Stability of Jahn-Teller distortion ordering in LaMn_(1-x)Sc_xO_3

摘要

We have investigated the role of Mn~(3+) Jahn-Teller distortion in driving ferromagnetism in the LaMn_(1-x)Sc_xO_3 series. The replacement of Mn by Sc in LaMnO_3 decreases the orthorhombic distortion of the Pbnm cell, but the unit cell remains distorted even in the LaScO_3 sample. The analysis of the x-ray diffraction patterns indicates a continuous evolution from the typical Jahn-Teller distorted octahedron in LaMnO_3 into a nearly regular one in LaScO_3. Surprisingly, x-ray absorption spectroscopy measurements at the Mn and Sc K edges reveal the stability of both Jahn-Teller distorted MnO_6 octahedron and nearly regular ScO_6 octahedron along the whole Sc-substituted series. Moreover, the structure is described as a random distribution of Jahn-Teller distorted MnO_6 octahedra spatially ordered as in LaMnO_3 and nearly regular ScO_6 octahedra. This result contrasts with the pseudocubic phase and the appearance of regular MnO_6 octahedra in LaMn_(1-x)Ga_xO_3 for x ＞ 0.5. Thus the occurrence of Jahn-Teller distortion strongly depends on the distorted orthorhombic crystallographic structure of the matrix in which the Mn~(3+) atom is allocated. Besides, a ferromagnetic ground state is observed for x ＞ 0.5 in both series independently of the presence (or not) of Jahn-Teller distortions around Mn~(3+), which discards either the spin flipping or the vibronic superexchange models proposed for the ferromagnetism in LaMn_(1-x)B_xO_3 (B = Sc or Ga).