Multiferroic h‑LuFeO3 Thin Films on (111) and (100) Surfaces of YSZ Substrates: An Experimental and Theoretical Study

Thin films of in-bulk unstable multiferroic hexagonal LuFeO3 were synthesized on coherent (111) and for the first time on incoherent (100) YSZ and Pt/YSZ surfaces by the metal–organic chemical vapor deposition (MOCVD) technique. The obtained films were thoroughly studied by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), atomic force microscopy (AFM), piezoresponse force microscopy (PFM), and theoretical simulations. The substrate surface symmetry has a crucial role in the formation of the epitaxial film’s structure. Also, the molecular mechanics calculations were adapted for film/substrate interface simulation and, for the first time, the number of variants was predicted by the number of minima on the energy profile as well as it was proved that that the formation of h-LuFeO3 is more energetically preferable than o-LuFeO3, even on the incoherent surface. It was shown that h-LuFeO3 films deposited on the YSZ(111) surface have formed a single in-plane rotational variant structure, while those deposited on the YSZ(100) surface have formed a bivariant structure. PFM results of bivariant h-LuFeO3(001)//Pt­(111)//YSZ­(100) show half the size of ferroelectric domains (∼100 nm) and twice as large the values of piezoelectric response compared to h-LuFeO3(001)//Pt­(111)//YSZ­(111).