A Coherently Strained Monoclinic [111]PbTiO₃ Film Exhibiting Zero Poisson's Ratio State

[111]-Oriented perovskite oxide films exhibit unique interfacial and symmetry breaking effects, which are promising for novel quantum materials as topological insulators and polar metals. However, due to strong polar mismatch and complex structural reconstructions on (111) surfaces/interfaces, it is still challenging to grow high quality [111] perovskite heterostructures, let alone explore the as-resultant physical properties. Here, the fabrication of ultrathin PbTiO₃ films grown on a SrTiO₃(111) substrate with atomically defined surfaces, by pulsed laser deposition, is reported. High-resolution scanning transmission electron microscopy and X-ray diffraction reveal that the as-grown [111]PbTiO₃ films are coherent with the substrate and compressively strained along all in-plane directions. In contrast, the out-of-plane lattices are almost unchanged compared with that of bulk PbTiO₃, resulting in a 4% contraction in unit cell volume and a nearly zero Poisson's ratio. Ferroelectric displacement mapping reveals a monoclinic distortion within the compressed [111]PbTiO₃, with a polarization larger than 50 µC cm⁻². The present findings, as further corroborated by phase field simulations and first principle calculations, differ significantly from the common [001]-oriented films. Fabricating oxide films through [111] epitaxy may facilitate the formation of new phase components and exploration of novel physical properties for future electronic nanodevices.