Ultrasonic study on the Jahn–Teller effect near different phase transitions in La1/3Sr2/3MO3 (M=Mn, Fe, Co)

The longitudinal ultrasonic velocity (Vl), magnetization and resistivity of polycrystalline La1/3Sr2/3MO3 (M=Mn, Fe, Co) have been measured between 20 and 300 K. Dramatic anomalies in Vl were observed near the temperature of the charge-ordering transition (CO), charge disproportionation transition (CD), and ferromagnetic transition (FM), which are explained by the Jahn–Teller effect originating from the M ions (M=Mn, Fe, Co). However, the detailed form of these anomalies is different, which is strongly depended on the M ionʹs unique electronic structure. For La1/3Sr2/3MO3 (M=Mn, Fe), the Vl exhibits a valley around the CO transition temperature because of the localization of the Jahn–Teller active ions (Mn3+, Fe4+). And due to the instability of Fe4+, the CD transition occurs in La1/3Sr2/3FeO3, which results in another softening in Vl, while only normal increase is observed in La1/3Sr2/3MnO3. For La1/3Sr2/3CoO3, the local lattice distortion via the Jahn–Teller effect of intermediate spin Co3+ leads to the velocity anomaly in the ferromagnetic metallic state.