RE-REFINEMENT OF COMPOSITE MODULATED NB2ZRX-2O2X-RADIATION DATA(1 (X=8) USING SYNCHROTRON)

The crystal structure of Nb2Zrx - 2O2x + 1, x = 8, has been re-refined as a composite modulated structure using synchrotron radiation data m easured at energies below the Zr K absorption edge. The structure comp rises one-atom thick layers of two substructures stacked alternately a long 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 oxyg en 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 r eciprocal space, based on the more strongly scattering metal substruct ure, 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 supers pace-group symmetry is Amma(alpha 10)0s0. Two sets of intensity data r ecorded at room temperature, 8 eV (lambda(n) = 0.6892 Angstrom) and 14 50 eV (lambda(f) = 0.7492 Angstrom) below the Zr K absorption edge, we re refined to an overall R(lambda(n)) = 0.0377 and R(lambda(f)) = 0.04 39. The displacive modulation wave amplitudes obtained from these refi nements agree well with those previously obtained for an x = 12 member . Difference Fourier maps show that metal-atom ordering is virtually n egligible.