Thermal conductivity switching through reversible crystallographic transformations in CaFeO_xand SrFeO_xthin films.

The development of functional materials whose thermal conductivity can be switched between different states plays a key role in several technologies, such as nanoelectronics or thermoelectric energy conversion.

Three-state reversible transformations between perovskite (PV) AFeO_3-x, brownmillerite (BM) AFeO_2.5, and infinite layer (IL) AFeO₂ (A=Ca²⁺, Sr²⁺) oxides can be induced by topotactic oxygen exchange at moderate temperatures under reducing/oxidizing conditions. Here we report the reversible change of thermal conductivity associated to such structural changes in epitaxial (Ca/Sr)FeO_2.5 thin films. The PV to BM can be induced, locally, by applying local electric fields through a voltage-biased AFM tip. Using Frequency Domain Thermoreflectance (FDTR), we were able to map the thermal conductivity associated to each of these phases with micrometer spatial resolution. The time-dependent thermal conductivity allowed us to characterize the oxygen diffusion in these films.

The reversibility of the structural transformations, along with the large thermal conductivity contrast, suggest that (Ca/Sr)FeO_x ferrites are suitable candidates for solid-state thermal switching devices.