Oxygen vacancy mediated local symmetry breaking drives intense Raman modes in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>BaZrO</mml:mi> <mml:mn>3</mml:mn> </mml:msub> </mml:math>
Avinash Agrawal, Poonam Mathur, Priti Kumar Roy, Rajashri Urkude, Isha, Pragati Sharma, M. N. Singh, Aniket Chowdhury, Shovan Kumar Majumder, Arvind Yogi, V. G. Sathe, Uday Deshpande, Debalaya Sarker
Abstract
${\mathrm{BaZrO}}_{3}$ is known to be one of the very few perovskites that retains its ideal centrosymmetric $Pm\overline{3}m$ structure down to 2 K, despite showing intense Raman modes over the entire temperature range. Arguably, the Raman signals are either due to localized octahedral tilts/phase transition or due to second-order Raman-active phonon modes. By combining ab initio atomistic thermodynamics within the framework of density functional theory and experimental observations, we show that local disorder mediated by charged oxygen vacancy lifts this inherent cubic symmetry. The vacancies become more stable at real growth conditions viz. higher temperatures and lower oxygen partial pressures and a pseudo ${a}^{0}{a}^{0}{c}^{\ensuremath{-}}$ tetragonal-type structure develops in the short-range vicinity. However, the average long-range structure of ${\mathrm{BaZrO}}_{3}$, as determined by Bragg's diffraction, remains cubic. The defect states manifest as a shallow donor level below the conduction band minima in the electronic density of states and are responsible for a secondary transition at $\ensuremath{\approx}3.86$ eV in the UV-vis spectra. Our O $1s$ x-ray photoelectron spectra and radial distribution function analysis from extended x-ray absorption fine-structure, further validate the presence of stable anion vacancies in this perovskite. Thus, the key to the mystery of the intense Raman signal in this otherwise long-range-ordered ``centrosymmetric'' perovskite is related to local structural disorder created by lattice oxygen vacancies.