Bonding patterns and instabilities in the m=2 Ruddlesden–Popper phases: LaSr2Mn2O7 and La2CaCu2O6

Abstract

We present the results of high level band structure calculations on the important layered compounds LaSr₂Mn₂O₇ and La₂CaCu₂O₆ ; both of which have the structure of the m=2 member of the Ruddlesden–Popper phases and comprise intergrowths of double perovskite slabs with rock-salt slabs. The cuprate additionally has an ordered oxygen defect. For further comparison, the band structure of tetragonal La₂CuO₄ has also been calculated. The manganite is a material exhibiting giant magnetoresistance (GMR) effects and the cuprates can be doped to give superconductors. The calculations have been performed using the TB-LMTO-ASA programs within the local (spin) density approximation L(S)DA; for the manganite, spin-polarized calculations result in a magnetic ground state; calculations on the ferromagnet and two antiferromagnetic structures are compared. The calculations show up similarities and differences between the bonding patterns of these two structurally related compounds that display widely different ground states when they are doped. The instabilities on or near the Fermi surface possibly implicated in the stabilization of the different ground states are, however, quite similar in character.