Magnetic structure of the high-density single-valent eg Jahn-Teller system LaMn7 O12

We studied the structural and physical properties of powder samples of the high-pressure LaMn7 O12 compound by means of neutron diffraction, dc magnetization, specific-heat, and dc electrical resistivity measurements. Our structural refinement shows that LaMn7 O12 is the counterpart with quadruple perovskite structure of the well-known simple-perovskite LaMnO3 because both compounds are single-valent Mn3+ systems sharing a similar pseudocubic network of buckled corner-sharing MnO6 octahedra distorted by the Jahn-Teller (JT) effect. Besides this similarity, LaMn7 O12 exhibits the following structural differences: (i) a monoclinic I2/m, instead of orthorhombic Pnma, structure; (ii) a much larger buckling of the MnO6 octahedra, corresponding to a Mn-O-Mn bond angle as small as ψ=180°-φ=136°. At TN,B =78K, the neutron data show evidence of an antiferromagnetic (AFM) structure of the octahedral Mn3+ ions (B sites), which consists of ferromagnetically coupled antiferromagnetic ac planes. This structure, commonly referred to as C type, markedly differs from the A -type structure of LaMnO3, which consists of antiferromagnetically coupled ferromagnetic ac planes. We argue that this difference may be due not only to the different buckling, because the magnetic superexchange interaction is sensitive to φ, but also to the existence of two distinct JT distorted B sites, a characteristic feature of the quadruple perovskite structure not found in LaMnO3. A further feature of this structure is the presence of a pseudocubic sublattice of JT Mn3+ ions with square coordination (A′ site) in addition to the B -site sublattice. At TN, A′ =21K, the A′ sublattice of LaMn7 O12 is found to form a second AFM structure consisting of AFM coupled ferromagnetic planes. This additional ordering appears to occur independently of that of the B sublattice. Finally, we observed a thermally activated insulating behavior of the resistivity similar to that previously reported for LaMnO3, which confirms the hypothesis that the superexchange interaction is predominant in single-valent systems while the double-exchange interaction is relevant only in mixed-valence or disordered systems. © 2009 The American Physical Society.