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Epitaxially strained LaCoO 3 (LCO) thin films were grown with different film thickness, t, on (001) oriented (LaAlO 3 ) 0.3 (SrAl 0.5 Ta 0.5 O 3 ) 0.7 (LSAT) substrates. After initial pseudomorphic growth the films start to relieve their strain partly by the formation of periodic nano-twins with twin planes predominantly along the <100> direction. Nano-twinning occurs already at the initial stage of growth, albeit in a more moderate way. Pseudomorphic grains, on the other hand, still grow up to a thickness of at least several tenths of nanometers. The twinning is attributed to the symmetry lowering of the epitaxially strained pseudo-tetragonal structure towards the relaxed rhombohedral structure of bulk LCO. However, the unit-cell volume of the pseudo-tetragonal structure is found to be nearly constant over a very large range of t. Only films with t > 130 nm show a significant relaxation of the lattice parameters towards values comparable to those of bulk LCO. 1Measurements of the magnetic moment indicate that the effective paramagnetic moment, m eff , and thus the spin state of the Co 3+ ion does not change for films with t ≤ 100 nm. However, the saturated ferromagnetic moment, m s , was found to be proportional only to the pseudo-tetragonal part of the film and decreases with increasing rhombohedral distortion. The measurements demonstrate, that ferromagnetism of LCO is strongly affected by the rhombohedral distortion while the increased unitcell volume mainly controls the effective paramagnetic moment and thus the spin state of the Co 3+ ion. 2 I. IntroductionThe perovskite-type lanthanum cobaltate LaCoO 3 (LCO) has recently attracted much attention due to its unusual electronic and magnetic properties at ambient pressure 1,2 and the observation of ferromagnetism in epitaxially strained thin films. 3,4 At low temperature, T ≤ 35 K, LCO is a nonmagnetic semiconductor with a ground state of Co 3+ ions in a low-spin (LS) configuration (t 2g 6 e g 0 , S = 0). 5,6 This is believed to change to a primarily intermediate-spin (IS) (t 2g 5 e g 1 , S = 1) state 7 in the temperature range 35 K < T < 100 K, and further to a mixture of IS and high-spin (HS) (t 2g 4 e g 2 , S = 2) states in the interval 300 K < T < 600 K. The crossover between spin states with increasing temperature arises from a delicate interplay between the crystal-field splitting, ∆ CF , between the t 2g and e g energy levels, and the intra-atomic exchange interaction (Hund`s rule coupling), ∆ EX . The balance between ∆ CF and ∆ EX can be affected by, e. g., hole or electron doping 8 and by chemical or external pressure. 910 Since ∆ CF is very sensitive to changes of the Co-O bond length, d, 11 and the Co-O-Co bond angle, γ , structural changes with respect to both easily modify the spin state of the Co 3+ ion. We have shown that the population of higher spin states is enhanced in tensile strained LCO films.3Calculations based on the generalized gradient approximation to the density functional theory indeed show that a magnetic state is more stable tha...

Numerous preparation techniques for perovskite-based MIEC materials have been developed over the last years. Apart from the conventional methods of cathode preparation, including high temperature calcination, mechanical grinding of powders, screen printing and sintering of thick fi lm layers, alternative synthesis and deposition routes are available. Nanocrystalline LSC or La 1−x Sr x Co 1−y Fe y O 3− δ thin fi lms on electrolyte substrates were already fabricated by pulsed laser deposition, [6][7][8][9] DC, and RF sputtering, [ 10,11 ] spray pyrolysis, [ 12 ] and sol-gel deposition. [13][14][15][16][17] In this work the chemical and structural properties and the stability of nanoscaled La 0.6 Sr 0.4 CoO 3− δ LSC thin fi lm cathodes on polycrystalline Gd 0.1 Ce 0.9 O 1.95 (CGO) substrates were investigated thoroughly. The fabrication by a low-temperature sol-gel technique is favorable for tailoring the grain size, porosity and cathode-fi lm thickness. [ 18 ] The electrochemical properties of the nanoscaled LSC thin fi lm cathodes were already reported by Hayd et al. who presented an outstanding electrochemical performance and extraordinary low area specifi c resistances as low as ASR chem = 0.023 Ω cm 2 at an operating temperature of 600 ° C. [ 19,20 ] Interestingly enough, theoretical models predicted ASR chem values up to one order of magnitude larger than the experimental data. For obvious reasons, the excellent performance of our nanoscaled cathodes does not solely depend on microstructure (porosity, increased inner surface area) as reported in previous work, [ 21 ] but presumably on the enhanced catalytic properties of the La 0.6 Sr 0.4 CoO 3− δ . However, the outstanding electrochemical performance motivates the analysis of the microstructure and phase composition of the LSC cathodes presented in this work.The nanoscaled LSC thin-fi lm cathodes were studied by transmission and scanning transmission electron microscopy (TEM/STEM) combined with energy-dispersive X-ray spectroscopy (EDXS). Moreover, high-angle annular dark-fi eld (HAADF) STEM tomography was used to study the distribution of the pores and quantify the porosity, which was not reported for nanoscaled (La,Sr)CoO 3 -based materials in literature before. Although several techniques like μ -tomography, [ 22 ] FIB-tomography, [ 23,24 ] transmission X-ray microscope based X-ray tomography, [ 12 , 25 ] mercury intrusion, [ 22 , 26 ] nitrogen adsorption, [ 27 ] or the Archimedes method [ 22 , 28 ]

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