Anomalous pre-nucleation volume expansion of amorphous BaTiO3

Abstract

The mass-density evolution of self-supported films of sputtered amorphous BaTiO₃ was investigated. Upon heating, these films undergo a pre-nucleation structural transformation that is accompanied by an anomalously large volume expansion (up to 10%). This behavior has not been observed for any other amorphous materials, in which crystallization is generally accompanied by monotonic shrinkage. Our data suggest that any factor affecting pre-nucleation structural changes in the amorphous phase has a decisive influence on the kinetics and products of crystallization. A layer of 20 nm MgO on one surface of the amorphous BaTiO₃ film strongly inhibits the expansion. If BaTiO₃ is allowed to expand freely, it crystallizes into the hexagonal polymorph, whereas in the presence of MgO the perovskite phase forms.