Evidence of a splitting of the Mn-O distance and of a large lattice disorder in the charge-ordered phase of LiMn₂O₄ obtained by EXAFS

We measured the extended x-ray-absorption fine-structure (EXAFS) spectrum of LiMn₂O₄ below room temperature in the charge-ordered phase and for comparison at room temperature in the cubic phase. By means of a standard fit procedure we verified that, as reported by neutron-scattering experiments, also at the local level there are two different Mn-O distances below room temperature, which correspond to the surroundings of well-defined Mn³⁺ and Mn⁴⁺ ions. This result is different from the ones obtained from previous EXAFS measurements and confirms the physical picture of the phase transition caused by the ordering of charges in contrast to a cooperative Jahn-Teller phenomenon. Moreover a large lattice disorder in the charge-ordered state, which determines a significant static contribution to the EXAFS Debye-Waller factor, has been found. This last result can be considered as the EXAFS spectral mark of charge-order transitions, even in those materials in which there is no clear evidence of the splitting of bond lengths.