Structure and electrical properties of oxygen-deficient hexagonal BaTiO3

Abstract

Rietveld refinements using neutron diffraction data have been used to determine the crystal structure of a series of oxygen-deficient barium titanate powders, BaTi ^IV _1–x Ti ^III _x O _3–x/2 (0<x<0.30). The powders were prepared by reduction of stoichiometric, tetragonal BaTiO ₃ in a vacuum furnace at temperatures above 1300 °C and under an oxygen partial pressure of 0.1 mbar. The 6H-BaTiO ₃ 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-BaO ₃ layers which separate pairs of occupied face-sharing octahedra, Ti ₂ O ₉ . There is no evidence of O(2) vacancies associated with corner sharing TiO ₆ octahedra. The Ti(2)-Ti(2) separation within face sharing dimers increases from 2.690(4) Å for x=0 to 2.7469(30) Å for x=0.30. BaTiO _2.85 is a band-gap semiconductor at 300 K with a resistivity of ca. 1 Ω 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.