  Thin films and heterostructures of hexagonal manganites as promising
multiferroic materials have attracted a considerable interest recently. We
report structural transformations of high quality epitaxial h-TMO/YSZ(111)
films, analyzed by means of various characterization techniques. A phase
transition from P63mc to P63mcm structure at TC~800 K was observed by
temperature dependent Raman spectroscopy and optical ellipsometry. The latter
probing directly electronic system, indicates its modification at the
structural phase transition likely due to charge transfer from oxygen to Mn. In
situ transmission electron microscopy (TEM) of the lamella samples displayed an
irreversible P63mc-P63mcm transformation and vanishing of ferroelectric domains
already at 410 K. After the temperature cycling (300K-1300K-300K) the room
temperature TEM of h-TMO films revealed an inhomogeneous microstructure,
containing ferroelectric and paraelectric nanodomains with P63mc and P63mcm
structure, respectively. We point out a strong influence of stress relaxation,
induced by temperature and by constrained sample geometry onto the structure
and ferroelectricity in strain-stabilized h-TMO thin films.
