Crystal structure and electronic property modification of Ca2RuO4 thin films via fluorine doping

Layered ruthenium oxyfluorides have various crystal structures and Ru oxidation states and exhibit unique physical properties. While various layered ruthenates have reportedly been topochemically fluorinated with Sr as A sites, investigation of the fluorination of layered ruthenates containing smaller Ca ions is lacking. In this paper, we fabricated phase-pure and single-crystalline thin films of Ca2RuO2.5F2 on LaSrAlO4 (001) substrates via topochemical fluorination of the Ca2RuO4 precursor using polyvinylidene fluoride. The obtained fluorinated thin films had a chemical composition of Ca2RuO2.5F2 with the Ru3+ state, as determined by energy-dispersive x-ray spectroscopy and x-ray photoemission spectroscopy, whereas the film prepared via Sr2Ru4+O4 fluorination had a composition of (Sr2Ru4+O3F2). Scanning transmission electron microscopy revealed that Ca2RuO2.5F2 has only 1 F- site in CaO rock-salt blocks, whereas Sr2RuO3F2 has two inequivalent F- sites in the SrO layers. The Ca2RuO2.5F2 film was insulating, with a resistivity (rho) of 8.6 x 10(-2) Omega cm at 300 K. Moreover, the temperature behavior of rho was well described by the two-dimensional variable range hopping model. These results demonstrate that local distortion is an important factor that governs the topochemical fluorination of ruthenates and affects the crystal and electronic structures of the reactants.

Automated summary

Layered ruthenium oxyfluorides possess various crystal structures and Ru oxidation states, exhibiting unique physical properties. This study investigates the fluorination process of layered ruthenates containing smaller Ca ions and fabricates phase-pure and single-crystalline thin films of Ca2RuO2.5F2. It finds that local distortion plays a crucial role in the topochemical fluorination of ruthenates and affects the crystal and electronic structures of the reactants.