Investigations on the thermo-chemical stability and electrical conductivity of K-doped Ba_{3 - X}K_xCaNb ₂O_{9 - δ} (x = 0.5, 0.75, 1, 1.25)

In this paper, we report the synthesis, crystal structure and electrical transport properties of new K-doped Ba₃CaNb₂O₉ (BCN) and investigate their chemical stability in H₂O and pure CO₂ at elevated temperature. The powder X-ray diffraction (PXRD) of Ba_2.5K_0.5CaNb₂O_{9 - δ}, Ba_2.25K_0.75CaNb₂O _{9 - δ}, Ba₂KCaNb₂O_{9 - δ}, and Ba_1.75K_1.25CaNb₂O_{9 - δ} showed the formation of a single-phase double perovskite (A₃BB ^/₂O₉)-like cell with a lattice constant of a ∼ 2a_p (where a_p is a simple perovskite cell of ∼ 4 Å). Perovskite-like structure was found to be retained after treating with CO₂ at 700 °C and also after boiling H₂O for 120 h. The lattice constant of CO₂ and H₂O treated samples was found to be comparable to that of the corresponding as-prepared compound. The total electrical conductivity of all the investigated K-doped BCN increases with increasing K content in BCN in various atmospheres, including air, dry H ₂, wet N₂ and wet H₂. The electrical conductivity in dry and wet H₂ atmospheres was found to be higher than that of air in the temperature range of 300-700 °C, while in wet N ₂ a slightly lower value was observed. Among the compounds investigated in the present study Ba_1.75K_1.25CaNb ₂O_{9 - δ} showed the highest total electrical conductivity of 1 × 10^{- 3} S/cm in dry H₂ at 700 °C with an activation energy of 1.28 eV in the temperature range of 300-700 °C.