Hexagonal BaTiOs

Fig. 2.2 MOg octahedra arrangements in (a) perovskite-type structures, (b) Ti02 and (c) hexagonal BaTiOs.

Hexagonal BaTiOs-type ABOs-x, 0 < < Known for BaNi03 x... 

Ordered removal of metal Hexagonal BaTiOs-type Unknown Known... 

The coordination numbers of metal ions range from I. as in ion pairs such as Na CP in the vapor phase, to 12 in some mixed metal oxides. The lower limit. I. is barely within the realm of coordination chemistry, since the Na a ion pair would not normally be considered a coordination compound, and there are few other examples. Likewise, the upper limit of 12 is not partteularly important since it b rarely encountered in discrete molecules, and the treatmem of solid crystal lattices such as hexagonal BaTiO and perovskiie as coordmation compounds b not done frequently. The lowest and highest cofxdinalion numbers found in typical coordination compounds are 2 and 9 with the intermediate number 6 bemg the most importam. 

Summarizing the features of the hexagonal fluoroperovskites it should be noted, that the structures of the BaTiOs- and BaRuOs-types are but different mixed forms of both, the purely cubic perovskites, e.g. CsCdFs with 3 layers in sequence ABC, and the purely hexagonal perovskites , e.g. CsNiFs with 2 layers in sequence AB. The dimensions of the c-axes are given by the number of layers and are therefore larger in the case of the mixed structures than for the basic types (e.g. CsMnFa 6 layers, CsCoFs 9 layers). 

The complex oxide BaTiOs ( barium titanate ) is remarkable in having five crystalline forms, of which three are ferroelectric. The structure of the high-temperature hexagonal form, stable from 1460°C to the melting point (1612°C), has already been described. The other forms are ... 

It was observed that perfect dislocatioDS in the basal plane, embedded in an a-type extended planar SF hexagonal (h-) BaTiOs with a Burgers vector of bn = 1/3(1210), were dissociated into prism plane half-partials with Burgers vectors bp = l/3(0ll0). 

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. 

When the temperature is raised above 1460 °C, cubic BaTiOs performs another phase transition to a hexagonal structure. This hexagonal phase can be quenched... 

Fig. 58. BaTio jVo.gSj. Lattice parameters vs. com- Fig. 59. BaVSj. Lattice parameters vs. temperature position. T, s phase transition temperature (orthorhon bic - hexagonal) [80Wad 1].

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