Group Theory Analysis to Study Phase Transitions of Quasi-2D Sr3Hf2O7

Abstract

We present an ab-initio study performed in the framework of density functional theory, group-subgroup symmetry analysis and lattice dynamics, to probe the octahedral distortions, which occur during the structural phase transitions of the quasi-2D layered perovskite Sr3Hf2O7 compound. Such a system is characterized by a high-temperature I4/mmm centrosymmetric structure and a ground-state Cmc21 ferroelectric phase. We have probed potential candidate polymorphs that may form the I4/mmm → Cmc21 transition pathways, namely Fmm2, Ccce, Cmca and Cmcm. We found that the band gap widths increase as the symmetry decreases, with the ground-state structure presenting the largest gap width (∼5.95 eV). By probing the Partial Density of States, we observe a direct relation regarding the tilts and rotations of the oxygen perovskite cages as the transition occurs; these show large variations mostly of the O p-states which contribute mostly to the valence band maximum. Moreover, by analyzing the hyperfine parameters, namely the Electric Field Gradients and asymmetric parameters, we observe variations as the transition occurs, from which it is possible to identify the most plausible intermediate phases. We have also computed the macroscopic polarization and confirm that the Cmc21 phase is ferroelectric with a value of spontaneous polarization of 0.0478 C/m2. The ferroelectricity of the ground-state Cmc21 system arises due to a second order parameter related to the coupling of the rotation and tilts of the O perovskite cages together with the Sr displacements.

Pressure-Induced Phase Transformations of Quasi-2D Sr3Hf2O7

“…14 In addition to HIF, the octahedral reorientations in layered perovskites are of continuing interest because these effects also trigger many interesting electronic properties, such as high-T c superconductivity, colossal magnetoresistance, metal-insulator transitions, and magnetic ordering. 15 Recent theoretical works from Liu et al 16 and da Silva et al 17 confirmed that Sr 3 Hf 2 O 7 (SHO), a n = 2 quasi-2D RP layered perovskite system, with a ferroelectric ground state Cmc2 1 (S.G. n. 36), is similar to the previously reported HIF systems, such as Ca 2 Mn 3 O 7 and Ca 2 Ti 3 O 7 , in ref 15. SHO is also characterized by a high-temperature I4/mmm (S.G. n. 139) centrosymmetric paraelectric structure.…”

“…Such a transition can occur by applying an external perturbation, such as pressure, in order to induce the condensation of the X 1 − zone boundary mode from the centrosymmetric system. 17 We show in Figure 1 the four structural phases we have studied in this investigation. They are:…”

“…The high-symmetry structure is centrosymmetric and belongs to the I4/mmm space group (S. G. n. 139) . By decreasing the temperature, lower symmetry structural phases may be generated by inducing tiltings and/or rotations of the O octahedra cages.…”

“…Since the Ccce phase is not related by group-symmetry analysis to the I4/mmm → Cmc 2 1 transition and there is no single mode connecting the phases, we infer that this structural phase may occur as a first-order phase transition. Such a transition can occur by applying an external perturbation, such as pressure, in order to induce the condensation of the X 1 – zone boundary mode from the centrosymmetric system …”

“…SHO is also characterized by a high-temperature I4/mmm (S.G. n. 139) centrosymmetric paraelectric structure. Previous studies showed that by applying group theoretical analysis, it was possible to probe potential intermediate polymorphs that form as the temperature is lowered, enabling a transition pathway as I4/mmm → Cmcm (S.G. n. 63) → Cmca (S.G. n. 64) → Cmc 2 1 (S. G. n. 36). , The Ccce (S.G. n. 68) structural phase is a possible polymorph; however, it is not evidenced as being an intermediate structure, occurring for these structural RP symmetries, according to group theoretical analysisa condensation of the X 1 – zone boundary mode would have to occur upon symmetry breaking of the centrosymmetric system. It was also found by computing the phonon dispersion curves of the Ccce phase that the system is dynamically unstable at 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.…”