Charge order transition and elastic/anelastic properties of LiMn2O4

We studied the charge ordering transition of LiMn2O4 by means of anelastic spectroscopy, measuring the Young's modulus, E', and the. elastic energy loss function, Q(-1). Our measurements confirm the occurrence of a phase transition at 296 K on cooling, showing a large hysteresis on heating. The anelastic spectroscopy results also indicate that the transformation is characterized by the coexistence of two phases. The shape of the real part of the Young's modulus seems to indicate that the elastic constant which is more strongly affected by the transition is c(44), like in the case of Fe3O4. The time evolution of E' in the coexistence region was measured. The. characteristic time, tau, of this evolution rapidly decreases from 296 to 290 K and then increases on further cooling. This behaviour of tau can possibly be related to the temperature dependence of E. The absolute value of the Young's modulus of LiMn2O4 obtained considering the effect of the porosity of the sample was estimated to be of the order of 10 GPa. Finally a new thermally activated peak, centred around 100 K at a measurement frequency of similar to I kHz, has been observed, which cannot be described by the Debye or Fuoss-Kirkwood model. This relaxation may be ascribed to the stress-induced jumps of charges from Mn3+ to Mn4+ sites, similarly to an analogous process found in Fe3O4, or to possible small movements of the octahedra and tetrahedra composing the crystal structure.