Thickness-dependent structural phase transition of strained SrRuO ₃ ultrathin films: The role of octahedral tilt

We grew epitaxial SrRuO₃ (SRO) films on SrTiO₃ (STO) (001) substrates with SRO layer thicknesses (t) between 10 and 200 pseudocubic unit cells (uc). Using the square net of the cubic STO surface, we were able to epi-stabilize the tetragonal SRO phase at room temperature for ultrathin films with t ≤ 17 uc. On the other hand thicker films with t ≥ 19 uc have an orthorhombic crystal structure similar to that of bulk SRO at room temperature. With increasing temperature, the orthorhombic films undergo a structural transition to the tetragonal phase at T_OT. The value of T _OT and the orthorhombicity factor at room temperature are reduced with decreasing film thickness. We also observed half-order Bragg reflections, indicating that the tetragonal structure arises from the suppression of the tilt angle of RuO₆ octahedra. The observed critical thickness around t_c ∼ 18 uc is much larger than the recent theoretical prediction (i.e., less than 2 uc). This work thus demonstrates that the lattice symmetry mismatch at the interface plays an important role in determining the structural properties of perovskite films.