Re-Refinement of Composite Modulated Nb2Zr x−2O2x+1 (x = 8) Using Synchrotron Radiation Data

Abstract

The crystal structure of Nb2Zr_{x-2}O_{2x + 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) Å, 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 a^{\ast}_{M}, b*, c* and q M . The primary modulation wavevector q M = ^{\ast}_{M} − 2a^{\ast}_{M} + b* - 1/8 a^{\ast}_{M} + b* for x = 8. The overall superspace-group symmetry is Amma(\alpha10)0s0. Two sets of intensity data recorded at room temperature, 8 eV (\lambda_n = 0.6892 Å) and 1450 eV (\lambda_f = 0.7492 Å) below the Zr K absorption edge, were refined to an overall R(\lambda_n) = 0.0377 and R(\lambda_f) = 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.