Using Solid State Chemistry to understand the structure-property relationships of
ferroelectric BaTiO3
Derek C Sinclair
Department of Materials Science & Engineering, University of Sheffield, Mappin Street, Sheffield UK, S1 3DJ
Since its discovery in the 1940’s, ferroelectric BaTiO3 with the perovskite structure has
attracted considerable attention from both the academic and industrial sectors due to its useful
electrical properties, including; high relative permittivity and voltage tunability for capacitor
applications; piezo-electricity for sensor and actuator applications; positive temperature coefficient of
resistance (ptcr) effect in ceramics for thermistor applications; and large electro-optic coefficient and
birefringence for electro-optic wave guides and modulators. Ferroelectricity is a rather unusual
physical state, whereby a material possesses a spontaneous polarisation that can be reversed by an
external electrical field.
Alkaline and Rare Earth (RE) ions are often used to chemically dope BaTiO3 to modify its
electrical properties for commercial applications; however, in many cases the role(s) of the dopants are
poorly understood. In part, this is because some dope only onto the Ba- or Ti- site whereas others dope
onto both sites (self compensation) and in many cases ionic and/or electronic charge compensation
mechanisms are required to maintain electroneutrality. Here we present recent results from our
laboratory based on a combination of methods commonly employed in solid state chemistry, eg
atomistic simulations, equilibrium phase diagrams and electrical (dielectric) property measurements
for a range of RE3+ ions and for Ca2+. We highlight; (i) the influence of RE ion size on the doping
mechanism(s) and the rich variety of electrical properties displayed, including n-type, p-type or
intrinsic conduction and ferroelectric relaxor behaviour dependent on the composition; (ii) the
influence and usefulness of cation variance (strain) in A-site (isovalent) Ca-doped materials for
developing the next generation (X8R) of multi-layer capacitors; and (iii) an alternative explanation for
the so-called ’Donor (La) doping’ mechanism for semiconductivity based on oxygen-loss as opposed
to direct electronic compensation.
The overall aim of the seminar is to highlight the rich defect chemistry and functionality of
ferroelectric BaTiO3 and that despite being discovered in the 1940’s and heavily exploited in oxidebased electronic components since the 1960’s we still have much to learn about this particular
perovskite.