Hexagonal manganites

These materials have the ilMnOs R = Sc or small, rare earth cation) stoichiometry,and have been erroneously referred to as hexagonal perovskites. The compounds do not exhibit the perovskite structure. The Mn cations are not octahedrally coordinated, rather the cation is surrounded by five oxide anions in a trigonal prismatic coordination environment. Also the R cations are not 12-coordinate, as would be the case in a perovskite, but are in seven-fold coordination. The materials are multi-ferroic, with anti-ferromagnetic and ferroelectric properties.The nature of the polarity and therefore the ferroelectric behaviour was only recently described. Careful structural studies indicated that although the dipole moments are attributable to the R-O bonds and not the Mn-O bonds, the R-cations are not directly responsible for the ferroelectric behaviour. The noncentrosymmetry is attributable to the tilting of the MnOs polyhedra, which in conjunction with the dipole moments in the R-O bonds results in ferroelectric behaviour. Thus the ferroelectric behaviour in these materials is termed improper and occurs by a much different mechanism than BaTiOs or even BiFeOs. 

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The three-state extended pseudo spin model is equivalent to the Potts model well-known in solid state physics. Its application to JT structural phase transitions was developed by Hock et al. [63]. Also, as a useful tool, it was mentioned by Kugel and Khomskii [9]. It was applied to hexagonal perovskites by Crama and Maaskant [64]. It has multiple applications to layered manganites (e.g., see [65]). 

The structure of the Rh-doped lanthanum manganite is modified when stabilised in the reduced state the XRD patterns corresponds now to the stoichiometric form LaMn03.oo (orthoihombic, ICDD n° 35-1353), which is slightly different fiom that of (hexagonal, thombohedrally... 

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