Elastic/anelastic behaviour during the phase transition in spinel LiMn₂O₄

Both the Young`s modulus sound velocity (VE) and the internal friction (Q-1) have been measured for a polycrystalline sample of LiMn2O4 in the temperature range between 35 and 350 K, using a vibrating-reed technique. The resonance frequency (fr) employed for the measurement ranged from 0.81 kHz to 2.69 kHz at 300 K. At fr(300 K)=0.81 kHz, as temperature is lowered from 350 K, VE decreases down to 225 K (TA) Then, VE increases rapidly but monotonically with lowering temperature, though the magnitude of the slope (-dVE/dT) reduces. The overall increase in the velocity between TA and 100 K is about 68%. In the Q-1-versus-T curve, a large peak is clearly observed at around 210 K, as is the forming of a large shoulder on the low-temperature side; additionally, there exists a small Q-1 peak at around 120 K, though VE indicates no significant change at around 120 K. Furthermore, both the VE-versus-T curve and the Q-1-versus-T curve shift toward the higher-temperature side, by ~45 K and ~25 K, as fr(300 K) is increased from 1.32 kHz to 2.69 kHz. The observed elastic anomalies suggest that a phase transition occurs at temperatures below 290 K. This phase transition is probably due to an ordering of Mn3+ and Mn4+ ions on the octahedral 16d site in the spinel lattice. On the other hand, the origin of the Q-1 peak is considered to lie in a stress-induced relaxation of the distributions of Mn ions or Li ions.