Solid Solutions Rb0.95NbxMo2–xO6.475–0.5x (x = 1.31–1.625) with Orthorhombic β-Pyrochlore Structure: Thermal Behavior and Electronic Structure of β-Pyrochlores Compounds Based on [Nb(Ta)/Mo] Octahedral Framework

Solid solutions Rb0.95NbxMo2-xO6.475-0.5x (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 Rb0.95Nb1.375Mo0.625O5.79 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 [MoO5□···MoO5□]. 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 CsNbMoO6 and CsTaMoO6 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 F43m cell. Under heating to 437 °C and 401 °C for CsNbMoO6 and CsTaMoO6, respectively, they undergo transition into centrosymmetric Fd3m modification. This is accompanied by the SHG signal disappearing, as well as the 402 reflection, which is characteristic of the F43m space group. The positions of the valence and conduction bands were determined by reflectance spectra and XPS analysis for structure-related β-pyrochlores CsNbMoO6, CsTaMoO6, and Rb0.95Nb1.375Mo0.625O5.79.