To whom correspondence should be addressed. Journal of Solid State Chemistry 156, 428 } 436 (2001) doi:10.1006/jssc.2000.9018, available online at http://www.idealibrary.com on The Fine Structure of YCuO 2x Delafossite Determined by Synchrotron Powder Diffraction and Electron Microscopy G. Van Tendeloo,* O. Garlea,- C. Darie,- C. Bougerol-Chaillout,- and P. Bordet- *EMAT, University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium; and - CNRS, Laboratoire de Cristallographie, BP 166X, 38042 Grenoble Cedex 9, France Received July 12, 2000; in revised form October 16, 2000; accepted October 27, 2000 YCuO 2 delafossite crystallizes into two stacking variants; hexagonal 2H or rhombohedral 3R, depending on the prepara- tion conditions. The structure of the fully oxygenated material YCuO 2.50 has been determined as orthorhombic (a o  6.1961 A s ; b o  11.2158 A s ; c o  7.1505 A s ; space group Pnma). The struc- ture is based on the hexagonal 2H structure (a o  a H 3; b o  c H ; c o  2a H ). Upon incomplete oxidation, a di4erent YCuO Z phase with ideal composition YCuO 2.33 and lattice para- meters a H 3, a H 3, c H is also formed. Di4raction patterns are often very complex because of the presence of planar defects and intergrowth of both phases. Under electron beam irradiation, oxygen is released from the structure and one phase gradually transforms into the other.  2001 Academic Press Key Words: delafossite; X-ray di4raction; electron micros- copy; oxygen ordering. 1. INTRODUCTION Delafossites of the type ¸nCuO  (¸n"Y or La) have been studied in fair detail by Cava et al. (1). The basic structural formula is A B O  , where the A  is always Cu  in the present case. The structure can be described as a succession of ¸nO  layers, with the ¸n in octahedral coordination and where neighboring octahedra share edges, so as to form a hexagonal arrangement of ¸n. Successive ¸nO  layers are connected by a Cu plane, where the Cu is in a linear stick coordination with the oxygens above and below (see Figs. 1 and 2 in (1)). These double-layer (¸nO  }Cu) sandwiches with a thickness of 5.6 A s can be stacked in di!erent ways, creating a number of polytypes (2, 3). LaCuO  for example shows a 3R stacking (2) while YCuO  has a 2H stacking (3). High-resolution electron microscopy (HREM) observations of Cava et al. (1) show a large variety of stacking variants for YCuO  , which ap- parently points toward a low stacking fault energy and a small stacking energy di!erence between the di!erent phases. Delafossites of the type LaCuO  and YCuO  are also known to easily intercalate extra oxygen in the Cu planes, leading to a structural formula of the type ¸nCuO  (x being of the order of 0.5). These oxygens tend to adopt an ordered arrangement within the CuO  plane and give rise to di!erent superstructures (see (1)). This oxygen ordering af- fects the symmetry within the plane and lowers the space- group symmetry; two of these superstructures have been identi"ed by Cava et al. (1), one being orthorhombic with lattice parameters 2a, a3, c, and the other being hexagonal but with lattice parameters a3, a3, c. TEM revealed the presence of several planar defects and phases with a di!erent superstructure. These e!ects hampered a direct interpreta- tion of the di!raction patterns and no detailed structure could be proposed. The idea of this contribution is to study the di!erent YCuO  structure(s) in more detail by X-ray di!raction, electron di!raction, and HREM and to relate the di!erent oxygenated materials. A basic requirement, however, is to be able to prepare homogeneous, single-phase samples. 2. EXPERIMENTAL A polycrystalline sample of YCuO  delafossite was pre- pared by solid-state reaction technique. The di$culty of this synthesis consists in isolating the 3R and 2H polytypes of YCuO  . These polytypes are very often intergrown prob- ably due to a very small di!erence in the thermodynamical stability region of these two phases. As a common "rst step, the Y  Cu  O  precursor was synthesized from reagent grade Y  O  and CuO. The start- ing materials were mixed by grinding in a mechanical mor- tar for 1 h and the powder mixture was "red in air at 10503C for 1 day. To obtain high-purity powder of the 2H phase, the calci- nated Y  Cu  O  powder was pressed into pellets and buried in powder of the same composition. This product put in 498 0022-4596/01 $35.00 Copyright  2001 by Academic Press All rights of reproduction in any form reserved.