Raman measurements of temperature dependencies of phonons in LiMnPO4

Research highlights: → Temperature induced changes of phonon frequencies were observed in LiMnPO4. → The Grueneisen parameters were determined for 5 phonon modes. → The change of frequency of the symmetric Agν1 mode was small. → The thermal dependencies of the antisymmetric modes (Agν3 and Agν4) were stronger. - Abstract: We present results of Raman and infrared absorption spectroscopy research on phonons in LiMnPO4-a new material for high capacitance rechargeable lithium-ion batteries. There is a significant interest in the structural and electrical properties of this material, because the battery performance depends strongly on the rate of lithium diffusion. Nanopowder of LiMnPO4 was obtained via a modified sol-gel method from salts of lithium and manganese. This method is cheap and effective so it is promising for the most popular applications. The material showed sharp phonon peaks in Raman and infrared spectra. In the Raman spectra, the strongest peak was Agν1 mode at energy 117.77 meV (950.1 cm-1), at 4 K. At room temperature, its energy decreased (due to phonon-phonon interaction) to 117.5 meV (947.5 cm-1). The Grueneisen parameter found for this oscillation mode was relatively low, γAgν1=0.5, at about 300 K. Since the mode consisted mainly of the symmetric PO4 tetrahedra oscillations, the low γAgν1 value indicated that the temperature influenced rather Li-O and Mn-O bonds than the P-O bonds forming the LiMnPO4 structure. The thermal dependencies of the antisymmetric modes (Agν3 and Agν4) were stronger (γAgν3=0.7, γAgν4=1.4) what suggested that these modes experienced stronger coupling. The thermal broadening of the Agν1 mode could be described in wide temperature range by exponential dependence with activation energy of 65 meV (about two times smaller than the Agν1 energy), what suggested a symmetric two-phonon decay.