Synthesis and structural characterization of Ba₁₄Pd₃Ir₈O₃₃

Abstract

Ba₁₄Pd₃Ir₈O₃₃ has been prepared in polycrystalline form and characterized by X-ray, neutron and electron diffraction, lattice imaging and magnetometry. The structure can be described as the n=5,m=9 member of the (A₃O₉)_n(A₃A′O₆)_m series which derives from the n=∞, m=0 2H perovskite structure. Ir and Pd cations occupy chains of octahedral and trigonal-prismatic sites in a disordered manner. An alternative description of the structure in (3+1)-dimensional superspace is also presented. No magnetic phase transitions were observed above 5 K.

Introduction

Compounds with general formula A_wA_x′B_yO_z with structures related to Sr₄PtO₆ (A=A′=Sr) [1] have received an increasing amount of attention in recent years [2], [3], [4], [5] because of the expectation that they will exhibit novel, one-dimensional electronic properties. The possibility of such a behavior arises because the A′ and B cations are located in interstices which form chains along the [001] direction of the trigonal unit cell. These closely packed interstices lie within face-sharing O₆ polyhedra, each chain consisting of an ordered sequence of prismatic and octahedral sites; in the ideal case A′ and B cations occupy the former and latter, respectively. The relatively large A cations are usually considered to occupy a second sub-lattice in the inter-chain space. Recognition of the structural similarity between these phases and the 2H perovskite structure enabled Darriet and Subramanian [6] to describe these structures as deriving from the hexagonal stacking of pseudo close-packed layers of stoichiometry A₃O₉ and A₃A′O₆, and these ideas were subsequently extended by Blake et al. [7]. Many members of this group of compounds have been shown to adopt incommensurate crystal structures and it has now been recognized [4], [7] that these materials can be described as modulated composites consisting of two interacting trigonal subsystems with a common ab-plane, but with different repeat distances along z; the (A′, B)O₃ polyhedral chains constitute one subsystem, and the A cations the other. It is thus possible to analyze the structures of the materials in a (3+1)-dimensional (D) formalism. We have previously synthesized the phase Ba₁₄Cu₃Ir₈O₃₃ [7], and in this paper we describe the preparation and characterization of Ba₁₄Pd₃Ir₈O₃₃. The substitution of Pd²⁺ for Cu²⁺ was carried out in order to investigate the consequences for the electronic properties of having cations with relatively extensive 4d or 5d valence orbitals occupying all the sites in the polyhedral chains. We apply both the conventional supercell model and the (3+1)-D model to our powder diffraction data and show that the superspace formalism does indeed provide an appropriate structural description of this compound. We also describe a study of this compound by electron microscopy.