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. 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 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.
@article{arxiv.2402.09945,
title = {Probing $Ca_3Ti_2O_7$ crystal structure at the atomic level: Insights from $^{111m}Cd/^{111}Cd$ PAC spectroscopy and ab-initio studies},
author = {P. Rocha-Rodrigues and I. P. Miranda and S. S. M. Santos and G. N. P. Oliveira and T. Leal and M. L. Marcondes and J. G. Correia and L. V. C. Assali and H. M. Petrilli and J. P. Araújo and A. M. L. Lopes},
journal= {arXiv preprint arXiv:2402.09945},
year = {2025}
}