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

摘要

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, a(M) = 5.1348 (2), b(M) = 4.9590 (2), c(M) = 5.2908 (3) Angstrom, and the oxygen substructure (0), space group Imam, a(O) = 2.4164, b(O) = b(M) = b, c(O) = c(M) = c, respectively. The four basis vectors used to index reciprocal space, based on the more strongly scattering metal substructure, are a(M)*, b*, c* and q(M). The primary modulation wavevector q(M) = a(O)* - 2a(M)* + b* = 1/8 a(M)* + b* for x = 8. The overall superspace-group symmetry is Amma(alpha 10)0s0. Two sets of intensity data recorded at room temperature, 8 eV (lambda(n) = 0.6892 Angstrom) and 1450 eV (lambda(f) = 0.7492 Angstrom) 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.