NASICON framework structure

More recently, lithium vanadium phosphates (LisV2-(P04)s and Li3FeV(P04)3, with open NASICON framework structures, have also been studied. Reversible electrochemical lithium deintercalation/re-intercalation at a higher potential (in comparison to the couples seen for the oxides) of between 3... 

XIV- Complex Oxides with the sodalite and Nasicon Framework Structure, Br. Ceram. Trans. /., 90 (1991) 64-69,... 

The use of framework structures to minimize AH for alkali-ion electrolytes has been demonstrated to provide a means of opening up the bottlenecks to cation motion in a number of oxides (Goodenough, Hong and Kafalas, 1976). Framework structures may provide one-dimensional tunnels as in hollandite, two-dimensional transport in planes as in the )S-aluminas, or three-dimensional transport as in NASICON and LISICON. Since one-dimensional tunnels are readily blocked, the two-and three-dimensional conductors are the more interesting. 

A novel LiNiV04 positive material, having a voltage of approximately 4.8 with respect to Li, has been characterized and proposed for high voltage lithium ion batteries. However, its practical use may be limited by the lack of electrolytes capable of withstanding its high oxidation potential. Other recent developments include NASICON-related framework structures, such as a Fe2(S04)3-based compound. 

Fig. 3.8 NASICON (NA Superionic CONductor) framework structure, which is the same as that of hexagonal Fe2(S04)3...

The conductivity, due to Na ions, passes through a maximum at intermediate x. It is optimised at x 2, where the values approach those of Na / "-alumina, especially at high temperature, >300°C, Fig. 2.11. At the solid solution limits, x = 0 and 3, the conductivity is very low, for the same reasons given in the discussion of Fig. 2.3. The crystal structure of NASICON is a framework, built of (Si, P)04 tetrahedra and ZrOg octahedra which link up in such a way as to provide a relatively open, three-dimensional network of sites and conduction pathways for the Na ions, Fig. 2.12(a). Two Na sites are available, Nal and Na2. The former is a six-coordinate site while the latter is an irregular eight-coordinate site. These sites are partially occupied at intermediate x. 

Figure7.15 Crystal structure ofthe rhombohedral Nasicon phase Na3Zr2Si2POi2 at 35O C [138]. (a) The framework (sodium cations omitted) (b) Sodium sites only (framework omitted).

Figure 7.21 Orthorhombic crystal structure of Li3Sc2(PO4)3 (Pbcn) alSOO C [209], (I) A M2T3O6O12/2 lantern , a common building block of NASICON and Li3M2(PO4)3 family (II) A fragment of Sc2(PO4)3 framework the shaded and unshaded areas are identical lanterns in...

With this background in mind, we chose vanadium as our paraffin activating element and the NASICON structure as the framework into which the vanadium and other catalytic moieties would be incorporated. The reaction to be studied was chosen to be the oxidation of n-butane. 

Phosphate-substituted zeolites have attracted commercial interest in the past two decades and it is claimed that incorporation of P increases the efficiency of these cavity-containing compounds both as catalysts and as sequestering agents for Ca + and Mg-+. Although in some cases these compounds are silicates with cavity-occluded orthophosphate groups (i.e. silicate/phosphates), in others the phosphate is incorporated in the silicate framework to form genuine silicophosphates with Si-O-P linkages. An important silicate/phosphate cavity structure is NASICON (Section 12.20). 

Transport of alkali metal ions through the tunnels in Nasicons can be extremely rapid, particularly at elevated temperatures, although the electronic conductivities are low. For these reasons, these materials were originally proposed for use as solid ionic conductors (e.g., to replace 3" alumina in high temperature Na/S batteries). In spite of their low electronic conductivities, researchers recognized that Nasicon structures with redox-active transition metals and related three-dimensional framework compounds could function as electrode materials as early as the late 1980s [229-231] and numerous materials were investigated [232, 233]. In many cases, the electrochemical... 

It should be noted that whilst the sodium conducting NASICON phases can be modified by partial replacement of the phosphorous cations with aliovalent species that readily form MO4 units, such as Si or Ge, this approach has not been successful for the lithium analogues that are of interest here. All reported crystal structures of lithium NASICON compounds contain framework tetrahedra that are fully and uniquely occupied by phosphorous. 

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