This perspective describes the use of pyrochlore oxides in photocatalysis with focus on the strategies to enhance their activity.

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Previously the non-cubic β-pyrochlore polycrystalline compounds Rb 0.95 Nb x Mo 2-x O 6.475-0.5x (x = 1.31-1.663) have been obtained in the ternary system Rb 2 O-Nb 2 O 5 -2MoO 3 . However, using special synthesis condition leads to the formation of cubic Fd-3m β-pyrochlore Rb 0.9 Nb 1.625 Mo 0.375 O 5.62 with small size particles and surface area of 15 m 2 /g. The Rb 0.9 Nb 1.625 Mo 0.375 O 5.62 powder has been investigated by X-ray powder diffraction analysis and scanning electron microscopy with X-ray microanalysis. The crystal structure refinement has been performed using the Rietveld method. The electronic structure and optical band gap of Rb 0.9 Nb 1.625 Mo 0.375 O 5.62 have been studied in comparison to similar non-cubic composition Rb 0.95 Nb 1.625 Mo 0.375 O 5.66 . The photocatalytic properties of Rb 0.9 Nb 1.625 Mo 0.375 O 5.62 have been investigated on the example for dyes solution oxidation under UV irradiation. The dependence on the photocatalytic properties of dye type and catalyst mass has been studied. It has been observed that the main oxidative species for dye oxidation is hydroxyl radicals. The phase and composition stability of catalyst powder have been confirmed.

Solid solutions Rb 0.95 Nb x Mo 2−x O 6.475−0.5x (x = 1.31−1.625) having a β-pyrochlore structure with an orthorhombic system were synthesized by solid-state reaction. The elemental composition was confirmed by X-ray microanalysis. The Rb 0.95 Nb 1.375 Mo 0.625 O 5.79 structure refinement was performed using the Rietveld method. The crystal structure consists of ordered O−Mo−O chains partly occupied by Nb atoms. The oxygen vacancies are necessary to save the electroneutrality of the unit cell. It predominantly appears between Mo atoms that lead to form two disconnected defect octahedra [MoO 5 □•••MoO 5 □].The structural defects cause the low thermal stability; the compounds obtained decompose in the 748−758 °C temperature range. The high-temperature phase transition of the CsNbMoO 6 and CsTaMoO 6 nonlinear optical β-pyrochlores has been studied by differential thermal analysis, differential scanning calorimetric analysis, high-temperature X-ray diffraction analysis, and second harmonic generation analysis. At room temperature the compounds possess the cubic noncentrosymmetric F4̅ 3m cell. Under heating to 437 °C and 401 °C for CsNbMoO 6 and CsTaMoO 6 , respectively, they undergo transition into centrosymmetric Fd3̅ m modification. This is accompanied by the SHG signal disappearing, as well as the 402 reflection, which is characteristic of the F4̅ 3m space group. The positions of the valence and conduction bands were determined by reflectance spectra and XPS analysis for structure-related β-pyrochlores CsNbMoO 6 , CsTaMoO 6 , and Rb 0.95 Nb 1.375 Mo 0.625 O 5.79 .