| Title: |
First-principles soft-mode lattice dynamics of PbZr₀.₅Ti₀.₅O₃ and shortcomings of the virtual crystal approximation |
| Authors: |
Baker, JS; Bowler, DR |
| Source: |
Physical Review B , 100 (22) , Article 224305. (2019) |
| Publisher Information: |
AMER PHYSICAL SOC |
| Publication Year: |
2019 |
| Collection: |
University College London: UCL Discovery |
| Description: |
A comparative study of PbTiO 3 , PbZrO 3 , and the solid solution PbZr 0.5 Ti 0.5 O 3 is performed on the soft-mode lattice dynamics within the first Brillouin zone. We consider the six unique B-site orderings for PbZr 0.5 Ti 0.5 O 3 representable within the 2 × 2 × 2 primitive perovskite supercell as well as the virtual crystal approximation (VCA) to extract the phonon dispersion relations of a high-symmetry cubic-constrained form using density functional perturbation theory. We find that the most unstable modes in the rock-salt ordered structure and the VCA, like pure PbZrO 3 , are antiferrodistortive (AFD) while lower symmetry arrangements are dominated by Γ -point ferroelectric (FE) instabilities like pure PbTiO 3 . Despite similarities in the phonon dispersion relations between the rock-salt ordered supercell and the VCA, the character of modes at high symmetry points are found to be different. In particular, the a 0 a 0 c − and a 0 a 0 c + AFD instabilities of the rock-salt ordering are replaced with a − b − c − and a + b + c + instabilities within the VCA. Such a rotation pattern is not seen in any of the supercell-based calculations, thus serving as a quantitative example of the inability of the method to accurately represent local structural distortions. Single modes are found exhibiting dual order parameters. At the zone center, some arrangements show mixed FE and antipolar soft modes (due to Pb motion transverse to the polar axis), and at long wavelengths all arrangements have soft modes of a mixed antipolar and AFD character. These are described with direct analysis of the eigendisplacements. |
| Document Type: |
article in journal/newspaper |
| File Description: |
text |
| Language: |
English |
| Relation: |
https://discovery.ucl.ac.uk/id/eprint/10088885/1/PhysRevB.100.224305.pdf; https://discovery.ucl.ac.uk/id/eprint/10088885/ |
| Availability: |
https://discovery.ucl.ac.uk/id/eprint/10088885/1/PhysRevB.100.224305.pdf; https://discovery.ucl.ac.uk/id/eprint/10088885/ |
| Rights: |
open |
| Accession Number: |
edsbas.1DB9FBBA |
| Database: |
BASE |