An ultrasonic study on complex charge ordering phenomena for La1-xSrxFeO3

The complex charge ordering phenomena for polycrystalline La1-xSrxFeO3 (1/3 <= x <= 2/3) have been studied by measuring the low temperature magnetization, resistivity and the longitudinal ultrasonic velocity (V-1). At low doping levels (1/3 <= x <= 0.5), a dramatic velocity increase is observed below 210 K, and the relative stiffening of V-1 is proportional to the Sr concentration. The analysis suggests that this feature may correspond to the short-range charge ordering state of Fe3+ and Fe4+. At high doping levels (0.5 < x <= 2/3), the resistivity shows a sharp increase around 200 K, and the relative stiffening of V-1 decreases with increasing x. This implies that the charge disproportionation transition 2Fe(4+) -> Fe3+ + Fe5+ occurs. In particular, the V-1 of La1/3Sr2/3FeO3 undergoes another softening transition below 181 K, which is attributed to the breathing-type distortion of Fe-O octahedron in the sequence ... Fe3+Fe5+Fe3+ .... The detailed form of the sound velocity anomalies in La1-xSrxFeO3 (x = 1/3, 2/3) is different, revealing the presence of complex charge ordering phenomena. To complete the ultrasonic characterization in La1-xSrxFeO3, sound measurements were performed on La0.1Sr0.9FeO3, whose ground state is a typical antiferromagnetic order, and the anomaly is small.