Structure and electrical properties of oxygen-deficient hexagonal BaTiO3

Rietveld refinements using neutron diffraction data have been used to determine the crystal structure of a series of oxygen-deficient barium titanate powders, (BaTi1-xTixO3-x/2)-Ti-IV-O-III (0 < x < 0.30). The powders were prepared by reduction of stoichiometric, tetragonal BaTiO3 in a vacuum furnace at temperatures above 1300 degrees C and under an oxygen partial pressure of 0.1 mbar. The 6H-BaTiO3 hexagonal perovskite structure is retained throughout and partial reduction of Ti-IV to Ti-III is accompanied by the formation of O(1) oxygen vacancies in the h-BaO3 layers which separate pairs of occupied face-sharing octahedra, Ti2O9. There is no evidence of O(2) vacancies associated with corner sharing TiO6 octahedra. The Ti(2) Ti(2) separation within face sharing dimers increases from 2.690(4) Angstrom for x = 0 to 2.7469(30) Angstrom for x = 0.30. BaTiO2.85 is a band-gap semiconductor at 300 K with a resistivity of ca. 1 Ohm cm and activation energy 0.16 eV. A switch in conduction mechanism to variable range hopping (VRH) of electrons between Ti-III and Ti-IV ions occurs on cooling below 240 K.