Double perovskites ordering

In perovskites of the type AA BB Oe, ordering of the B-site ions may occur if the differences between ionic size and formal charge is sufficiently large. The crystal chemistry of these complex perovskites has been reviewed recently. More than 300 materials can be identified as double perovskites so only a few compounds will be described here. Attention... 

Figure 33 Unit cell of the B-site ordered double perovskite, A2B B"06.(Fm3m). (a) The light grey octahedral represent the B" sublattice, the dark grey octahedra the B sublattice, and the white spheres the A2 sublattice, (b) The fee sublattice for either the B or B" sublattice...

In the last decade, study of double perovskites has accelerated with the discovery of tunneling magnetoresistive (TMR) effects in Sr2FeMo06.i56N. Both the magnetism and transport in these oxides is strongly influenced by the cationic nature and the order/disorder of the ions at the B-site. Ferromagnetism and half-metallic properties are often... 

The observation of room temperature CMR in the ordered double perovskite Sr2peMo06 has sparked renewed interest in this class of compounds. Similar effects are seen when Fe is replaced by Fe, Co or Mn, or Mo partially replaced by W. i ki79... 

Moreover, the environmental systems demonstrate unique diversity and versatility of the processes depending on the actual conditions, viz. the reaction directions and rates are sensitive to many diverse parameters, sometimes even difficult to be perceived. The example may be the dependence of the photocatalytic activity of Sr—A1—Nb—0 double perovskite on the cation ordering in the oxides (267), or the effect of the in-plane twist of the quinoline-based co-ligand on the thermal stability and yield of NO photorelease from the [Ru(NO)] nitrosyl complexes (96). 

R642 K. Asai, Studies of Ferromagnetic Oxides with Ordered Double Perovskite Structure by Nuclear Probes , KURRI-KR, 2000, (KURRI-KR-49, Proceedings of the Specialist Research Meeting on New Developments in Solid State Physics with Probes of Radiations and Nuclei, 1999), 229... 

I. Zvereva et al., Complex aluminates RE2SrA1207 (RE = La, Nd, Sm-Ho) Cation ordering and stability of the double perovskite slab-rocksalt layer P-2/RS intergrowth. Solid State Sci. 5(2), 343-349 (2003). 

According to Campbell (1992), the double perovskite, LaCaMnCoOf, was of interest as a cyclic mode methane coupling catalyst due to its structure and redox properties. This mixed oxide has an ordered perovskite structure showing multiple occupations of both A (La, Ca) and B (Mn, Co) sublattices. The crystalline material presented some ordered domains, while in other areas the cations were distributed at random. For the ordered domains, the most probable structural model was an AB03 perovskite-type structure in which Mn4 and Co3+ ions occupy B positions in adjacent AB03 units while La3+ and Ca2+ ions alternate in A positions. Two ions of this structure can be reduced ... 

Figure 2.1 The octahedral framework of the rock-salt ordered double perovskite structure. The A-site cations are omitted for clarity...

The symmetry of ordered rock-salt A2(BB X) double perovskites, taking into account octahedral tilting, has been systematically derived via group theoretical analysis. If the stmcture is cubic, with a tilt system aVa , the space group is Ftriitn (225), the new axes run parallel to the simple cubic perovskite axes, and the lattice parameter is double the simple cubic lattice parameter a. Thus, in the case of Ba FeMoO, a=0.81865 nm Relatively small A cations usually lead to octahedral tilting, as... 

The superconducting double perovskite (NaQjsKg jlBajBi Ojj has a complex ordering of A-site cations. The structure is cubic space group Im3m (229), a=0.8550nm. The A-sites are subdivided so that one set is occupied by (Na V), where V represents an A-site vacancy, while the other (A -sites) are fully occupied by Ba. The Bi cations occupy the B-site octahedra. 

In the unusual double perovskite CaMnTi Og, the Mtf cations occupy the A-sites. The Mn + and Ca are ordered in columns along the c-axis, with structural distortions allowing the Mn " to take tetrahedral and square planar positions in the cage sites. The phase CaFeTi Og appears to be similar, but with Fe only occupying tetrahedral positions. These phases are structurally related to the AA B Oj -related phases described in the following section. 

The A-site ordered double perovskite (Nag jKg jlBajBi Ojj has a superconducting transition temperature T of approximately 27 K. Tlie structure is cubic, space group Imlm (229), a=0.8550nm, with ordering of the (Na, K) and Ba atoms in layers to give a doubled perovskite unit cell. As the formula suggests, one set of A-sites (Al) is only partially occupied and contains the Na " and K " cations and a... 

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