Ionic and electronic conductivity of La9.83−xPrxSi4.5Fe1.5O26±δ apatites

The incorporation of praseodymium in the apatite-type lattice of La9.83−xPrxSi4.5Fe1.5O26±δ (x=0–6) decreases the unit cell volume, suppresses Fe4+ formation according to Mössbauer spectroscopy, and increases p- and n-type electronic contributions to total conductivity, studied by the impedance spectroscopy and modified faradaic efficiency (FE) and electromotive force (EMF) methods at 973–1223 K. The additions of praseodymia have no essential effect on the ionic transport, with an activation energy of 99–109 kJ/mol, under oxidizing conditions. Contrary to the Al-containing analogue, La9.83Si4.5Al1.5O26, exhibiting p(O2)-independent conductivity at oxygen pressures from 10−20 to 0.5 atm, the ionic conductivity of La9.83−xPrxSi4.5Fe1.5O26±δ decreases on reducing p(O2) below 10−14–10−12 atm. The observed behavior suggests a presence of hyperstoichiometric oxygen, critical for the level of ionic conduction and compensated by the formation of Fe4+ or Pr4+. The ion transference numbers in air vary in the range 0.979–0.994 for La9.83−xPrxSi4.5Fe1.5O26+δ, whilst for La9.83Si4.5Al1.5O26 the p-type electronic contribution to the total conductivity is lower than 0.5%. The average thermal expansion coefficients (TECs) of silicate-based solid electrolytes are (8.8–9.4)×10−6 K−1 at 300–1200 K and (14.2–15.8)×10−6 K−1 at 1200–1350 K.