Experimental and theoretical study of the strong dependence of the microstructural properties of Sr_xBa_1−xNb₂O₆ thin films as a function of their composition

We report an experimental and theoretical study of the strong dependence of the crystalline texture of thin films of Sr_xBa_1−xNb₂O₆ grown on MgO(001) on the Sr-content x between 0.35⩽x⩽0.75. Synchrotron-based x-ray diffraction measurements have identified three film-to-substrate crystal orientations, with contributions from domains at 0°, ±18.43°, and ±30.96°. The relative contributions from these domains change dramatically with x. Surprisingly, the ±18.43° orientation dominates, particularly for high x, although it has the largest lattice mismatch to the underlying substrate. These results can only be explained by detailed modeling of the electrostatic forces between the ions at the film-substrate interface, and not with simplistic lattice mismatch arguments. Therefore, molecular dynamics simulations of the ionic heterogeneous interface structure of thin films of such large oxide systems have been performed in an attempt to explain the diffraction data. The simulations predict that, depending on x, an initial ultrathin layer of SrNb₂O₆ is favored, such that its Nb-O network forms the first ionic layer on the MgO(001) substrate. This provides the necessary template for the change in crystallographic orientation with x. Such layers were subsequently identified by further synchrotron-based x-ray-diffraction measurements. This information has important consequences for ferroelectric and electrooptic applications, on the optimal conditions required to grow thin films with large crystalline domains of the desired orientation.