Anisotropic oxygen diffusion properties in epitaxial thin films of La₂NiO_4+δ

Abstract

We report on the development and validation of a new methodology for the determination of anisotropic tracer diffusion and surface exchange coefficients of high quality epitaxial thin films in the two perpendicular directions (transverse and longitudinal), by the isotopic exchange technique. Measurements were performed on c-axis oriented La 2 NiO 4+d films grown on SrTiO 3 (100) and NdGaO 3 (110) by pulsed injection metal organic chemical vapour deposition (PIMOCVD), with different thicknesses ranging from 33 to 370 nm. The effect that the strain induced by the film-substrate mismatch has on the oxygen diffusion through the film was evaluated. Both tracer diffusion coefficients, along the c-axis and along the ab plane, were found to increase with film thickness, i.e., as the stress of the film decreases, while the thickness seems to have no effect on the tracer surface exchange coefficient. Best fits were obtained when considering the thickest films composed by two regions with different c-axis tracer diffusion coefficient values, a higher and constant D* close to the film surface and a variable decreasing D* closer to the substrate. As expected, the tracer diffusion and surface exchange coefficients are thermally activated and are approximately two orders of magnitude higher along the ab plane than along the c-axis. The low activation energies of D* compared with bulk values for both directions at low temperatures seem to confirm the contribution of a vacancy mechanism to the ionic conduction.

Mechanism for enhanced oxygen reduction kinetics at the (La,Sr)CoO3−δ/(La,Sr)2CoO4+δ hetero-interface

“…6,12,13,[17][18][19][20][21][22][23] In support of our second hypothesis, lattice strain was recently shown to alter the oxygen defect chemistry as well as the oxygen reaction and diffusion kinetics on perovskite oxides.…”

Interstitialcy diffusion of oxygen in tetragonal La2CoO4+δ

“…6,10 For La 2 NiO 4 , previous experimental studies determined activation energies for oxygen diffusion in the range of 0.19 eV (for epitaxial thin films) to 0.90 eV (for single crystals). 10,15,17 Interestingly, a number of diffusion mechanisms were proposed for the oxygen transport in this material. 6,10,15 A summary of these results is given in Table 1.…”

“…3,4 Additionally, the first members of the RP series with n = 1, A 2 BO 4 , that exhibit the K 2 NiF 4 structure are also being considered for oxygen sensors and oxygen separation membranes. [5][6][7][8][9][10][11][12][13][14][15][16][17] These materials have been extensively investigated since the observation of high-temperature superconductivity in La 2Àx Ba x CuO 4 . 18 An interesting and recent example of a composite SOFC cathode has been the work of Sase et al, 19 which demonstrated higher transport and an oxygen exchange rate that was higher by three orders of magnitude at the interface of the perovskite (La,Sr)CoO 3 and the RP phase (La,Sr) 2 CoO 4 .…”

Chemomechanics of ionically conductive ceramics for electrical energy conversion and storage

“…However, the effect of strain was not clearly demonstrated or explained. On another material system, Burriel et al investigated epitaxially grown La 2 NiO 4 (c-axis perpendicular to the substrate surface) on SrTiO 3 (100) and NdGaO 3 (110) with thicknesses from 33 nm to 370 nm [57], with seemingly contradictory results to those shown on the strained LSC films. While the La 2 NiO 4 films grown on STO were compressively strained, those grown on NdGaO 3 were under tensile strain.…”

“…These results provide insight into the strain and reactivity correlation of SOFC cathode materials. Isotope exchange depth profiling (IEDP) in combination with time of flight secondary ion mass spectroscopy (ToF-SIMS) has been used to determine the diffusion coefficients and oxygen surface exchange rates for several materials relevant to SOFCs [2,56,57]. Zapata et al used this method to determine the anisotropic diffusion coefficients and surface exchange rates of oxygen in thin films of the double perovskite GdBaCo 2 O 5+δ .…”