## 3 Results and Discussion
Thin films of BCO, BCY10, and BCY20 consisted of randomly oriented large grains with the shape of a triangular prism, growing on top of each other in the columnar way (Figure 1a-c) while BZO's and BZY's microstructures are contained in small circular-shaped grains (Figure 1d,e). Figure 1f demonstrates that even a small amount of cerium dopant influences the formation of the randomly oriented triangular grains instead of the circular-shaped grains. Such a polycrystalline orientation is expected for perovskite materials barium cerates and barium zirconates [24]. The grain size distribution performed by image processing software revealed that the addition of yttrium decreased the surface grain size of barium cerates, from 0.33 to 0.20 \(\upmu\)m, on average. The surface grain sizes of barium zirconates were smaller than barium cerates, yet the same tendency of the grain size decrease in the thin films doped with yttrium was observed, from 0.14 to 0.07 \(\upmu\)m, on average. Similar tendencies are seen in the research of [PERSON]. [PERSON] et al. [25]. Cross sectional images (Figure 2) confirm the densely packed microstructure of barium cerates and zirconates independently of their composition. According to the Thornton structural model, the columnar growth of dense barium cerate thin films with expressed grain boundaries is observed and the films may be considered at zone II, which is characteristic when the ratio of the substrate temperature (\(T_{s}\)) and the material's melting temperature (\(T_{m}\)) is higher than 0.4 (Figure 2a-c). In such a condition, surface and bulk diffusion becomes substantial, resulting in the formation of large, oriented grains [20]. On the other hand, barium zirconate thin films were formed in zone T, which is denser than zone II (Figure 2d-f). An effect of the dopant on the roughness of barium zirconates can be seen from SEM images (Figure 2). The roughness was reduced with the addition of yttrium or cerium. Roughness can be created by charged particles of high energy that reach the surface that results in etching effects in some areas. The roughness can be reduced by enhancing the mobility of the depositing atoms or by low-energy ion bombardment during the deposition [26]. Figure 1: SEM images of the surface of the thin films: BCO (**a**), BCY10 (**b**), BCY20 (**c**), BZO (**d**), BZY20 (**e**), and BZCY (**f**). It is known that one of the disadvantages of barium cerates and zirconates is their high reactivity with their environment and further degradation to the carbonates. The main sources of degradation are: (1) unreacted BaO that reacts with CO\({}_{2}\) and causes disintegration of the film [27], (2) amorphous phases that contain not very well packed regions and are usually found on the grain boundaries [28; 29] that add free space to carbon molecules to enter, and (3) the high basicity of BaCeO\({}_{3}\) that makes it prone to react with CO\({}_{2}\)[30] above 400 \({}^{\circ}\)C. In order to verify the stability of the formed thin films, the crystallographic nature of the deposited films was measured twice: as deposited and after two weeks. The samples did not show any signs of degradation at room temperature. XRD identified that the BCO thin films deposited at a substrate's temperature of 500 \({}^{\circ}\)C had a predominant orthorhombic phase (PDF: 01-078-4102) with (002), (213) (231), and (004) (422) orientations (Figure 3a). The films were highly oriented towards the (002) plane. A crystallite size of 106.2 nm was calculated from Rietveld refinement and the Le Bail method (Table 2). This value was higher compared to the BCO pellets after sintering at high temperatures [31; 32], with values between 30 and 75 nm. The high orientation is attributed to the orientation of the metal substrate alloys and e-beam formation conditions [33] (e.g., power and temperature). BCO-based oriented films formed by physical vapor deposition are usually reported as pseudocubic [34] or cubic [35].