Synthesis, sintering, transport and thermal properties of Na2Fe2Ti6O16 freudenbergite

Na2Fe2Ti6O16 freudenbergite has been synthesized using a simple solid-state route and conventionally sintered to reach 94% density ceramics. High temperature electrical and thermal transport properties have been investigated for the first time. In the temperature range of 468 K-1000 K, the electrical conductivity of Na2Fe2Ti6O16 can be well fitted with small polaron hopping model. For a further verification of the model, we measured the room-temperature optical reflectivity and derived the optical conductivity (σ(ω)) with Kramers-Kronig analysis. A mid-infrared peak in σ(ω) confirms the presence of small polarons. From 468 K to 1000 K, Na2Fe2Ti6O16 is an n-type oxide and exhibits a temperature-independent Seebeck coefficient (∼−590 μV K−1), in agreement with the small polaron hopping mechanism. The thermal conductivity decreases with increasing temperature from 16.5 W m−1.K−1 (373 K) to 3.4 W m−1.K−1 (1000 K). Na2Fe2Ti6O16 shows a maximum figure of merit (ZT) ∼ 3 × 10−4 at 1000 K.