Influence of Bi³⁺ substitution on spin-phonon coupling in La_0.7Sr_0.3MnO₃ Manganites: A Raman spectroscopy study

Interesting structural and physical properties observed in La_0.7-xBi_xSr_0.3MnO₃ are attributable to the Bi³⁺ concentration (x). In order to understand the role played by Bi³⁺ substitution in influencing lattice dynamics and magnetic ground state, we carried out Raman spectroscopy studies on a series of La_0.7-xBi_xSr_0.3MnO₃ (x = 0.0 – 0.70) samples. As the Bi³⁺ concentration increases from x = 0.0 to 0.30, the magnetic ground state transforms from dominantly ferromagnetic metallic to the coexistence of ferro and antiferromagnetic phases. Correspondingly a significant increase in the number of Raman modes has been noticed, which specifies a transition in the crystal structure. The Raman spectra of ferromagnetic La_0.7Sr_0.3MnO₃ (i.e., x = 0.0) has been accounted for considering the rhombohedral symmetry (D_3d⁶). Whereas for Bi³⁺ substituted samples (i.e., x ≥ 0.30), the Raman spectra show the phonon modes corresponding to orthorhombic (D_2h¹⁶) symmetry. To compare with experimental values, theoretical Raman modes were calculated using Quantum Espresso code. The variation in the Raman spectra with Bi³⁺ substitution suggests a strong influence of lattice distortion on the electrical and magnetic behavior, highlighting a strong structure-property correlation in the system. Additionally, phonon modes show softening across the magnetic ordering temperature indicating the spin-phonon coupling of the system. The the theortical frame work using Balkanski model confirms the above conclusion.