Ca$_3$Mn$_2$O$_7$ structural path unraveled by atomic-scale properties: A combined experimental and ab initio study

The structural phase transition path from the low-temperature polar structure up to the highest symmetric phase in the hybrid improper ferroelectric Ca$_3$Mn$_2$O$_7$ compound is here investigated at atomic scale. Measurements using the perturbed angular correlation local probe technique are combined with ab initio electronic structure calculations to observe the evolution of the electric field gradient parameters at the Ca site within the 10–1200 K temperature range. The results show that polar-phase clusters persist at temperatures as high as 500 K. In addition, evidence is given for a structural phase transition occurring above 1150 K. The high-temperature symmetry is here confirmed to be $I4/mmm$.