The pentavalent bismuthate BaBi 2 O 6 was prepared by a lowtemperature hydrothermal reaction using NaBiO 3 .nH 2 O and Ba (OH) 2 .8H 2 O as starting materials. The X-ray powder diffraction pattern was indexed with a hexagonal cell (space group: P-31 m), the same as a PbSb 2 O 6 -type crystal. The crystal structure of this compound was refined using synchrotron X-ray powder diffraction data. The cell parameters were found to be a = 5.57534(6) and c = 5.7381(1) Å and the final reliability (R) factors obtained by Rietveld analysis were R wp = 5.15 % and R p = 3.62 %. This compound exhibited profound photocatalytic activity for phenol decomposition under visible light irradiation.

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The crystal structure of NaBiO· nHO was refined using synchrotron powder X-ray diffraction and was assigned to a trigonal unit cell (space group P3̅) consisting of layered structures formed by edge-sharing BiO octahedra and consisting of an interlayer composed of water molecules sandwiched between two layers of sodium atoms, perpendicular to the c axis. An intermediate phase was observed during the dehydration of the hydrated compound. Density of state calculations showed hybridization of the Bi 6s and O 2p orbitals at the bottom of the conduction bands for both the hydrated and the dehydrated phases, which narrows the band gap and promotes their photocatalytic activity in the visible region.

The electronic band gap shrinks from the UV to visible region of cubic halide KCaCl3 perovskite under pressure, making it easier to move electrons from the VB to the CB, which improves optoelectronic device efficiency.

The current study diligently analyzes the physical characteristics of halide perovskites AGeF3 (A = K, Rb) under hydrostatic pressure using density functional theory. The goal of this research is to reduce the electronic band gap of AGeF3 (A = K, Rb) under pressure in order to improve the optical characteristics and assess the compounds’ suitability for optoelectronic applications. The structural parameters exhibit a high degree of precision, which correlates well with previously published work. In addition, the bond length and lattice parameters decrease significantly leading to a stronger interaction between atoms. The bonding between K(Rb)–F and Ge–F reveal ionic and covalent nature, respectively, and the bonds become stronger under pressure. The application of hydrostatic pressure demonstrates remarkable changes in the optical absorption and conductivity. The band gap becomes lower with the increment of pressure, resulting in better conductivity. The optical functions also predict that the studied materials might be used in a variety of optoelectronic devices operating in the visible and ultraviolet spectrum. Interestingly, the compounds become more suitable to be used in optoelectronic applications under pressure. Moreover, the external pressure has profound dominance on the mechanical behavior of the titled perovskites, which make them more ductile and anisotropic.

The pyrochlore-type Ca 2 Bi 2 O 7 and Sr 2 Bi 2 O 7 have been synthesized from a low-temperature hydrothermal route using NaBiO 3 •nH 2 O as a starting material. The crystal structures of these compounds were refined using synchrotron powder X-ray diffraction data. The cell parameters were found to be a = 10.75021 (5) Å and 10.94132 (6) Å for Ca 2 Bi 2 O 7 and Sr 2 Bi 2 O 7 , respectively. Density functional theory calculations showed the metallic band structure, but the negligible mixing of O2 2p bands with the A-site alkaline-earth-metal states and weak overlap with the conduction bands result in the semiconducting behavior.

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