Modal disorder in rubidium tungstoniobate RbNbWO6 confirmed by EXAFS

Abstract

Two forms of RbNbWO6 crystal stable at room temperature were found based on single crystal syntheses: a cubic form (Fd3¯m space group) with the modal disorder, and a tetragonal form (I4¯2d space group). The nature of the disordered cubic crystal, in which the modes break the symmetry of the Fd3¯m group causing large, specifically oriented and correlated atomic shifts of Nb/W atoms and small shifts of O atoms, was described based on the analysis of diffuse scattering in our previous paper (J. Solid State Chem. 230 (2015) 325). Now, we present a local analysis of atomic displacements performed using EXAFS method. Tungsten L3-edge EXAFS spectra measured for the modal cubic and for the tetragonal sample show strong similarities which indicate on the common nature of the atomic displacements. EXAFS fits performed on the modal cubic sample reveal at least three, but presumably more, different W–O distances in deformed WO6 octahedra and confirm our previous qualitative X-ray diffraction results.