Jahn-Teller structural phase transition around 280K in LiMn2O4

The structural phase transition of spinel LiMn2O4 (Fd3m at room temperature) was investigated using thermal analysis and powder x-ray diffraction. The phase transition was observed around Tt=280K with a temperature hysteresis of about 10K. It was revealed that the low temperature phase (T < Tt) is a mixture of Fd3m (c/a = 1) and I41/amd (c/a = 1.011). The phase transformation from Fd3m to I41/amd proceeds with the decrease in temperature until the volume fraction of the I41/amd phase saturates at 65% at around 260K, just below Tt. The single phase of I41/amd never appeared even at 220K, far below Tt. This phase transition is of a first-order character and is owing to the cooperative Jahn-Teller effect of Mn3+. Cell performance utilizing the LiMn2O4 cathode is also discussed.