Speaker
Description
We present an ab-initio study performed by means of Density Functional Theory (DFT) and lattice dynamics to probe the octahedral distortions, which occur during the structural phase transitions of the quasi-2D layered perovskite Sr$_3$Hf$_2$O$_7$ compound. Such a system is characterized by a high-temperature I4/mmm (space group n. 139) centro-symmetric structure and a ground-state Cmc2$_1$ (space group n. 36) ferroelectric phase. We have probed potential candidate polymorphs that may form the I4/mmm towards the Cmc2$_1$ transition pathways [1] from which the lower symmetry structural phases may be generated by inducing tiltings and/or rotations of the O octahedral cages. We mainly focus our attention to the Ccce (space group n. 68) structural phase, since it has been experimentally evidenced in systems with similar stoichiometry, i.e. Ca$_3$Mn$_2$O$_7$ [2]. This phase may occur through a first-order phase transition when temperature decreases towards room temperature, breaking the center-of-symmetry of the tetragonal phase. By observing the phonon dispersion curves of the Ccce phase [1] we find that the system is dynamically unstable at the given conditions of the calculation (0 K and 0 GPa), evidencing negative phonon modes localized at two of the high symmetry points of the Brillouin-zone (BZ): Γ- and Y-points.
As a continuation of the work done so far in Sr$_3$Hf$_2$O$_7$, we apply an external perturbation to study, from a theoretical perspective, the possibility of stabilizing the respective Ccce phase at room conditions. Pressure is an important thermodynamic variable which enables the understanding of the properties of materials, even at room pressure, since it allows for a precise control over the interatomic distances and hence the atomic interactions. We will hence show in this work the variation of the structural, vibrational and electronic properties of the Ccce structure as a function of applied hydrostatic pressure.
Acknowledgements
This research was supported by the project NECL under NORTE-01-0145-FEDER-022096, Projects CCO 30420916, POCI-01-0145-FEDER-029454, POCI-01-0145-FEDER-032527 and CERN/FIS-PAR/0005/2017. The authors acknowledge computing resources from GRID@FEUP and Advanced Computing Projects (FCT/CPCA/2020/01), CPCA/A2/7087/2020.
References
[1] E. Lora da Silva et al., Nanomaterials 2021, 11, 897. https://doi.org/10.3390/nano11040897
[2] P. Rocha-Rodrigues, et al. Phys. Rev. B 101, 064103 (2020).
