The incorporation of Pr in the apatite-type lattice of La9.83−xPrxSi4.5Fe1.5O26±δ (x = 0–6) decreased the unit cell volume, suppresses Fe4+ formation according to Mössbauer spectroscopy, and increased p- and n-type electronic contributions to total conductivity, studied by the impedance spectroscopy and modified faradaic efficiency and electromotive force methods at 973 to 1223K. The additions of praseodymia have no essential effect on the ionic transport, with an activation energy of 99–109kJ/mol, under oxidizing conditions. Contrary to the Al-containing analogue, La9.83Si4.5Al1.5O26, exhibiting PO2-independent conductivity at O pressures from 10−20 to 0.5atm, the ionic conductivity of La9.83−xPrxSi4.5Fe1.5O26±δ decreased on reducing PO2 below 10−14 to 10−12atm. The observed behavior suggested the presence of hyper-stoichiometric O; critical for the level of ionic conduction and compensated by the formation of Fe4+ or Pr4+. The ion transference numbers in air varied from 0.979 to 0.994 for La9.83−xPrxSi4.5Fe1.5O26+δ while, for La9.83Si4.5Al1.5O26, the p-type electronic contribution to the total conductivity was lower than 0.5%.

Ionic and Electronic Conductivity of La9.83-xPrxSi4.5Fe1.5O26±δ Apatites. A.A.Yaremchenko, A.L.Shaula, V.V.Kharton, J.C.Waerenborgh, D.P.Rojas, M.V.Patrakeev, F.M.B.Marques: Solid State Ionics, 2004, 171[1-2], 51-9