DENSITY-FUNCTIONAL STUDY OF LATTICE DISTORTION AND ORBITAL ORDERING IN LA1-XCA1+XMNO4

To study the correlation among lattice distortion, orbital ordering an d magnetic ordering in La-0.5 Ca1.5MnO4, we have performed total energ y calculations by using the first principles molecular dynamics method s with ultra-soft pseudo-potential within local density approximation (LDA) and LDA+U method. As results, it is found that a strong hybridiz ation between e(g) orbitals of Mn3+ and Mn4+ ions along zigzag ferroma gnetic chain leads to an insulating ground state and the orbital order ing similar to the one in La0.5Ca0.5MnO3 occurs in a way to maximize t he hybridizations. The equilibrium internal coordinates of oxygens are determined to be consistent with the orbital ordered structure. Throu gh the comparison of total energies of the AF and FM states, we found that the AF state becomes stable relative to the FM state only when th e corresponding lattice distortion is present. It is concluded that th e magnetic and orbital ordering of the ground state of La0.5Ca1.5MnO4 occurs cooperatively together with the corresponding lattice distortio ns.