ISSN 1063-7761, Journal of Experimental and Theoretical Physics, 2006, Vol. 103, No. 1, pp. 54–59. © Pleiades Publishing, Inc., 2006. Original Russian Text © V.A. Khomchenko, I.O. Troyanchuk, O.S. Mantytskaya, M. Tovar, H. Szymczak, 2006, published in Zhurnal Éksperimental’noœ i Teoreticheskoœ Fiziki, 2006, Vol. 130, No. 1, pp. 64–70. 54 1. INTRODUCTION Manganese-based complex magnetic oxides exhibit a variety of physical phenomena: colossal magnetore- sistance, orbital and charge ordering, structural and magnetic phase separation, and concentration and tem- perature insulator–metal and antiferromagnet–ferro- magnet transitions [1, 2]. Their investigation is a prior- ity direction in the modern physics of condensed mat- ter. Although manganites were studied in numerous works, there exist a number of disputable questions, including the problem of the magnetic properties of bis- muth-containing manganites. The manganites LnMnO 3 of lanthanum or rare-earth elements are antiferromag- netic, whereas bismuth manganite BiMnO 3 is charac- terized by ferromagnetic ordering of the spin magnetic moments of the Mn 3+ ions. Moreover, the substitution of a rare-earth metal (Ca 2+ , Sr 2+ , Ba 2+ ) for a lanthanide, which generates tetravalent manganese ions, results in the transition from an antiferromagnetic insulating state to a ferromagnetic metallic state [3, 4]; however, simi- lar substitution for BiMnO 3 -based manganites breaks the ferromagnetic order [5, 6]. Ferromagnetism in BiMnO 3 (T C = 105 K) is consid- ered to be caused by specific ordering of the orbitals of the Mn 3+ ions [7]. The crystal structure of this com- pound has the monoclinic space group C2 and contains d z 2 three nonequivalent positions of manganese cations. Each of the three MnO 6 polyhedra is characterized by an axial elongation, which is typical of the Jahn–Teller cations in perovskite structures. The relative spatial position of the orbitals is so that two-thirds of the total number of exchange couplings are positive and that the resulting structure is ferromagnetic. The crystal structure and orbital ordering in BiMnO 3 are caused by the stereochemistry of an isolated pair of the 6s 2 elec- trons of the Bi 3+ ions [7], which plays a key role in the appearance of spontaneous dipole ordering below the ferroelectric Curie temperature T E = 770 K [8]. Unlike bismuth manganite, orbitals in LaMnO 3 undergo antiferrodistorsive ordering, which manifests itself in the O' orthorhombic structure of the crystal (space group Pnma) [9]. Antiferrodistorsive orbital ordering leads to the formation of an A-type antiferro- magnetic structure below T N = 140 K [10]. Since the types of orbital ordering and the magnetic structures of bismuth and lanthanum manganites are radically different, it is interesting to study the struc- tural characteristics and physical properties of solid solutions based on these manganites. The earlier stud- ies of bismuth-containing lanthanum manganites revealed the evolution of their magnetic and transport d z 2 d z 2 Crystalline and Magnetic Structures of La 1 – x Bi x MnO 3 + d Manganites V. A. Khomchenko a , I. O. Troyanchuk a , O. S. Mantytskaya a , M. Tovar b , and H. Szymczak c a Joint Institute of Solid State and Semiconductor Physics, National Academy of Sciences of Belarus, ul. Brovki 17, Minsk, 220072 Belarus e-mail: khomchen@ifttp.bas-net.by b Berlin Neutron Scattering Center (BENSC), Hahn-Meitner Institute, Glienicker Str. 100, Berlin, D-14109 Germany c Institute of Physics, Polish Academy of Sciences, Lotnikov str. 32/46, Warsaw, 02-668 Poland Received January 2, 2006 Abstract—The crystalline and magnetic structures and magnetic properties of La 1 – x Bi x MnO 3 + δ (0.4 ≤ x ≤ 0.6, 0 ≤ δ ≤ 0.06) manganites have been studied. The solid solutions having the stoichiometric oxygen content are shown to be orbitally ordered A-type antiferromagnets. An increase in the oxygen content above the stoichio- metric value is found to cause Mn 4+ ions in the perovskite lattice, to remove the cooperative Jahn–Teller distor- tions, and to form a long-range ferromagnetic order. This order becomes broken as the concentration of the tet- ravalent manganese ions increases further. The tendency toward breaking the ferromagnetic order increases with the bismuth content. The magnetic properties are interpreted in terms of superexchange interactions on the assumption of local lattice distortions induced by anisotropy of the 6s 2 (Bi 3+ )–2p 6 (O 2– ) chemical bonds. PACS numbers: 75.30.Kz, 75.47.Lx, 61.50.Ks DOI: 10.1134/S1063776106070077 ORDER, DISORDER, AND PHASE TRANSITIONS IN CONDENSED SYSTEMS