Structure and Properties Investigations of the La 2 Co 1+z (Ti 1-x Mg x ) 1-z O 6 Perovskite System [Elektronisk resurs]

• Perovskite based materials have great potentials for various energy applications and the search for new materials for uses in SOFCs has largely been concentrated to this class of compounds. In this search, we have studied perovskite phases in the system La 2 Co 1+z (Ti 1-x Mg x ) 1-z O 6 , with 0  x 0.9 and z = 0.0, 0.2, 0.4, 0.6. Crystal structures were characterized by XRD and, for selected compositions, also by NPD and SAED. They exhibit with increasing x, as well as increasing z, a progressive increase in symmetry from monoclinic to orthorhombic to rhombohedral. The main focus in this work has been on the investigation of structure-property relations for compositions with 0.0 x 0.5 and z = 0. The nominal oxidation state of Co increases for these with increasing x, from Co 2+ for x = 0 to Co 3+ for x = 0.5. Magnetic measurements and XANES studies showed that the average spin state of Co changes linearly with increasing x, up to x = 0.5, in accordance with varying proportions of Co with two fixed oxidation states, i.e. Co 2+ and Co 3+ . The data suggests that the Co 3+ ions have an IS spin state or a mixture of LS and HS spin states for all compositions with nominally only Co 2+ and Co 3+ ions, possibly with the exception of the composition with x = 0.1, 0.2 and z = 0, for which the data indicate that the spin state might be HS. The XANES data indicate furthermore that for the perovskite phases with z = 0 and x > 0.5, which in the absence of O atom vacancies contain formally Co 4+ , the highest oxidation state of Co is Co 3+ , implying that the substitution of Ti 4+ by Mg 2+ for x ³ 0.5 effects an oxidation of O 2- ions rather than an oxidation of Co 3+ ions. The thermal expansion was found to increase nearly linearly with increasing oxidation state of Co. This agrees well with findings in previous studies and is attributable to an increase in the ionic radius of Co 3+ ions with increasing temperature, due to a thermal excitation from a LS to IS or LS/HS spin states. High temperature electronic conductivity measurements indicate that the electronic conductivity increases with an increase of both relative and absolute amount of Co 3+ . The latter can be attributed to an increase in the number of Co-O-Co connections. Additional high temperature magnetic measurements for selected samples, whose susceptibilities did not follow a Curie law behaviour up to room temperature, showed effective magnetic moments that did approach plateaus even at high temperatures (900 K). Interpretations of these data are, however, hindered by the samples losing oxygen during the applied heating-cooling cycle. The present study has shown that the investigated system is suitable for further studies, of more fundamental character, which could provide further insight of the structure-property relationships that depend on the oxidation state of Co. 

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Indexterm och SAB-rubrik

solid oxide fuel cell

SOFC

LaCoO3

cobaltates

cobalt oxide

perovskite

XANES

XRD

Neutron powder diffraction

thermal expansion

electronic conductivity

magnetism

citrate

electron diffraction

synthesis

phase diagram

spin state

fast fas oxid bränslecell

LaCoO3

koboltoxid

perovskit

XANES

XRD

neutron diffraktion

elektron diffraktion

termisk expansion

elektrisk ledningsförmåga

magnetism

citrat

SOFC

syntes

fasdiagram

spin tillstånd