An infrared spectroscopic study of the insulator–metal transition and charge-ordering in rare earth manganates, Ln_1−xA_xMnO₃ (Ln=Rare Earth, A=Ca, Sr, Pb)

Abstract

Infrared absorption spectra of rare earth manganates of the type Ln_0.7A_0.3MnO₃ (A=Pb, Sr, Ca) show that the compositions which are metallic at room temperature do not exhibit phonon bands, while the insulating compositions show sharp bands. La_0.7Ca_0.3MnO₃ shows some changes in the phonon frequencies across the insulator–metal transition, but more importantly, the temperature variation of the relative band intensities parallels that of the electric resistivity. It appears that infrared absorption in these materials occurs when the resistivity is higher than Mott's maximum metallic resistivity. Infrared absorption spectra of Nd_0.5Ca_0.5MnO₃, Y_0.5Ca_0.5MnO₃, and Nd_0.5Sr_0.5MnO₃ have been investigated through their charge-ordering transition temperatures. The band frequencies increase across the charge-ordering transition accompanied by a significant increase in the band intensities. The stretching and bending bands of the MnO₆ octahedra show splittings in the charge-ordered manganates consistent with the occurrence of octahedral distortion. Some systematics in band frequencies are found with the average radius of A-site cations in Ln_0.5A_0.5MnO₃.