  Perturbed angular correlation spectroscopy combined with $ab-initio$
electronic structure calculations is used to unravel the structural phase
transition path from the low-temperature polar structure to the
high-temperature structural phase in $Ca_3Ti_2O_7$, a hybrid improper
ferroelectric. This procedure explores the unique features of a local probe
environment approach by monitoring the evolution of the electric field gradient
tensor at the calcium sites. The local environments, observed above 1057 K,
confirm a structural phase transition from the $A2_1am$ symmetry to an
orthorhombic $Acaa$ symmetry in the $Ca_3Ti_2O_7$ crystal lattice, disagreeing
with the frequently reported avalanche structural transition from the polar
$A2_1am$ phase to the aristotype $I4/mmm$ phase. Moreover, the EFG temperature
dependency, within the $A2_1am$ temperature stability, is shown to be sensitive
to the recently proposed $Ca_3Ti_2O_7$ ferroelectric polarization decrease
within the 500-800~K temperature range.
