Double perovskites

Search for HMF and HMAFM properties in other classes of materials led to the emergence of double perovskite oxides as potential candidates. Computational searches based on density functional theory predicted several new double perovskites with HMAFM properties (details on structure and properties of double perovskites are in Section 5.5). 

1 Half-Metallic Antiferromagnetic (HMAFM) Double Perovskites 

HMAFM in oxides with double perovskite structure was proposed by Warren Pickett for the first time in 1998. Band structure calculations based on local spin-density approximations (LSDA), predicted La2MnV06 double perovskite as a promising candidate for exhibiting HMAFM. The calculations were based on the assumption that both Mn and V ions are trivalent and more importantly, that the Mn (d ) occurs in a low spin configuration (t2g Cg ). The band structure of La2MnVOg show that only the minority 2g bands of both V and Mn contribute to the DOS near the The antiparallel alignment of low spin Mn + (tzg Ttzg i) and states would yield a net zero 

La2MnVOg, which is not an HMAFM due to the absence of complete B-site order and apparent presence of high-spin Mn and Considering the negUgible size difference between Mn (0.645 A) and (0.64 A), their ordering in the double perovskite structure is unexpected. Recently, it was shown that even with a more reduced Mn than 

2 Half-Metallic Ferro-/Ferrimagnetic (HMF) Double Perovskites 

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Huang Y-H, Dass RI, Xing Z-L, and Goodenough JB. Double perovskites as anode materials for solid oxide fuel cells. Science 2006 312 254—256. 

Similarly to the case of direct-oxidation anode materials, sulfur-tolerant anode materials based on sulfides [6, 7] or double-perovskite oxides have special requirements for their processing into SOFC layers. For example, nickel sulfide-promoted molybdenum sulfide is tolerant to high sulfur levels [7], However, it has a low melting temperature [6] that has resulted in the development of cobalt sulfide as a stabilizer of the molybdenum sulfide catalyst [6], CoS-MoS2 admixed with Ag has an even higher performance in H2S-containing fuels than in pure H2 [6]. However, processing methods such as PS, infiltration, or sol-gel techniques that can process... 

SYNTHESIS AND CHARACTERIZATION OF THE OXYGEN DEFICIENT DOUBLE-PEROVSKITE Sr CrNbOssj... 

In this work we have attempted to prepare the oxygen deficient double-perovskite... 

According to the Curie-Weiss law the theoretically predicted paramagnetic moment for a d d ferrimagnetic system should be around 4.90 pg rather than the observed Pe of 2.81 pg. However, the analysis of reported magnetic susceptibility data for a variety of ferrimagnetic double-perovskites [6,10-13] shows that the observed paramagnetic... 

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). 

Fig. 4 Unit cell of double perovskite. Small open spheres M/M, large open spheres Sr, shaded spheres O. 

A technology of Sr2FeMoObui (SFMO) nanosized films deposition by ion-beam sputtering is described. Optimization of deposition conditions on formation of structural ly-perfect SFMO double perovskite films is presented. Several problems arise with the use of the ion-beam sputtering method concerning the films inhomogeneity, the presence of multiple phases and Femo and Mope antistructural defects. It is shown that they are solved by means of complex selection of parameters substrate temperature, deposition rate and subsequent thermal processing. 

The crystallinity of MW04 (M=Ba, Pb) thin films was studied by IR spectroscopy.223 High-pressure Raman spectra of A12(W04)3 showed phase transitions at 0.28 0.07 and 2.8 0.1 GPa.224 The IR spectrum of the double perovskite Sr2FeW06 includes characteristic bands at 143, 227, 377 and 625 cm-1.225... 

Part of the neutron powder pattern (A =1.909 A) of cubic double perovskite HNbOs, showing very different profile shapes and widths. No excellent fit of that pattern has ever been produced from a realistic model, though the most probable faulting inside of the sample is a simple three dimensional antiphase effect due to [Nb06] octahedra inversion of the tilting angles of connection. 

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 ... 

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