A series of doped Ruddlesden-Popper phases, of general formula Sr3Ti2-xMxO7
-delta (M = Al, Ga, Co), were synthesized and their electrical conductivity
characterized as a function of temperature and oxygen partial pressure. Fo
r fixed-valent dopants, p-type conductivity predominates at p(O-2) > 10(-5)
atm, followed by a p(O-2)-independent electrolytic regime, and n-type elec
tronic conductivity at very low p(O-2) The electrolytic regime exhibits act
ivation energies in the range 1.7-1.8 eV. Doping with transition metals suc
h as Co results in a very significant increase in fetal conductivity with a
p-type conductivity at high p(O-2). Furthermore, an apparent ionic regime
at intermediate p(O-2) is observed characterized by high conductivity (> 10
(-2) S/cm at 700 degrees C) and low activation energy (0.6 eV). This interp
retation is consistent with iodometric measurements as interpreted by a def
ect chemical model. Other measurements are in progress to confirm this conc
lusion. (C) 1999 Elsevier Science Limited. All rights reserved.