Cooperative oxygen displacements

Cooperative displacements of the oxygen atoms from the middle of a (180° - 0) M—0—M bond may be superimposed on the structural modifications introduced by the mismatch of the mean equilibrium (A—O) and (M—0) bond lengths. This situation was first proposed [12] for Mn(III) t e ions in LaMnOs and is now well-established where cooperative oxygen displacements remove a ground-state orbital degeneracy at a localized-electron configuration. This situation represents a cooperative Jahn-Teller orbital-ordering distortion. 

Where the cooperative oxygen displacements are static and long-range-ordered, they can be established by a diffraction experiment. However, above the long-range ordering temperature, we can expect dynamic local cooperative... 

Fig. 4a-c. Cooperative oxygen displacements of oxygen towards one neighboring atom and away from the other found in low-temperature a CaFeOs (disproportionation) b PbTiOs (ferroelectric) c LaMnOs (Jahn-Teller orbital order). Cooperative rotations of the MOg/2 octahedra may coexist with these displacements... 

Above Tc, the conductivity of the O phase fits the adiabatic small-polaron model a - (A/T) exp(-Ea/kT) over the entire doping range [162]. As already discussed, an electron tunnels from an Mn(III) to either an Mn(IV) ion or a Zener Mn(III)—O—Mn(IV) pair in a time Zh > where is the period of the locally cooperative oxygen displacements that, for x < 0.5, dress the mobile holes as Mn(IV) ions or as Mn(III)—O—Mn(IV) pairs. Fast electron transfer (zh < occurs within a Zener polaron. 

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