Charge disproportionation in La1-xCaxFeO3−δ (x = 0.4 and 0.5) investigated by mechanical spectroscopy

The nature of charge disproportionation in orthorhombic lanthanum orthoferrites La1-xCaxFeO3-δ (x = 0.4 and 0.5) has been investigated by kilohertz mechanical spectroscopy. Contrast to rhombohedral La1-xSrxFeO3-δ (x = 0.5), no elastic softening was observed in La1-xCaxFeO3-δ above charge disproportionation. This different behavior of elastic modulus in La1-xCaxFeO3-δ and La1-xSrxFeO3-δ unveils the implicit orbital effect. Below charge disproportionation, an internal friction peak around 140 K was observed in La1-xCaxFeO3-δ, which is similar to that in La1-xSrxFeO3-δ. This peak is ascribed to an elastic manifestation of charge freezing and can be used as an indirect indication of CD in doped LaFeO3.