Re-refinement of composite modulated Nb2Zrx-2O2x+1 (x=8) using synchrotron radiation data

The crystal structure of Nb2Zrx-2O2x+1, x = 8, has been re-refined as a composite modulated structure using synchrotron radiation data measured at energies below the Zr K absorption edge. The structure comprises one-atom thick layers of two substructures stacked alternately along the b direction. The two component substructures are, in general, mutually incommensurable along their common a-axis directions and are referred to as the metal substructure (M), space group Amma, aM = 5.1348 (2), bM = 4.9590 (2), cM = 5.2908 (3) A, and the oxygen substructure (O), space group Imam, aO = 2.4164, bO = bM = b, cO = cM = c, respectively. The four basis vectors used to index reciprocal space, based on the more strongly scattering metal substructure, are aM*, b*, c* and qM. The primary modulation wavevector qM = aO* - 2aM* + b* = 1/8aM* + b* for x = 8. The overall superspace-group symmetry is Amma(α10)0s0. Two sets of intensity data recorded at room temperature, 8 eV (λn 0.6892 A) and 1450 eV (λf = 0.7492 A) below the Zr K absorption edge, were refined to an overall R(λn) = 0.0377 and R(λf) = 0.0439. The displacive modulation wave amplitudes obtained from these refinements agree well with those previously obtained for an x = 12 member. Difference Fourier maps show that metal-atom ordering is virtually negligible. (orig.)