Extended Data Fig. 5: Crystalline quality and film strain characterized with reciprocal space mapping (RSM) and X-ray diffraction (XRD).

(a) Synchrotron-based 3D RSM taken at the (113)_pc peak of DyScO₃ with corresponding slices along the (b) H-L, (c) K-L, and (d) H-K reciprocal planes. The H-K plane was sliced aligned with the NaNbO₃ peak as shown in (a) and displays four satellite peaks due to the periodic domain structure, as is commonly seen in ferroelectrics. (e, f) Line scan cuts of the H-K plane along the dashed lines from (d), which can be used to extract a domain periodicity of 38 nm along H and 44 nm along K. (g) Synchrotron-based 002-line scan measurement showing strong and clear Laue oscillations, confirming the quality of our NaNbO₃ films. (h, i) 2D RSM taken at the (033)_pc and (303)_pc peaks, respectively, showing that the film is coherently strained along both in-plane directions. From a refinement of these results, we extract lattice parameters for NaNbO₃ of a = 5.519 Å, b = 5.516 Å, and c = 15.774 Å and unit cell angles of α = 90°, β = 90°, and γ = 91.3°. We can thus calculate the strain for our NaNbO₃ film to be 0.8% tensile along ab_DSO and 1.6% tensile along c_DSO.