Interstitial fluorine atom

Interstitial positions, positions in a crystal not normally occupied by an atom, are denoted by the subscript i. For example, F would represent an interstitial fluorine atom in, say, a crystal of fluorite, CaF2. 

An interconnected anion displacement occurs in the process of M i xR xp2+x solid solution formation to avoid very short F-F interatomic distances, which would arise if interstitial fluorine atoms would be statistically distributed in the fluorite structure. Then some fluorine atoms have to shift from their initial crystallographic F positions. To compensate for the charge of interstitial anions, a part of cations should be replaced by Additional fluorine atoms occupy interstitial positions only and and cations practically retain their positions in the fee lattice. 

Fig. 2 (a) The [1 0 3] and [1 0 4] clusters in CeN F3 3, and PrNjj F3 3, . Small circles represent fluorine atoms (nitrogen atoms dotted) and large circles metal atoms. Full circles are fluorine interstitials in (x,x,x) X = 0.41 and dashed circles represent an anion vacancy at (x, x, x)x = 0.2S. (b) The projection along [001] of fig. 2a. Arrows indicate relaxed fluoride positions of the anions (F" ) Numbers indicate heights in the z-coordinate. [From Vogt et al. (1989) with permission.]... 

F ", fluorine atom also displaced from the normal position in direction < 111 >. The last one is not a real interstitial position but just a relaxed position - normal with very small displacement. There are different systems for marking interstitial positions in the scientific literature [21,35,36]. 

Substitution Variants. The fluorite-type structure is maintained in principle when alkaline earth elements are replaced partially by rare-earth elements. Charge compensation is achieved by occupation of additional interstitial anionic sites.The coordination of the metal atoms may increase from 8 to 9 or even 10 by this. Another way of charge balance is the partial replacement of fluorine by oxygen to form oxyfluorides. Since the possible interstitial positions provide pathways for anion disorder and movement, this class of materials shows fluoride ionic conductivity. 

Tin Oxide (Sn02) - Tin oxide has a tetragonal rutile structure with a unit cell of six atoms. The electrical conductivity of n-doped Sn02 is due to the presence of oxygen vacancies, interstitial tin (in excess), or added dopants such as fluorine, chlorine, or antimony. Carrier concentrations can be increased to approximately 10 cm through doping from the intrinsic concentration of approximately 10 cm for Sn02 [19]. 

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