Structure and magnetic order in undoped lanthanum manganite

The undoped lanthanum manganite system of nominal composition LaMnO₃ has been analyzed by neutron powder diffraction for different sample heat treatment methods. Four distinct crystallographic phases have been identified: (i) an orthorhombic phase of space group Pnma and lattice parameters (at 300 K) a=5. 7385 (3), b=7. 7024 (3), c=5. 5378 (2), produced by annealing in a reducing atmosphere. The system develops long-range antiferromagnetic order below T₍=140 K with the Mn^3+ spins coupled ferromagnetically in the a-c plane and antiferromagnetically along b, with the spin direction along a. The volume of this phase increases monotonically with increasing temperature, but both the a and c lattice parameters exhibit negative thermal expansion in alternate temperature regimes. (ii) A second (previously unreported) orthorhombic phase that exhibits a smaller splitting, also of space group Pnma and lattice parameters (at 300 K) a=5. 4954 (3), b=7. 7854 (4), c=5. 5355 (3), produced by annealing in an oxygen (or air) atmosphere. This system orders with a simple ferromagnetic structure at T₂=140 K, with the spin direction along c. Phases (i) and (ii) can be transformed reversibly by suitable heat treatment of the same sample, and exist with a range of lattice parameters and compositions. The unit-cell volume for the antiferromagnetic phase is considerably larger than for the ferromagnetic phase, which agrees with the double-exchange model proposed for this system. (iii) A monoclinic phase of space group P112₁/a and lattice parameters (at 200 K) a=5. 4660 (4), b=7. 7616 (7), c=5. 5241 (5), =90. 909^ (5) that orders ferromagnetically below 140 K. (iv) A rhombohedral phase of space group R3-barc and hexagonal lattice parameters (at 300 K) a=5. 5259 (2), c=13. 3240 (4), that is observed only above room temperature. Occupancy refinements show that phase (i) ideally has the stoichiometric composition LaMnO₃, while the results for the Mn-O bond distances suggest that phases (ii), (iii), and (iv) are progressively richer in oxygen (and thus Mn^4+). The results of our study strongly suggest the progressive development of cation vacancies in equal numbers on the La and Mn sites as the oxygen content is increased by heat treatment. In the monoclinic phase the Mn ions occupy two crystallographically independent sites, but no evidence of ordering of Mn^3+ and Mn^4+ was observed. The structures of the four phases are closely related to that of perovskite. The MnO₆ octahedra are tilted from the undistorted configuration, the tilt system being a^-a^-a^- in the rhombohedral structure and a^+b^-b^- in both orthorhombic modifications.