Jahn-Teller transition ofLiMn2O4studied by x-ray-absorption spectroscopy

The valence states and local structure of ${\mathrm{LiMn}}_2{\mathrm{O}}_4$ have been investigated in comparison with those of ${\mathrm{ZnMn}}_2{\mathrm{O}}_4$ and $\lambda -{\mathrm{MnO}}_2$ by using x-ray-absorption spectroscopy on the Mn $K$ edge, in order to study the Jahn-Teller (JT) transition in ${\mathrm{LiMn}}_2{\mathrm{O}}_4$ at $T_{\mathrm{a}}=280\mathrm{}\mathrm{K}.$ In the near-edge spectra, a systematic edge shift in accordance with the formal manganese valence was found, confirming the ${\mathrm{Mn}}^{3+}{/\mathrm{M}\mathrm{n}}^{4+}$ mixed states in ${\mathrm{LiMn}}_2{\mathrm{O}}_4.$ Extended x-ray-absorption fine structure reveals a structural disorder of ${\mathrm{MnO}}_6$ octahedra in the cubic ${\mathrm{LiMn}}_2{\mathrm{O}}_4$ above $T_{\mathrm{a}}.$ The local structure is analyzed using a structural model in which ${\mathrm{LiMn}}^{3+}{\mathrm{Mn}}^{4+}{\mathrm{O}}_4$ consists of isotropic ${\mathrm{Mn}}^{4+}{\mathrm{O}}_6$ octahedra and anisotropic ${\mathrm{Mn}}^{3+}{\mathrm{O}}_6$ 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 ${\mathrm{ZnMn}}_2{\mathrm{O}}_4$ (1.19). The local structure exhibits no change when the temperature is lowered below $T_{\mathrm{a}}.$ These results imply that the low-temperature phase is induced by ordering the local JT distortion.