Redetermination of Sr2PdO3 from single-crystal X-ray data

Sr2PdO3 adopts the Sr2CuO3 structure type. In comparison with previous determinations, the present redetermination results in improved precision of the structural parameters.


Chemical context
Low-dimensional transition-metal oxides with chain structures have received attention since they can enable interesting physical phenomena such as spin 1/2 antiferromagnetic Heisenberg coupling Takigawa et al., 1996), superconductivity (Hiroi et al., 1993), ultrafast nonlinear optical response (Ogasawara et al., 2000) or even glucose sensing (El-Ads et al., 2016). The particularly relevant sub-family based on square-planar MO 4 (M = divalent metal) primary building units is dominated by oxidocuprates(II), while the chemistry of respective palladates(II), showing the same preference for a square-planar coordination by oxygen, is much less explored.

Structural commentary
The crystal structure of Sr 2 PdO 3 is essentially the same as determined previously (Wasel-Nielen & Hoppe, 1970; Muller ISSN 2056-9890 & Roy, 1971Nagata et al., 2002). The lattice parameters (Table 1) are almost identical to those in the previous reports but with higher precision. The Pd II atom occupies the 2d crystallographic sites with mmm site symmetry. We would like to point out that we chose a different cell setting as compared to all the previous reports, where the Pd II atom was chosen to be located at the cell origin (site 2a; 0, 0, 0; hence the different site designations). The Pd II atom forms distorted PdO 4 square planes, which are linked by sharing oxygen atoms in the transposition to form infinite chains extending along the b-axis direction as shown in Fig. 1. Corresponding to this connectivity pattern, the Pd-O bond lengths are longer for the shared oxygen atoms, 2.052 (2) Å , and shorter for the terminal ones, 1.9911 (2) Å . The Sr atom is situated at the 4j Wyckoff site having mm2 site symmetry. It is seven-coordinate in a monocapped trigonal-prismatic fashion by oxygen with three different bond lengths (Table 1, Fig. 2). In addition to the square-planar first coordination of Pd II with oxygen, the second consists of eight Sr II atoms present at the corner of a cuboid with dimension 3.5342 (2) Â 3.7887 (2) Â 3.9822 (3) Å 3 (Fig. 2). Of the two kinds of oxygen atoms, both surrounded by six metal ions that form distorted octahedra, O1 is coordinated by one Pd II atom [2.052 (2) (Fig. 3). In our current structure determination, much more precise values of the cell parameters along with the z parameters of Sr and O1 have been determined, consequently, yielding very precise values for the bond lengths (see Table 1). The quality of the current refinement is also clearly reflected by better reliability factors (see Table 2) as compared to the previous refinements. The atomic arrangement described here is same as provided by Wasel-Nielen & Hoppe (1970).
The structural features discussed above are closely related to those of the K 2 NiF 4 type of structure, which is regarded as the prototype structure for all the high T c cuprates. K 2 NiF 4 consists of layers of corner-shared NiF 6 octahedra extending in the ab plane. One can derive the Sr 2 PdO 3 structure from the K 2 NiF 4 structure by systematically removing the bridging F atoms from the NiF 6 octahedra lying in the a-axis direction (Fig. 4). This would reduce the dimensionality of the layer, resulting in linear chains of square planes connected by edges along only one direction.

Synthesis and crystallization
Millimeter-sized block-shaped crystals of dark-yellow colour with composition Sr 2 PdO 3 as confirmed by SEM-EDS, were obtained from a mixture of different phases while attempting to synthesize SrPd 3 O 4 using a KOH flux (Smallwood et al., 2000). SrCO 3 and Pd metal powder were mixed in the molar ratio of 2:3, placed in an alumina crucible, and 15 grams of KOH pellets were added on top. The crucible was heated in a muffle furnace to 1023 K in 24 h with a 6 h dwell time. The

Figure 2
Coordination around the Sr II (left) and Pd II atoms (right). All atoms are drawn with displacement ellipsoids at the 80% probability level. Distances are in Å .

Figure 3
Coordination polyhedra of two types of oxygen atoms, O1 (left) and O2 (right). All atoms are drawn with displacement ellipsoids at the 80% probability level. Distances are in Å .

Figure 1
Crystal structure of Sr 2 PdO 3 viewed along the a axis (left) and along the b axis (right).
furnace was then cooled slowly to 873 K over 125 h after which it was switched off and allowed to cool naturally. The product was washed several times with water to remove the solidified flux and subsequently rinsed with ethanol.

Distrontium palladium trioxide
Crystal data Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.