Topochemical Synthesis of Mixed-Anion Oxide Epitaxial Thin Films

Strongly correlated transition metal oxides exhibit fascinating physical and chemical properties, including superconductivity, colossal magnetoresistance, ferroelectricity, and photocatalytic abilities. Since these properties are strongly affected by bonding interactions between the <i>d</i> orbital of transition metal cations and the <i>p</i> orbital of oxide anions, moderate replacement of O^2- by other anions such as H^- and F^- can drastically change the properties. The effects of anion doping on strongly correlated oxides include not only the chemical pressure effect and carrier doping effect, which can also be achieved by cation doping, but also tuning of crystal field splitting and band gap, and geometrical effects associated with <i>cis</i>/<i>trans </i>degree of freedom, local coordination asymmetry, and dimensional reduction.<br/>One of the most excellent methods to obtain oxyhydrides and oxyfluorides is topochemical synthesis using reagents and electrochemical reaction, where guest species can be introduced into a host crystalline structure without destroying the initial crystalline matrix. Especially, topochemical reactions of thin-film samples are expected to have several advantages over bulk: considerably higher reactivity owing to the larger surface area/volume ratio, stabilization of the crystal framework by epitaxial effect, and modification of physical properties by epitaxial strain.<br/>So far, we have succeeded in topochemical reactions for various transition-metal oxide thin films, such as H^- doping, OH^- (H⁺) doping, F^- doping, and Cl^- doping, and found interesting electronic properties in the obtained mixed-anion oxide thin films. We have also demonstrated that anion arrangement can be controlled by changing the epitaxial strain and the conditions of topochemical reactions, and that a certain anion can be selectively doped into the top layer of an epitaxial bilayer film without changing the properties of the bottom layer. In this talk, we will present our recent studies on the topochemical synthesis of mixed-anion oxide epitaxial thin films and the modulation of their physical properties by anion doping.