Proton transport and structural relations in hydroxyl-bearing BaTiO3 and its doped compositions synthesised by wet-chemical methods

Hydrothermally synthesised powders of BaTiO3 and its Fe- or Nd-doped analogues contain hydroxyl groups in the lattice substitutional to oxide ion, as confirmed from TGA/DTA, IR spectral analysis of D2O-treated powders, EGA-MS, the contraction in lattice constant with heat treatment by XRD and surface examination by XPS. Electrical resistivity measurements were carried out on the pellets from 298 to 1000 K by ac impedance spectroscopy and dc methods in dry or moist air and 8% H2+Ar environments. The electrical conductivity observed for unsintered pellets between 298 and 500 K, is in the range of 10−3 to 10−7 S/cm and can be attributed to the extrinsic hydroxyls in BaTiO3. The acceptor-doped composition, BaTi0.9Fe0.1O3−δ: 2δ(OH) exhibits higher electrical conductivity than BaTiO3 or the donor-doped Ba0.9Nd0.1TiO3−δ: 2δ(OH) in moist air. The hydrothermally prepared powders heat treated below 1000 K having cubic symmetry at room temperature, possess higher proton conductivity and reabsorption capability for hydroxyls on exposure to moisture than the powders sintered at 1673 K (tetragonal symmetry). The conductivity at 298–500 K is due to the mobility of proton along OHO octahedra in the perovskite lattice. The conduction at 550–1000 K is a combined effect of proton as well as oxygen vacancy mobility in BaTiO3 and Ba0.9Nd0.1TiO3; electron hole (Ti4+, Ti3+, Fe3+, Fe2+) participation is the additional contribution in acceptor-doped composition in this temperature range.