REDETERMINATION OF THE STRUCTURE OF GD2CUO4-A SITE POPULATION ANALYSIS

In view of recent interest in compounds of the type Ln2CuO 4 (where Ln = lanthanide), a single-crystal X-ray study has been performed for the redetermination of the structure of Gd2CuO 4 including a site population analysis. M r = 442.04, tetragonal, I4/mmm, * Group INC-4, MS C346. I" Group P10, MS K764. 0108-2701/88/091518-03503.00 a = 3.892 (1), c = 11.878 (3) A, V= 179.91 A 3, Z = 2, D x=8 .15gcm -a, MoKa~, 2 = 0 . 7 0 9 2 6 A , # = 423.5 cm -~, F(000) = 378, T = 295 K, R = 2.8%, 102 unique reflections used for refinement. The structure consists of a two-dimensional edge-linked square-planar network of [CuO2] 2groups which are linked by planes of Gd, O and Gd atoms. The oxygen coordination environment around the Gd 3+ cations is cubic. The © 1988 International Union of Crystallography KIMBERLY A. KUBAT-MARTIN, ZACHARY FISK AND ROBERT R. RYAN 1519 temperature factor for the O atom in the [CuO212plane is much larger than those of the other atoms in the system. A least-squares refinement of the population parameters indicates that all sites in Gd2CuO a are

The water-molecule geometry is typical for this species in hydrated inorganic compounds; it is held in the y = 0.25 plane by a weak hydrogen bond between D2 and 03. The presence of this structural feature fits with the strong absorption at 1620cm -t in the IR spectrum which is due to the H20 symmetric bend.
The coordination geometry of Mo is similar to that observed in molecular Mo[(CH3)2NCHO]202CI 2 (Flo-Abstract. In view of recent interest in compounds of the type Ln2CuO 4 (where Ln = lanthanide), a single-crystal X-ray study has been performed for the redeter- lntroduetlon. New interest in compounds of the type LnzCuO 4 (Ln = lanthanide) has been generated by the discovery of high-temperature superconductivity in doped La2CuO 4 (Bednorz & Miiller, 1986). The superconducting properties of this system generally have been considered to be dependent on the oxygen stoichiometry of the material and/or a variability of valencies for the Cu atoms (Alp et al., 1987).
In view of the recent attention focused on Ln2CuO 4 materials, we have carried out a redetermination of the structure of Gd2CuO 4. Although a single-crystal structural determination has been reported for GdzCuO 4 (Grande, MiiUer-Buschbaum & Schweizer, 1977), the published data indicate only that this system does not possess the KzNiFa-type structure of LazCuO 4 (Grande, M/Jller-Buschbaum & Schweizer, 1977), but instead is isostructural with NdzCuO 4 (Miiller- Buschbaum & Wollschl~iger, 1975). In particular, these data do not contain information on anisotropic thermal parameters nor are statistical uncertainties reported for lattice or positional parameters. As disorder and possible site vacancies have been suggested as contributing to the superconductor phenomena in the related La2CuO 4 system, we have also carried out a site population analysis for GdzCuO 4.
Experimental. Synthesis. Gd2CuO 4 single crystals were grown from a PbO-based flux. A mixture of the oxides in the proportions 0-09 GdO3/2:0.37 PbO: 0.54 CuO was heated in a Pt crucible to 1520 K in air, held 2 h and cooled to 1070 K at 7 K h -~, then removed from the furnace. The solidified melt was tapped from the crucible and the cuprate crystals were separated from the CuO-PbO flux using very dilute acetic acid.

