Epitaxially induced high temperature $(>900K)$ cubic-tetragonal structural phase transition in $BaTiO3$ thin films

For $(001)c$ oriented $BaTiO3$ thin films, it has been found that epitaxial constraint can result in a dramatic increase in the temperature of a tetragonal $(T)$ structural phase transition. For $2000-Å$-thick films grown directly on $SrTiO3$ substrates, a $T→cubic$ $(C)$ phase transition was found on heating at $>950K$⁠, where the lattice constant changed smoothly with temperature. It was also found for films of the same thickness that the $T→C$ transition is nearly restored to that of bulk crystals by the use of a buffer layer, which relaxes the epitaxial constraint. These results provide evidence of an epitaxially induced high temperature structural phase transition in $BaTiO3$ thin films, where the ferroelectric (internal) and structural (external) aspects of the phase transition are decoupled.