Abstract
A new defect solid solution, the series Na 7 8 (Fe III 7 8 +x Ti IV 9 8 −2x Sb V x) O 4 , was synthesized. Its homogeneity range is rather wide: 0 <- x ≤ 0.33. The incorporation of Sb V gives rise to a progressive increase of the parameters of the orthorhombic unit cell. X-ray powder structure calculations point to a partial occupancy of the large double tunnels in a quadruple rutile-chain structure. A significant ordering of cations over the octahedral framework is observed, owing to a Ti IVSb V segregation. Electrical measurements emphasize a cationic conductivity, mainly related to a 1D motion of Na I cations. A transition from a low activation energy process— E A ≤ 0.20 eV—to a high activation energy one— E A ≈ 0.75 eV—systematically occurs at T ≈ 440°C, independent of the Sb V concentration. A possible skew motion from a half tunnel to another one is proposed as a tentative explanation of the high-temperature conductivity mechanism.
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