Phase transitions and Raman spectra correlations in ${\mathrm{La}}_{2-\mathbf{x}}{\mathrm{Sr}}_{\mathbf{x}}{\mathrm{CuO}}_4(0.0\mathrm{}<~\mathbf{x}<~0.45)$

The doping (x) and temperature dependence of the structural parameters and Raman spectra of high-quality ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ polycrystalline compounds with $x=0.0\mathrm{}–0.45$ have been investigated. For $x=0\mathrm{}$ the squared frequency and the linewidth of the soft mode attributed to the tilting vibrations of the ${\mathrm{CuO}}_6$ octahedra about the (110) axis vary linearly with the laser-induced temperature rise. A similar behavior is observed at low temperatures for $x>~0$ that correlates the softening of this mode with the tetragonal-to-orthorhombic transition for the Sr-doped compounds. Changes in the apex oxygen phonon characteristics are correlated with the tetragonal-to-orthorhombic phase transition. At room temperature the relative intensity of the La/Sr mode and another mode at ∼150 ${\mathrm{cm}}^{\mathrm{-}1}$ when compared to the apex one follows very closely the variation of the superconducting transition temperature with Sr content. This correlation is attributed to a charge redistribution at room temperature that affects the transition temperature and the bonding orbitals of the La/Sr, the apex, and the ∼150 ${\mathrm{cm}}^{\mathrm{-}1}$ phonons.