Anomalous pre-nucleation volume expansion of amorphous BaTiO3

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.