Structural and chemical analysis of colossal magnetoresistance manganites by Raman spectrometry

Colossal magnetoresistance manganites in a wide composition range can be systematically characterized by Raman spectrometry with respect to the cation substitution in the A sublattice of the perovskite structure, oxygen isotope exchange, and variations of the Mn³⁺/Mn⁴⁺ ratio. The effects can be clearly separated by analysis of the experimental band shifts of the Ag(2) soft mode as a function of the tolerance factor t and a parameter V1/3t, where V is the perovskite cell volume. This allows us to structurally and chemically analyze mixed-valence manganites in the series $({La}_{1{-}x}{Pr}ₓ{)}0.7{Ca}0.3{Mn}¹⁶{O}₃, ({La}_{1{-}x}{Pr}ₓ{)}0.7{Ca}0.3{Mn}¹⁸{O}₃ (x=0--1), ({La}0.25{Pr}0.75{)}_{1{-}x}{Ca}ₓ{MnO}₃ (x=0.2--0.5), {R}_{1{-}x}{Sr}ₓ{MnO}₃$ $(x=0.33,$ 0.45, $R=Nd, {Nd}0.5{Sm}0.5,$ Sm, Eu, Gd), ${La}0.7{Sr}0.3{MnO}₃{,La}0.35{Pr}0.35{Sr}0.3{MnO}₃{,La}0.5{Ca}0.5{MnO}₃{,LaMnO}₃.$ The band shift indicates in particular the existence of doped holes on Mn sites but not on the oxygen sites, thus providing validation for the basic assumptions of the Zener double exchange model. The mechanism of lattice strain in thin films of the compounds was tested; variations of the Jahn-Teller distortion and valence angle perturbations can in this case be separated by Raman spectrometry.