Pergamon zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Solid State Communications, Vol. 101, No. 11, 835-839, 1997 pp. 0 1997 Published by Elsevier Science Ltd Printed in Great Britain. All rights reserved 0038-1098/97 $17.00+.00 PII: S&038-1098(96)00682-S RANDOM STACKING OF A COMMENSURATE GUEST LAYER IN AN ORDERED HOST: Ni/Al LAYER-DOUBLE-HYDROXIDES D.R. Hines,” G.T. Seidler,’ M.M.J. Treaty” and S.A. Solina’b “NEC Research Institute, 4 Independence Way, Princeton, New Jersey 08540, U.S.A. ‘Blackett Laboratory, Imperial College of Science, Technology and Medicine, London SW7 2BZ, U.K. zyxwvutsrqponm (Received 2 October 1996; accepted 28 October 1996 by A.L. Efros) In the solid solution layer double hydroxides with the general composi- tion [Ni,_&(OH),] [(C03)fl*yH,0], 0 I x 5 0.4 the Ni2+/A13+ ions decorate a positively charged triangular lattice that is sandwiched between two hydroxyl layers to form brucite-like host layers. Guest layers of co-intercalated water molecules and carbonate anions oriented with their three-fold axes perpendicular to the basal plane occupy the galleries. For x = 0.30, the carbonate ions form a (a X J13)R13.9” in-plane super- lattice structure relative to the host layers which themselves exhibit an ABC stacking sequence that is only slightly (20%) disordered. Despite their commensurate structure, the guest layers yield Warren line shape X-ray reflections characteristic of a random stacking arrangement. This unusual result is attributed to the high (26-fold) degeneracy of the ground state structure of the guest layer. 0 1997 Published by Elsevier Science Ltd. All rights reserved Keywords: A. insulators, C. crystal structure and symmetry, X-ray scattering, D. order-disorder effects. The layer double hydroxides (LDHs) are members of a large class of minerals and synthetic compounds whose general chemical formula can be written in the form [RT~~R~f(OH)2]‘X[R~~.yHzO] where R2+ = Mg, Fe, Ni, . . . . R3+ = Al Fe Cr and R”- = [C0312-, zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGF W,12-, zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA W,l’- , .: . . il] Prototypical of these materials are the NilAl compounds with the general formula [Ni1-~Al,(OH)2][(C03)~~2~yH20], 0 Ix I 0.4. These intercalated layered solids not only exhibit a rich variety of structural and physical phenomena [2-41 but are also of major interest for battery applications [5]. With a focus on the former we have undertaken studies of the composition dependence of the physical properties of the NilAl LDHs [4] and have discovered a very unusual if not unique structural arrangement of the carbonate guest species. For the comparison n = 0.30, despite the highly ordered stacking of the host layers, the carbonate ions form a commensurate in-plane structure relative to the host layers but a laterally random structure relative to each other. The composition dependence of the host structure of the Ni/Al LDHs has been reported elsewhere [4]. Here we provide a full analysis and physical examination for the unusual intercalate stacking exhib- ited by the carbonate and water guest layers in [Ni1_&&(OH)2][(C03),2-yH20], x = 0.3. We also address the close interplay between the carbonate and water content and the attendant structural. The Ni/Al LDHs are derived from Ni(OH)2 which is composed of layers of edge shared Ni(OH)6 octahedra formed from a triangular lattice of Ni2+ ions sandwiched between a pair of identical triangular lattices of hydroxyl ions shifted laterally above and below the metal planes as shown graphically in the left inset of Fig. 1. The hydroxyl bonds point away from the Ni layer and extend into the gallery spaces on either side of the host layer [l]. The CO:- ions in the gallery space provide charge neutrality when the divalent Ni2+ by trivalent A13+. ions in Ni(OH)2 are replaced The host layers of the mixed-metal LDHs can adopt either a hexagonal 2H stacking arrange- ment with two layers per cell or a trigonal 3R stacking arrangement with three layers per cell [l]. The [Ni,_,Al,(OH)2][(C03),12-yH20] LDHs with 835