Layered Cobaltites: Synthesis, Oxygen Nonstoichiometry, Transport and Magnetic Properties

Abstract

Complex cobalt oxide perovskite-derived compounds with a general formula LnBaCo2O5+x (Ln = rare earth) attracted considerable interests because of their interesting properties: magnetic and metal-insulator transitions, giant magnetoresistance, ionic conductivity, and a structural similarity to high temperature superconductors. All the compounds are oxygen nonstoichiometric (0 < x < 1) and the cobalt cations can adopt different oxidation and spin states. Compounds with x ≈ 0.5 display a metal-insulator transition. We found that this transition is affected by an oxygen isotope substitution and is accompanied by structural changes and melting of the orbital ordering. Studies of the metal-insulator transition qualitatively support the models assuming rather carriers delocalization than a spin-state transition in Co 3+ .

High-Temperature Structural and Electrical Properties of BaLnCo2O6 Positrodes

“…The structure of double perovskite cobaltites has been the subject of numerous studies [7][8][9][10][11][12]. Those materials are reported to adopt both tetragonal (P4/mmm) [9,10,13] and orthorhombic (Pmmm) [9,12] structures.…”

“…Figure 1 presents the differences between tetragonal and orthorhombic structures with different oxygen content. The obtained crystal structure of Ba Ln Co 2 O 6−δ strongly depends on oxygen stoichiometry [ 4 , 7 , 8 ], as well as synthesis procedure [ 9 ]. As the difference between the two polymorphs, resulting from the oxygen vacancy ordering, is very subtle, the structural studies require very high-quality data such as synchrotron radiation X-ray powder diffraction collected with 2D detectors.…”

“…The lattice parameter doubling along the c -axis confirms A -site cation ordering, while the double b -parameter results from oxygen vacancy ordering [ 12 ]. BPC has been reported to form both tetragonal and orthorhombic structure, strongly depending on oxygen stoichiometry [ 4 , 6 , 7 , 8 , 20 ], as well as synthesis procedure [ 21 ], but is reported to be stabilized in orthorhombic structure when oxygen non-stoichiometry (δ) is between 0.25 and 0.6 [ 14 ].…”

Thermal properties of layered cobaltitesRBaCo2O5.5(Wieckowski1, Gutowska2, Szewczyk3 et al. 2012Phys. Rev. B

“…The driving force for the ordered occupation of the A positions is a difference between the ionic radii, specifically of Ba 2+ and R 3+ ions. 2,3 The oxygen content within the R-O planes can be modified over a wide range and by removing one oxygen ion from each other pair of the unit cells of the ideal perovskite one obtains the RBaCo 2 O 5.5 composition, in which all cobalt ions take the 3+ charge state. The created oxygen vacancies order at relatively high temperatures (e.g., at ∼770 K in the case of TbBaCo 2 O 5.5 ) and, in this way, the orthorhombic structure presented in Fig.…”