Structure. A dark parallelepiped-shaped crystal
(60 x 80 x 801am) was cleaved from a larger sample. Unit-cell parameters were derived from a least-squares analysis of 25 reflections (MoKa~ radiation, 2= 0.70926 A; range, 6 < 0 < 20 °) automatically centered on an Enraf-Nonius CAD-4 X-ray diffractometer. Data for four reciprocal-lattice octants (-5 _< h < 5, -5 _< k < 5, and 0 _< l <_ 16) were collected at room temperature over a 20 scan range of 0.0 to 60.0 ° (sin 0max/2 = 0.7049 A -~) using a variable-speed 0/20 scan mode and graphite-monochromatized MoKa radiation. The intensities and orientations of two standard reflections (006 and ii4) were monitored every 2h of X-ray exposure time and every 200 reflections, respectively. The standard reflections showed no significant intensity fluctuations; reorientation was not required. The intensity data were cor-    (5)  rected for Lorentz and polarization factors. Averaged azimuthal scan intensities for a reflection near Z = 90° and its Friedel pair measured at I0 ° increments about ~, showed a variation of Imin/lma x = 0.58. An absorption correction based on these data, multiplied by a spherical correction (0.04 mm), was applied to the intensities. Merging the 616 measured reflections (Rin t = 0.029) gave rise to 102 independent reflections [I _> 2cr(1)l which were used in the structural refinement.
Although normalized structure-factor distribution statistics favored a non-centrosymmetric structure, the structure was successfully refined in the space group I4/mmm (No. 139). Positions for the Gd and Cu atoms were based on the previous structure (Miiller-Buschbaum & Wollschl/iger, 1975). The O atoms were located using difference Fourier methods; all atoms were refined via standard least-squares techniques.
The scale factor, a secondary-extinction parameter (Zachariasen, 1967;Larson, 1967), atom coordinates and anisotropic temperature factors were ultimately refined. Neutral-atom scattering factors and appropriate anomalous-scattering terms were used (Cromer & Waber, 1974;Cromer, 1974). The final R I and wR I values are 2.8% and 3.7% with a goodness-of-fit parameter of 2.17 for the 102 reflections and 12 parameters. The ratio of the maximum least-squares shift to e.s.d, in the final refinement cycle is 5 x 10 -5. All calculations were performed on a CDC 7600 computer using an in-house package of programs. Atomic coordinates and anisotropic thermal parameters for GdzCuO 4 are listed in Tables 1 and 2.* * Lists of structure factors have been deposited with the British Library Document Supply Centre as Supplementary Publication No. SUP 44988 (2 p.). Copies may be obtained through The Executive Secretary, International Union of Crystallography, 5 Abbey Square, Chester CH 1 2HU, England.

Discussion.
As previously noted (Grande, MiJller-Buschbaum & Schweizer, 1977), the structure of Gd2CuO 4 is not the distorted K2NiF 4 type of La 2-CuO 4, but is of the type seen in Nd2CuO 4 (Miiller-Buschbaum & Wollschl~iger, 1975). An expanded view of Gd2CuO 4 is depicted in Fig. 1. The Cu atoms, rather than being octahedrally coordinated by O atoms as in La2CuO 4, exhibit a square-planar coordination. The framework of the Gd2CuO 4 system consists of a two-dimensional edge-linked square-planar network of [CuO2] 2-groups which are linked by planes of Gd, O and Gd atoms. The Cu-O(2) distance in the squareplanar array is 1.946 (1) A; the Cu-O(1) and Cu-Gd distances are 3.550 (1) and 3.285 (1)A, respectively. The Gd a+ cation sites are eight coordinate rather than the nine-coordinate environment found for the larger La 3+ cation in La2CuO4 (Grande, Miiller-Buschbaum & Schweizer, 1977). The O environment around each Gd a+ is nearly cubic with a Gd-O(1) distance of 2.275 (1)A and a Gd-O(2) distance of 2.646 (1)A.
Possible lattice site vacancies for the atoms in Gd2CuO4 were probed by a least-squares refinement of site occupation factors for the Gd and the two O atoms. The site occupancy for the Cu atom was assumed to be 1.0 and was not refined. Anisotropic thermal parameters for all atoms and the z fractional coordinate for the Gd were also refined. Convergence of the last least-squares refinement cycle resulted in a site occupancy of 0.99 (2) for the Gd, 1.04 (5) for O(1), and 0.98(6) for O(2), indicating that the compound Gd2CuO 4 is stoichiometric. The U~ thermal parameter for the 0(2) atom in the Cu-O square-planar array is much larger than the anisotropic thermal parameters associated with the other atoms in the structure. The direction of thermal motion for this atom is in the Cu-O layer. The anisotropy of this particular O atom can be contrasted with that seen for the analogous O atom in the Cu-O layer in tetragonal LaLssSro.~sCuO4 (Wang et al., 1987). In LaLssSro.~sCuO 4, the thermal motion for the corresponding O atom is perpendicular to the Cu-O plane. The structure of LaLssSr0.~sCuO 4 has also been determined at 300, 60 and 10 K by neutron diffraction powder profile analysis (Cava, Santoro, Johnson & Rhodes, 1987). The structure is tetragonal at 300 K, but at ca 200 K undergoes a tetragonal-to-orthorhombic distortion. In La2CuO4, the orthorhombic distortion involves a puckering of the Cu-O planes through this O atom in the direction of the anisotropy, which Birgeneau et al. (1987) describe in terms of a low-lying optical mode which exhibits classical soft-mode behavior at the X point (in 14/mmm) at T c = 425 K. Our results suggest that, if second-order lattice instabilities are discovered in the Nd2CuO 4 structural type, they will be of an entirely different nature than in LaL85-Sr0.15CuO4.