Single Crystal Refinement of the Incommensurately Modulated Mn0.55Ta0.45O1.7, an Oxygen Deficient Fluorite Type Compound

Abstract

A structural investigation of the incommensurately ordered compound Mn0.55Ta0.45O1.7 has been carried out, using single-crystal X-ray diffraction data. The basic structure is related to the fluorite type with a monoclinic distortion. Electron diffraction (ED) patterns for various crystal orientations were used for determination of unit cell, incommensurate modulation vector and superspace group. The unit cell parameters and the length and direction of the incommensurate vector were refined using X-ray powder diffraction (XRPD) data. P2/m was chosen as the three-dimensional space group for a subcell with parameters a = 3.5005(2) A (√/2/2 · af), b = 3.5730(2) (√2/2/ · af), c = 5.0015(2) A (af) and β = 91.677(7)°. From the systematic absences hklm: k + m ≠ 2n, the four-dimensional superspace group was determined to be B:P2/m(α0γ) with determined modulation vector components α = -0.1833(1) and γ = 0.3582(2). A total of 511 unique reflections (79 basic fluorite type, 268 first-order and 164 second-order satellites) were used in the structure refinement, which resulted in weighted R-values of 5.6% for the fluorite type sublattice reflections, 5.5% for the first-order satellites and 7.4% for the second-order satellites. The Mn and Ta atoms were found to be both positionally and occupationally modulated and the oxygen atoms to be highly disordered. The structure is the first fluorite-related modulated structure of an oxide that has been determined from single-crystal data. The structural relation to the previously characterised disordered cubic phase Mn0.6Ta0.4O1.65 and the diffuse scattering exhibited by it are discussed in the article. (Less)