  The phase diagram of Sr_1-xEu_xTiO_3 is determined experimentally by electron
paramagnetic resonance and resistivity measurements and analyzed theoretically
within the self-consistent phonon approximation as a function of x
([0.03-1.0]). The transition temperature of the structural instability of the
system increases nonlinearly to higher temperatures with increasing x. This is
interpreted theoretically by a substantial alteration in the dynamics caused by
a change in the double-well potential from broad and shallow to narrow and
deep.
