Vibrational study to analyze the A-site cation disordering in four-layered Aurivillius oxides ABi4Ti4O15 (ACa, Sr, Ba)

Abstract

The Raman and infrared (IR) wavenumbers were computed for the Aurivillius structure ABi₄Ti₄O₁₅ (ABT) (ACa, Sr, Ba) in an orthorhombic space group (A21am, No. 36, C₁₂^2v) using normal coordinate analysis. This study aimed to investigate the impact of A-site cations on vibrational phonons. The analysis of zone center phonons primarily involved numerous stretching and bending bonds, serving as force constants. These force constants were utilized to assign the calculated wavenumbers in the examined phase for the first time. The theoretical findings in this paper exhibited a favorable correlation with the wavenumbers reported in the literature. Comparisons of force constants, bond lengths, and wavenumbers were conducted to elucidate A-site cation disordering in these intricate compounds. The outcomes indicated that, in the studied complexes, the Sr atom exhibited an ideal radius for fitting into the structure. The mass of the A-site cations was identified as a factor contributing to tilts in the octahedra. An additional analysis was carried out to assess the impact of A-cations on both the affected equatorial and axial bonds, providing a clearer understanding of the structure. Outer octahedra displayed greater sensitivity to A-site cations compared with inner octahedra. A study of potential energy distribution was also conducted in this work to determine the significance of each force constant in all calculated wavenumbers. It was observed that higher wavenumbers were predominantly influenced by vibrations in oxygen atoms, while lower wavenumbers were mainly affected by A-site cations. Notably, overdamping was observed in the lowest frequency of the BBT compound.