Interstitial fluoride ions

Friedman and Low (6) have shown that the trivalent lanthanides dissolved in the alkaline earth fluorides can be compensated by interstitial fluoride ions at either adjacent or remote sites. If the interstitial is adjacent, the crystal field of the trivalent is axial but if it is remote, the crystal field of the trivalent is cubic. Measurement of the crystal field splitting of radiation-produced divalent lanthanide ions indicate cubic symmetry 16). More recent measurements by Sabisky (20) have shown a small percentage of non-cubic sites. It is thought that the trivalent ions in the cubic symmetry are the species predominantly reduced by radiation. 

Replacing divalent calcium ions with trivalent yttrium ions in the cationic sublattice results in excess fluoride ions, which are situated in the interstitial sites of the anionic sublattice. The interstitial fluoride ion thus generated can move through both the normal and the interstitial sites, leading to an increase in ionic conductivity with increasing concentrations of the rare-earth fluoride dopant. As the mobility of the interstitial fluoride... 

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. 

It is shown in [67] for Cdo.9Ro.iF2.i (R = La-Lu, Y) that ionic conductivity (ctsook) of fluorites of various RE elements correlates with concentration of interstitial fluoride ions (Fi(48g) + Fi(32Qi + Fi(32f)2) located on the periphery of clusters. These interstitial fluoride ion concentrations were measured by means of single-crystal X-ray diffraction (XRD). 

The fluoride ion interstitials again lead to an increase in ionic conductivity. At lower temperatures this increase is modest because the interstitials aggregate into clusters, thus impeding ionic diffusion. At higher temperatures the clusters tend to dissociate, resulting in a substantial increase in conductivity. 

Ionic Conductivity (see Ionic Conductors). Pluoride anionic conductivity is observed mainly in derivatives of fluorite (Cap2) and tysonite (Lap3). If Cap2 is doped by a tervalent rare-earth metal ion, the additional fluoride ions are positioned in interstitials where they become mobile by a hopping mechanism. 

In light of the work of Ure (23) showing the conductivity of calcium fluoride being mainly caused by the motion of fluoride ion, the solid state reduction reactions have been interpreted in terms of a model which allows reduction to occur by diffusion of the interstitial, charge compensating ion out of the crystal while an electron is injected to reduce the trivalent lanthanide. Under these conditions there is no hole or electron deficiency remaining in the crystal, as in the case of radiation reduction hence the divalent species is stable. 

A number of fluorite-based systems show anion interstitials. For example, fluorite itself, CaFj, exhibits a small solid solution range withtrivalent fluorides such as YFj, to give a solid solution Ca, xYxF2+x which contains interstitial F ions. The interstitial ions are accommodated in the vacant cubic sites (see Figure 3.24c). Similarly, the fluorite-based polymorphs of PbFj and BaFj may also be doped with suitable trivalent cations to give interstitial solid solutions. 

Fig. 5.3b In the CaF2 structure (white spheres Ca, black spheres F) the fluoride ions occupy sdl the tetrahedral interstices of a formedly close-packed sublattice. The octahedral spaces are interstitial sites (see asterisk). As indicated, these are also the centres of cubes formed by regular F ions (filled-in circles). (Niggli-formulae would be (Fi(CaF8/4)6/i) for the interstitial and (VF(CaF7/4)4/i)+ for the vacancy cluster.)...

The many possible oxidation states of the actinides up to americium make the chemistry of their compounds rather extensive and complicated. Taking plutonium as an example, it exhibits oxidation states of -E 3, -E 4, +5 and -E 6, four being the most stable oxidation state. These states are all known in solution, for example Pu" as Pu ", and Pu as PuOj. PuOl" is analogous to UO , which is the stable uranium ion in solution. Each oxidation state is characterised by a different colour, for example PuOj is pink, but change of oxidation state and disproportionation can occur very readily between the various states. The chemistry in solution is also complicated by the ease of complex formation. However, plutonium can also form compounds such as oxides, carbides, nitrides and anhydrous halides which do not involve reactions in solution. Hence for example, it forms a violet fluoride, PuFj. and a brown fluoride. Pup4 a monoxide, PuO (probably an interstitial compound), and a stable dioxide, PUO2. The dioxide was the first compound of an artificial element to be separated in a weighable amount and the first to be identified by X-ray diffraction methods. 

Anion Interstitials The other mechanism by which a cation of higher charge may substitute for one of lower charge creates interstitial anions. This mechanism appears to be favored by the fluorite structure in certain cases. For example, calcium fluoride can dissolve small amounts of yttrium fluoride. The total number of cations remains constant with Ca +, ions disordered over the calcium sites. To retain electroneutrality, fluoride interstitials are created to give the solid solution formula... 

This order is valid only for the fluorides. It is contingent on the assumption that the observed distribution curves reflect a homogeneous distribution of the impurity ions in the ice lattice without formation of channel networks or interstitial zones of high solute content. 

Differential ion transfer across the phase boundary is manifest in small differences of distribution coeflBcients for the species of an ion pair. The distribution coeflBcient for a given ion depends also on the other ionic species present in solution and their concentrations. The apparent distribution coeflBcients, determined from experiment, depend on both freezing rate and concentration. Differential diffusion appears to play only a secondary role. The apparent distribution coeflBcients for potassium and cesium fluoride are higher than those for HF solutions of the same concentration. They appear to increase with concentration while those for HF decrease. The increase is perhaps explained by the formation of regions of higher concentration at cell or grain boundaries, or it may be related to the possibility that most cations enter the ice lattice interstitially rather than substitutionally. The interpretation of solute distribution curves in ice is diflBcult. 

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