  Polycrystalline samples of SmIr2Si2 formed at room temperature both the low
temperature phase (LTP) and the metastable high temperature phase (HTP),
respectively, depending on the heat treatment. The samples were studied by
X-ray powder diffraction, DTA, specific-heat and magnetization measurements
with respect to temperature and magnetic field. The first order LTP to HTP
polymorphic phase transition has been determined showing the huge temperature
hysteresis of 264{\deg}C caused by the high energy barrier due to the change of
stacking of Sm, Ir and Si basal plane sheets within the transition. Both
polymorphs show indications of antiferromagnetic order at low temperatures. The
considerably different magnetic phase transitions determined for the LTP and
HTP confirm the strong influence of crystal structure symmetry on magnetism in
the two polymorphs. The magnetism in SmIr2Si2 exhibits typical features caused
by the specific behavior of Sm3+ ion characterized by energy nearness of the
ground state and first excited state and crystal field influence.
Interpretation of experimental results is corroborated by results of ab initio
electronic structure calculations.
