Structural and Electrical Investigation of Oxide Ion and Proton Conducting Titanium Cuspidines

The La 4 (Ga 2 - x Ti x O 7+ x /2 □ 1 - x /2 )O 2 ( x = 0−2) cuspidine series has been prepared as single phases in the full compositional range. The evolution of the cell parameters is smooth along the series although the symmetry changes from monoclinic P 2 1 / c for La 4 (Ga 2 O 7 □ 1 )O 2 to orthorhombic Pnam for La 4 (Ti 2 O 8 )O 2 . The crystal structure of La 4 (Ga 1 Ti 1 O 7.5 □ 0.5 )O 2 has been determined from a refinement of neutron and laboratory X-ray powder diffraction data at room and high temperatures. This compound crystallizes in the P 2 1 / c space group, Z = 4, with a = 7.9629(2) Å, b = 11.1000(2) Å, c = 11.8294(4) Å, β = 109.689(4)°, and V = 984.44(4) Å 3 . The neutron Rietveld disagreement factors at room temperature were R WP = 3.10% and R F = 3.36%. The Ga 3+ /Ti 4+ aliovalent substitution is accompanied by extra oxygen needed for the charge compensation. This oxygen transforms the isolated ditetrahedral groups in La 4 (Ga 2 O 7 □ 1 )O 2 into infinite trigonal bipyramid chains with interruptions due to the oxygen vacancies in La 4 (Ga 1.0 Ti 1.0 O 7.5 □ 0.5 )O 2 and into infinite trigonal bipyramid chains in La 4 (Ti 2 O 8 )O 2 . The electrochemical characterization under different atmospheres (static air, dry Ar, wet Ar, dry 5% H 2 −Ar, and wet 5% H 2 −Ar) indicates that there is a significant proton contribution to the overall conductivity below ∼950 K for samples with intermediate compositions. Above that temperature, the conductivities are predominantly due to ion oxide. The ion oxide transport numbers range between 0.99 and 0.93, depending upon the temperature and oxygen partial pressure. Small p- and n-type electronic contributions have been observed by thermal and electrochemical analysis under oxidizing and reducing conditions, respectively.