Ordered Fluorite-like Phases

The fact that almost all ions are concentrated in superclusters allows the crystal matrix to keep its structure similar to the initial one in a wide concentration interval. 

In all these phases, the cation position is split. It means that both M and R cations are ordered in the crystal lattice only in classical cation positions of the fluorite stmcmre. The main difference between initial and ordered phases consists in nonequivalence of anions packages in the crystal structure. 

Practically all the structures of the ordered phases studied by now, in fluoride systems with RE elements, belong to the class of phases with ordered cuboctahedral clusters. The position in the centre of cuboctahedron can be occupied by an anion (8 12 1) or can be vacant (8 12 0). In oxyfluoride ordered phases, the position in the centre of cuboctahedron is usually vacant, as for instance in PbZr3Fe04 [60], PbgY6p320 [61], Ba2.iBio.9(0,F)6.g [59]. 

Formula Crystal System Space group Parameters R m, M Ref. 

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The presence of defects clusters in disordered solid solutions was confirmed by direct experimental methods (neutron diffraction, F NMR, EXAFS, etc.) as well as indirectly, using results of X-ray and electron diffraction studies of ordered fluorite-like phases having close compositions [7]. 

Table 14.3 Compositions and unit cell parameters for some ordered fluorine-containing fluorite-like phases...

Figure 14.16 Solid solution in the BiF3-Bi203-BaF2 system at 873 K [34],R Ba4Bi3pi7 based phase, (3 (3-BiOyF3 2y based phase 0 fluorite-like BaF2 based solid solution, I, II tysonite-like solid solutions. III ordered tysonite-like phase BiOyFs 2y(0.13

The ionic conductivity of fluorite-like phases is connected to two types of anionic sublattice stnictural defects excess interstitial fluoride ions Fj and vacancies in regular positions Vp. The high defect concentration increases the conductivity. The introduction of excess fluoride ions into fluorite matrix by means of the heterovalent replacement leads to an increase in both the concentration and the mobility of charge carriers. As a result the conductivity can be increased by several orders of magnitude (Figure 14.18 [63]). In this figure and further in this section, MF2-RF3 solid solutions will be considered as examples. 

The following clustering and probably local cluster ordering (similar to fluorites, see Section 14.5.2) in the structure leads to a decrease of the charge carrier concentration and to a blocking of a part of the conductivity channels. It negatively affects the transport properties of these phases. AU these factors lead to the appearance of a maximum on the curve of conductivity, a=f(x) atx = 0.05 (Figure 14.38). Similar extremes were observed on concentration dependences of transfer characteristics of tysonite-like MxRi xF3 2x (M = Ca, Sr, R = La-Lu) solid solutions [115]. 

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