Solid Solutions Rb_0.95Nb_xMo_2–xO_6.475–0.5x (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

Abstract

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 .

The Photocatalytic Oxidation Ability of Rb0.9Nb1.625Mo0.375O5.62 with Classic β‐Pyrochlore Structure

“…Since the ratio of atoms with oxidation states 5+ and 6+ differs from 1 : 1 (or 8 : 8 per unit cell), the partial loss of oxygen atoms is necessary to maintain the electroneutrality of the unit cell. As was shown for the orthorhombic phase Rb 0.95 Nb 1.375 Mo 0.625 O 5.79 , [4a] it is most likely that the oxygen deficiency is realized through the formation of defective [MoO 5 ] octahedra.…”

“…Previously, in the ternary system Rb 2 O‐2MoO 3 ‐Nb 2 O 5 we have obtained the Rb 0.95 Nb x Mo 2‐x O 6.475‐0.5x (x=1.31–1.663) solid solutions with an atypical orthorhombic ( Pnma ) β‐pyrochlore structure [4a] . The details of crystal and electronic structure have been investigated for a powder consisting of ∼20 μm single crystals with the composition Rb 0.95 Nb 1.375 Mo 0.625 O 5.79 .…”

“…Previously, we have obtained solid solutions with the structure of β‐pyrochlore Rb 0.95 Nb x Mo 2‐x O 6.475‐0.5x (x=1.31–1.663), which possess an orthorhombic symmetry ‐ atypical for this structural type, [4a] since the classical unit cell is described in terms of cubic symmetry. The typical synthetic conditions of polycrystalline powders is solid state synthesis using stoichiometric reagents mixture; however, it is possible to change the size particles and morphology varying method of synthesis.…”

Structural Diversity of Ordered Pyrochlores

“…Our calculated structural formula for β-pyrochlores is consistent with neutron and X-ray structural refinement of CsTe 2 O 5.75 , considering that A 4c = Cs; B 4c = Te(1); B 4a = Te(2) and Te(3); X 4c = O(1) and O(2); X 8d = O(3) and O(4). Another similar example of orthorhombic β-pyrochlores is solid solutions Rb 0.95 Nb x Mo 2– x O 6.475–0.5 x ( x = 1.310–1.625) . Orthorhombically distorted β-pyrochlore phases were also found in different fluorides (see for example refs and ).…”

“…Another similar example of orthorhombic β-pyrochlores is solid solutions Rb 0.95 Nb x Mo 2−x O 6.475−0.5x (x = 1.310−1.625). 134 Orthorhombically distorted β-pyrochlore phases were also found in different fluorides (see for example refs 135 and 136).…”