Unusual solid-state transformations in LuFe2O4 films during their synthesis via MOCVD with further reduction

Thin films of lutetium ferrite LuFe2O4, potential components of multiferroic devices, are difficult objects for synthetic solid-state chemistry. In this paper, we report on the successful synthesis of epitaxial LuFe2O4 thin films on YSZ(111) single-crystalline substrates via a two-stage technique. The newly proposed synthesis route combines a novel variant of metal-organic chemical vapor deposition (MOCVD) with subsequent annealing at low oxygen partial pressure provided by the metal/oxide (Fe/FeO) getter. The phase composition and structural transformations during synthesis are studied. It is shown that the orienting effect of the substrate manifests itself in the fine structure of as-deposited films: the LuFe2O4.5 phase forms as a layer (∼5 ​nm thick) adjacent to the substrate. On top of it the layer of hexagonal LuFeO3 (h-LuFeO3) forms. The crystalline layer is then replaced by an amorphous mixture of lutetium and iron oxides. High-temperature annealing at 900 ​°C and partial oxygen pressure (pO2) laying in the region of thermodynamic stability of LuFe2O4 leads to the formation of films that are well-crystallized over the entire thickness of the sample. At the same time, it was discovered that a thin epitaxial layer (∼5 ​nm) of Lu2O3 forms at the interface with the substrate. The proposed scenario of the appearance of this layer implies the partial evaporation of the iron oxide (FeO) from the film surface during annealing, which is accompanied by diffusion of iron and oxygen ions from the interface to the upper surface of the film. It was also established that the solid-state reaction between h-LuFeO3 and FeO is an essential stage in the formation of LuFe2O4. Transmission electron microscopy (TEM) images of the model samples reveal that wustite penetrates the interlayer space between lutetium and iron-containing layers, which leads to the formation of well-ordered (FeO)2(LuO2)1 structure.