Structure and superstructure of BiNdSr2O6 and BiLuSr2O6 by X-ray powder diffraction and electron microscopy

The crystal structures of BiNdSr2O6 and BiLuSr2O6 have been investigated us ing both X-ray powder diffraction method and electron microscopy: selected area electron diffraction and high resolution transmission electron microsc opy connected with crystallographic image processing and multislice image s imulation technique. Both crystal structures have been refined from the X-r ay data by the Rietveld method in P12(1)/n1 space group giving atomic arran gements known earlier from the single crystal data for BiNdSr2O6, and the f ollowing lattice parameters: a=5.9433(2) Angstrom, b=6.0976(3) Angstrom, c= 8.4966(3) Angstrom, beta=90.207(3)degrees, and a=5.8564(2) Angstrom, b=5.92 45(2) Angstrom, c=8.3151(4) Angstrom, beta=90.094(4)degrees; for BiNdSr2O6 and BiLuSr2O6, respectively. Electron microscopy studies confirmed these re sults, but additionally for BiNdSr2O6 allowed to find the superstructure wi th the following superstructure unit cell: a(s)=6.097(2) Angstrom, b(s)=14. 609(3) Angstrom, c(s)=10.368(2) Angstrom, alpha=90 degrees, beta=90 degrees , gamma=90.26(2)degrees; and with a unit cell volume three times larger tha n the basic one. (C) 1999 Elsevier Science S.A. All rights reserved.