Probing Ca3Ti2O7 crystal structure at the atomic level: Insights from perturbed angular correlation spectroscopy and ab initio studies

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 Ca₃Ti₂O₇, a hybrid improper ferroelectric. The experimental procedure, conducted at ISOLDE, 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 A2₁am symmetry to an orthorhombic $Acaa$ symmetry in the Ca₃Ti₂O₇ crystal lattice, disagreeing with the frequently reported avalanche structural transition from the polar A2₁am phase to the aristotype $I4/mmm$ phase. Moreover, the EFG temperature dependency, within the A2₁am temperature stability, is shown to be sensitive to the recently proposed Ca₃Ti₂O₇ ferroelectric polarization decrease within the 500--800 K temperature range.