Spin and lattice dynamics at the spin-reorientation transitions in the rare-earth orthoferrite <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub> <mml:mi>Sm</mml:mi> <mml:mrow> <mml:mn>0.55</mml:mn> </mml:mrow> </mml:msub

Linear and nonlinear couplings of magnetic and lattice excitations are at the heart of many fascinating magnetophononic phenomena observed in rare-earth orthoferrites, the distinctive feature of which is the tendency to spin-reorientation transitions. Here we report the results of the experimental study of the spin and lattice dynamics in the Brillouin zone center of the rare-earth orthoferrite ${\mathrm{Sm}}_{0.55}{\mathrm{Tb}}_{0.45}{\mathrm{FeO}}_{3}$ by using polarized infrared reflectivity and Raman scattering spectroscopic techniques. The obtained results were supported by the first-principles calculations, which allowed us to reliably identify the parameters of most infrared- and Raman-active phonons. We reveal the spin-reorientation transitions ${\mathrm{\ensuremath{\Gamma}}}_{4}({G}_{a}{F}_{c})\stackrel{{T}_{1}}{\ensuremath{\leftrightarrows}}{\mathrm{\ensuremath{\Gamma}}}_{24}({\mathrm{G}}_{\mathrm{ac}}{\mathrm{F}}_{\mathrm{ac}})\stackrel{{T}_{2}}{\ensuremath{\leftrightarrows}}{\mathrm{\ensuremath{\Gamma}}}_{2}({\mathrm{G}}_{\mathrm{c}}{\mathrm{F}}_{\mathrm{a}})$ at ${T}_{1}\ensuremath{\simeq}220\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ and ${T}_{2}\ensuremath{\simeq}130\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ and carefully studied the following evolution of Raman scattering on magnetic excitations at these transitions. Notably, the intermediate magnetic structure ${\mathrm{\ensuremath{\Gamma}}}_{24}$ displays an exceptionally broad temperature range $\mathrm{\ensuremath{\Delta}}T={T}_{1}\ensuremath{-}{T}_{2}\ensuremath{\simeq}90\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ in mixed ${\mathrm{Sm}}_{0.55}{\mathrm{Tb}}_{0.45}{\mathrm{FeO}}_{3}$ compared to pure rare-earth orthoferrites. We attribute this broadening of the intermediate phase to the modification of the magnetocrystalline anisotropy as a result of the inhomogeneous magnetic structure caused by the random distribution of rare-earth ${\mathrm{Sm}}^{3+}$ and ${\mathrm{Tb}}^{3+}$ ions. We found neither change in the parameters of Raman-active ${B}_{1g}$ phonons nor the appearance of new phonons induced by spin-reorientation transitions, which have been reported in ${\mathrm{SmFeO}}_{3}$. We assume that our results provide a solid basis for more deeper understanding of magnetophononic phenomena in rare-earth orthoferrites.