Study of spin-phonon coupling in LiFe_{1 - X}Mn_xPO₄olivines

LiFe_{1 - x}Mn_xPO₄ olivines are promising material for improved performance of Li-ion batteries. Spin-phonon coupling of LiFe_{1 - x}Mn_xPO₄ (x = 0, 0.3, 0.5) olivines is studied through temperature-dependent Raman spectroscopy. Among the observed phonon modes, the external mode at ~263 cm^-1 is directly correlated with the motions of magnetic Fe²⁺/Mn²⁺ ions. This mode displays anomalous temperature-dependent behavior near the Néel temperature, indicating a coupling of this mode with spin ordering. As Mn doping increases, the anomalous behavior becomes clearly weaker, indicating the spin-phonon coupling quickly decreases. Our analyses show that the quick decrease of spin-phonon coupling is due to decrease of the strength of spin-phonon coupling, but not change of spin-ordering feature with Mn doping. Importantly, we suggest that the low electrochemical activity of LiMnPO₄ is correlated with the weak spin-phonon coupling strength, but not with the weak ferromagnetic ground state. Our work would play an important role as a guide in improving the performances of future Li-ion batteries.