The undoped, lightly doped and defective LaMnO₃ can be observed in four basic phases: orthorhombic O^*, orthorhombic O', rhombohedral R and monoclinic P. The differences in structure of these phases are governed by the [MnO₆] rotations and Jahn-Teller distortions. That is why the elastic properties of these phases are expected to differ strongly. The experimental investigation of elastic properties of high doped manganites shows step like changes in both elastic and sound propagation properties near structural phase transitions [1].

For elucidating the role of microstructure and JT interaction influence on elastic properties of lightly doped manganites we performed a calculation of elastic constants.
The calculation and microstructure analyses were performed in interionic pair potential and shell model approximation explicitly allowing for the JT term both in crystal energy and dynamical matrix [2]. The structure of each phase was obtained by crystal energy optimization procedure. The calculation of elastic module in O' and P phase was performed within static cooperative JT effect approximation, whereas cooperative dynamical JT effect approximation in R phase was used.

The calculation revealed some interesting features like presence of a giant anisotropy of elastic modulus in orthorhombic O* phase, i.e. (C₁₁-C₃₃)/(C₁₁+C₃₃) = -49%. The origin of such anisotropy is anomalous interaction between apex oxygen and lanthanum shifts, which is enhanced strongly by cooperative static JT effect. The giant anisotropy appears experimentally as anisotropic compressibility of orthorhombic LaMnO₃ [3].

This work was partially supported by CRDF grant REC-005 and RFBR U02-02-96412.
[1] T.W. Darling, A. Migliori et al, Phys. Rev., B57, No. 9, p. 5093, 1998
[2] A.E. Nikiforov, S.E. Popov, Phys. Solid. State, v43, No 6, p 1132, 2001
[3] I. Loa , P. Adler et al, Phys. Rev. Lett., V87, No. 12, p 125501, 2001

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