Phase transitions and structure determination of ferroelectric hafnia-based thin films

The origin of ferroelectricity in hafnia is thought to be an orthorhombic polar (o-) phase stabilized by doping, stress or strain, all facilitated at the nanoscale. Therefore, mostly very thin films (below 10 nm) are available in the polar state, making it extremely challenging to determine the true nature of the polar phase. We recently reported the epitaxial growth of Hf0.5Zr0.5O2 on perovskite substrates under large compressive strain with large ferroelectric polarization. The films are (111) oriented and pole figures show 1:3 multiplicity indicating a rhombohedral (r-) phase. We aim to: (a) elucidate the exact structure of the (r-) phase and determine if the two reported polar phases are truly difelow ferent (b) characterise the high temperature transition recently observed in r-phase films and (c) explore the non-polar-to-polar phase transition, identifying the symmetry of the primary instability.