We have studied electric-field-induced symmetry lowering in the tetragonal (001)-oriented heteroepitaxial (Ba$_{0.8}$Sr$_{0.2}$)TiO$_3$ thin film deposited on (001)MgO substrate. Polarized micro-Raman spectra were recorded from the film area in between two planar electrodes deposited on the film surface. Presence of textit{c}-domains with polarization normal to the substrate was confirmed from polarized Raman study under zero field, while splitting and hardening of the textit{E}(TO) soft mode and polarization changes in the Raman spectra suggest monoclinic symmetry under external electric field.
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