The term ‘high purity’ applied to oxides usually implies less than 1 in 104 atoms of impurities which consist mainly of the more generally abundant elements magnesium, aluminium, silicon, phosphorus, calcium and iron, and a much smaller content of the less abundant elements such as niobium, tantalum, cerium, lanthanum etc. The bulk of the predominant impurities in BaTiO3 are cations such as Mg2þ, Al 3þ, Fe 3þ and Ca2þ that form acceptors when substituted on Ti4þ sites. The resulting deficit in charge is compensated by oxygen vacancies which may therefore be present in concentrations of order 1 in 104, a far greater concentration than would be expected from Schottky defects in intrinsic material (cf. Section 2.5.2).