  We present the results of the high-temperature neutron and x-ray diffraction
experiments on the Ca3-xSrxTi2O7 (x = 0.5, 0.8, 0.85, 0.9) compounds. The
ferro- to paraelectric transition in these hybrid improper ferroelectric
materials arises from the so-called trilinear coupling. Depending on the
Strontium content, various structures and phase transitions, different from
theoretical predictions, emerge. The in-situ x-ray powder diffraction indicates
a direct ferro- to paraelectric transition between the orthorhombic A21am and
tetragonal undistorted I4/mmm phase for x < 0.6. We identified a reduction in
the trilinear coupling robustness by increasing the Sr-doping level to lead to
the emergence of the intermediate tetragonal P42/mnm phase and the gradual
suppression of the orthorhombic phase. The observed character of the structure
transitions and the Ca3-xSrxTi2O7 phase diagram are discussed in the framework
of theoretical models of other related hybrid improper ferroelectric systems.
