  Bulk EuPd$_2$Si$_2$ show a temperature-driven valence transisition of
europium from $\sim$+2 above 200 K to $\sim$+3 below 100 K, which is correlated
with a shrinking by approximatly 2 % of the crystal lattice along the two
a-axes. Due to this interconnection between lattice and electronic degrees of
freedom the influence of strain in epitaxial thin films is particularly
interesting. Ambient X-ray diffraction (XRD) confirms an epitaxial relationship
of tetragonal EuPd$_2$Si$_2$ on MgO(001) with an out-of plane c-axis
orientation for the thin film, whereby the a-axes of both lattices align. XRD
at low temperatures reveals a strong coupling of the thin film lattice to the
substrate, showing no abrupt compression over the temperature range from 300 to
10 K. Hard X-ray photoelectron spectroscopy at 300 and 20 K reveals a
temperature-independent valence of +2.0 for Eu. The evolving biaxial tensile
strain upon cooling is suggested to suppress the valence transition. Instead
low temperature transport measurements of the resistivity and the Hall effect
in a magnetic field up to 5 T point to a film thickness independent phase
transition at 16-20 K, indicating magnetic ordering.
