Ho-doped BaTiO3: Polymorphism, phase equilibria and dielectric properties of BaTi1−xHoxO3−x/2: 0 ≤ x ≤ 0.17

An extensive range of Ho-doped BaTiO₃ solid solution forms in which Ho substitutes for Ti with creation of oxygen vacancies. The effect of Ho substitution is to destabilise thermodynamically the high-temperature hexagonal polymorph of BaTiO₃. Nevertheless, at high Ho contents, the hexagonal polymorph forms as a kinetically stable intermediate before transforming to the thermodynamically stable cubic polymorph; its formation represents an example of Ostwald's rule of successive reactions. Samples fired at 1400 °C and cooled in air are insulating and transform from ferroelectric to relaxor ferroelectric behaviour with increasing x.