fitting indicates a direct bandgap semiconductor with an energy value of 2.34 ​eV, which consistent with the theoretical prediction. As a compare, the electronic bandgaps of three polymorphs were studied and shows a sequence of α -V 2 Se 2 O 9 < β -V 2 Se 2 O 9 <γ-V 2 Se 2 O 9. Density of states were also calculated to reveal the origin of the bandgaps and bond interactions. [Display omitted] γ-V 2 Se 2 O 9 was obtained by boric acid flux method in vanadate selenite system. Its crystal structure features novel 2D electroneutral [V 2 Se 2 O 9 ] layers composed of 1D [V 2 SeO 9 ]4- chains that are further bridged by Se (1)O 3 triangular pyramids. [V 2 SeO 9 ]4- chain consist corner-sharing of VO 6 and tridentate Se (2)O 3. UV–Vis diffuse reflectance spectroscopy coupled with Tauc's analysis indicates a direct semiconductor with a bandgap value of 2.34 ​eV, and its energy transition type was further supported by first-principles electronic structure calculation. • γ-V 2 Se 2 O 9 was obtained by boric acid flux method for the first time. • Unlike α - and β -V 2 Se 2 O 9 with isolated [V 4 O 18 ]16− tetramers, corner-sharing of VO 6 forms 1D [V 2 O 9 ]8- chains in γ-V 2 Se 2 O 9. • UV–Vis diffuse reflectance spectroscopy coupled with Tauc's fitting indicates γ-V 2 Se 2 O 9 as a direct bandgap of 2.34 ​eV. [ABSTRACT FROM AUTHOR]

Abstract: Borogermanates are a class of very important compounds in materials chemistry. In this paper, the syntheses, structures, and properties of metal borogermanates are reviewed. Organically templated borogermanates with zeolite-like open-frameworks show potential applications as microporous materials. Many compounds in alkali or alkaline-earth borogermanate systems are structurally acentric or polar, some of which exhibit excellent Second Harmonic Generation (SHG) coefficients, wide transparency regions, and high optical-damage thresholds as well as excellent thermal stability. Most of the lanthanide borogermanates are structurally centrosymmetric and not SHG active; however, they are able to emit strong luminescence in visible or near-IR region. In the B-rich compounds, BO3 and BO4 groups can be polymerized into a variety of discrete polynuclear anionic cluster units or extended architectures via B–O–B bridges; whereas in the Ge-rich compounds, GeO4 and GeO6 polyhedra can also be polymerized. The combinations of borate and germinate afforded rich structural and topological types. [Copyright &y& Elsevier]