Neutron diffraction investigation of the evolution of the crystal structure of oxygen-conducting solid solutions (Yb1-xCax)2Ti2O7 (x = 0, 0.05, 0.10)

The crystal structure of pyrochlore-like solid solutions (Yb1-xCax)2Ti2O7 (x = 0, 0.05, 0.10) synthesized by high-temperature annealing of mechanically activated initial oxides at temperatures of 1300-1500 deg. C is studied using neutron diffraction. It is found that the Ca2+ cations are located in the ytterbium sublattice, which apparently favors the splitting of one of the oxygen sublattices [O(2) (48f)] of the pyrochlore structure: the decrease in the occupancy of this sublattice is accompanied by the formation of a new sublattice O(3) (8b), whereas the other oxygen sublattice O(1) (8a) remains unchanged. This rearrangement of the anions in the oxygen subsystem due to the incorporation of an alkaline-earth cation explains the high ionic conductance (∼0.2 S/cm at 1000 deg. C) for (Yb0.9Ca0.1)2Ti2O7 ∼ 0.2, which is the maximum value observed to date for pyrochlores of the A2B2O7 type, where A = Ln and B is a Subgroup IVA element of the periodic system. The bulk and grain-boundary components of the conductivity of (Yb0.95Ca0.05)2Ti2O7 synthesized at temperatures of 1300, 1400, and 1500 deg. C are studied using impedance spectroscopy. It is found that the (Yb0.95Ca0.05)2Ti2O7 sample synthesized at 1500 deg. C has the highest total conductivity due to the increased grain-boundary component. The bulk component of the ionic conductivity of (Yb0.95Ca0.05)2Ti2O7 is hardly affected by the synthesis temperature and depends mainly on the degree of heterovalent substitution.