Jahn-Teller transition of LiMn2O4 studied by x-ray-absorption spectroscopy

The valence states and local structure of LiMn2O4 have been investigated in comparison with those of ZnMn2O4 and lambda-MnO2 by using x-ray-absorption spectroscopy on the Mn K edge, in order to study the Jahn-Teller (JT) transition in LiMn2O4 at T-a = 280 K. In the near-edge spectra, a systematic edge shift in accordance with the formal manganese valence was found, confirming the Mn3+/Mn4+ mixed states in LiMn2O4. Extended x-ray-absorption fine structure reveals a structural disorder of MnO6 octahedra in the cubic LiMn2O4 above T-a. The local structure is analyzed using a structural model in which (LiMn3+Mn4+O4) consists of isotropic Mn4+O6 octahedra and anisotropic (Mn3+O6) octahedra distorted uniaxially by the JT effect. The magnitude of the local JT distortion, the ratio of the longer Mn-O distance to the shorter one, is found to be 1.17 at 300 K, which is as large as that of tetragonal ZnMn2O4 (1.19). The local structure exhibits no change when the temperature is lowered below T-a. These results imply that the low-temperature phase is induced by ordering the local JT distortion. [S0163-1829(98)04126-5].