Heterovalent Replacement in the Cation Sublattice

Heterovalent replacement in the cation sublattice leads to an increase in the number of anion defects, for example vacancies - M + Vp) in Ri xMxF3 x solid solutions or interstitial ions - R + Fj) in Mi xRxF2+x solids solutions. Of course, this replacement greatly increases the conductivity of doped phases and will be discussed in detail hereinafter. 

Very wide homogeneity regions of the anion-excess fluorite-like Mi xRxF2+x (0 x 0.5) solid solutions [7] are another unusual feature that will also be considered. 

For the anion-deficit tysonite-type Bii xMx(0,F)3 (M = Na, Sr, Ba) solid solutions a rather strange influence of the dimensional factor on the homogeneity region width was detected. This was an increase of M cations content, which was found from x = 0.08 (in systems with M = Na, Sr having close cation radii to the matrix cation radius) to x = 0.17 (for the large M = Ba cation) at 873 K [11,22]. 

Possibly, the homogeneity region widening with an increase of the dopant ionic radius could be explained by two factors compensating for each other - the large dopant cation and the presence of anion vacancies. The first factor leads to the unit cell expansion and the second one leads to its reduction because the larger the unit cell, the higher its capacity to adopt anion vacancies. 

However, in the Ri xMxp3 x and Mi xRxF2+x examples mentioned above, the conductivity is affected, in addition to an increase in anion defect numbers, by two other factors the size factor (due to cation replacement and anion sublattice defects) and the change of the electronic structure (due to the introduction a dopant cation). 

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