**1. Introduction**

Rare-earth element perovskites with the formula ABO3 (A = alkaline/rare-earth element, B = 3d–5d transition metal) have received much attention in the field of heterogeneous catalysis [1–3]. The catalytic activity of these materials relates to the nature of the B-site element [2]. In addition, the partial substitution of the A-site alkaline/rare-earth element with a lower valency cation (typically Ca or Sr) may result in oxygen nonstoichiometry, which in turn induces specific effects on the catalytic performance [3]. Encouraging results for catalytic oxidation reactions have been obtained with LaCoO3 and La1−<sup>x</sup>A'xCoO3 (A' = Ca or Sr) [1,3]. However, the basicity of lanthanum makes such catalysts vulnerable to detrimental volume expansion due to lanthanum oxide hydration upon reaction with air and moisture [4,5]. Preliminary bench-scale catalyst performance tests of bulk Gd1−*<sup>x</sup>*Ca*x*CoO3 for ammonia oxidation show comparable catalytic performance to the corresponding La-based system, but without the undesired degradation of the catalysts due to hydroxide formation upon temperature cycling in the processing atmosphere [6]. We currently focus on GdCoO3-based catalysts of relevance for the ammonia slip reaction (i.e., the oxidation of minute quantities of NH3 in a process stream into nitrogen and steam), owing to the lower basicity, and thus improved resistance towards hydration, of such Gd-containing compounds in realistic processing environments [7,8]. Notably, we also explore deposition routes for Ca-substituted variants, providing means for oxygen vacancies.

Recent literature underlines the pertinence of using atomic layer deposition (ALD) in the design and study of coatings for heterogeneous catalysis [9–11]. The sequential nature of the ALD technique inherently rules out any gas phase reactions, and the self-limiting nature of the processes leads to controllable and reproducible synthesis of morphologically and chemically uniform materials [12–14]. A major advantage of ALD compared to conventional thin-film synthesis routes like sputtering or CVD is the possibility of obtaining high-surface-area supported catalysts by depositing chemically uniform thin films of the active phase on a high-surface-area support [15,16]. This is enabled by the self-limiting mechanism that allows for deposition beyond the line-of-sight.

ALD processes have been developed and reported for a wide variety of oxides, including around 30 functional perovskites [17,18]. The development of ALD processes for ternary and quaternary oxides has recently gained attention due to their high potential in a range of applications, such as ferroelectrics, photovoltaics, and battery technology [18,19]. However, owing to the complexity of multi-cation deposition, the available ALD processes for quaternary oxides are still limited to a few systems [20]. To the best of our knowledge, no reports have been published on the preparation of ALD films of GdCoO3 or the substituted variants thereof.

ALD of LaCoO3 using β-diketonates and ozone was reported in 1997 by Seim et al. [21]. No reports were made of any structural or functional characterization of the product films. More recently, ALD of the quaternary La1−*<sup>x</sup>*Sr*x*CoO3-<sup>δ</sup> system for the composition range 0.3 < *x* < 0.7 was achieved by Ahvenniemi et al., using the same type of process [20]. One of the challenges of introducing cobalt in complex oxide ALD is catalytic decomposition of ozone and the metal-organic precursors by CoO*x* species. This challenge can be overcome by tuning the precursor flux and precursor sequence, similar to recent reports on the deposition of lanthanum cuprate, for which CuOx species exhibit the same detrimental catalytic precursor decomposition [22].

In this work we report for the first time the controlled thin-film growth and characterization of the ternary GdCoO3 and quaternary Gd1−*<sup>x</sup>*Ca*x*CoO3 rare-earth cobaltites using β-diketonates and ozone as precursors on flat and high-aspect ratio substrates. The current investigation is a step towards the growth and tailoring of highly selective complex thin films for heterogeneous catalysis.
