Nasicon framework

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... 

There are many examples of rational synthesis. A good example is Sialon, in which A1 and oxygen were partly substituted for Si and nitrogen in silicon nitride, Si3N4. The fast Na+ ion conductor Nasicon was synthesized based on understanding the coordination preferences of cations and the nature of oxide networks formed by them. The zero-expansion ceramic CaQ5Ti2P30j2, possessing the Nasicon framework, was later synthesized based on the idea that the property of zero expansion would be exhibited by two- or three-coordination polyhedra linked in to leave substantial empty space in the network [3]. 

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

NASICON, the acronym for Na superionic conductor, is a non-stoichio-metric framework zirconophosphosilicate (Kreuer et al, 1989). It is... 

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. 

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. 

Reversible electrochemical lithium deintercalation from 2D and 3D materials is important for applications in lithium-ion batteries. New developments have been realized in two classes of materials that show exceptionally promising properties as cathode materials. The first includes mixed layered oxides exemplified by LijMn Nij, Co ]02, where the Mn remains inert to oxidation/reduction and acts as a framework stabilizer while the other elements carry the redox load. Another class that shows much potential is metal phosphates, which includes olivine-type LiFeP04, and the NASICON-related frameworks Li3M2(P04)3. 

NASICON (Na3Zr2PSi20i2) is a well-known framework oxide (Figure 7) with high Na+-conductivity. The substitution of 2Zr + M + + M + in NASICON is possible, yield-... 

ADP = Ammonium Dihydrogen phosphate, NH4H2PO4 AlPO = A framework aluminophosphate HUP = Hydrogen uranyl phosphate KDP = Potassium Dihydrogen phosphate, KH2PO4 KTP = Potassium Titanyl Phosphate, KTi0P04 NASICON = Sodium Superionic Conductor. 

Appreciable ionic conductivity is found in open framework or layered materials containing mobile cations (see Ionic Conductors). Several phosphates have been found to be good ionic conductors and are described above NASICON (Section 5.2.1), a-zirconium phosphates (Section 5.3.1), HUP (Section 5.3.3), and phosphate glasses (Section 5.4). Current interest in lithium ion-conducting electrolytes for battery apphcations has led to many lithium-containing phosphate glasses and crystalline solids such as NASICON type titanium phosphate being studied. ... 

Nanjundaswamy, K.S., Padhi, A.K., Goodenough, J.B., Okada, S., Ohtsuka, H., Arai, H., and Yamaki, J. Synthesis, redox potential evaluation and electrochemical characteristics of NASICON-related-3D framework compounds. Solid State Ionics, 92, 1, 1996. 

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).

Although a great number of other NASICON-type Na -ion conductors have been studied [1,2,4,146,147], all have exhibited lower conductivities (see Table 7.2), mostly due to geometric constraints, as their formula volumes (203-261 A ) are less than the optimum value (see Figure 7.16), or due to a non-optimum sodium content However, neither these factors nor the electronegativity can explain the decrease in conductivity when Ge is substituted for Si [140,146], when As is substituted for P [140], or when In is substituted for Sc and Ga is substituted for Cr in Nai+Zr2 x(PC)4)3 [147]. In all of these pairs, lower conductivities were observed with framework cations having filled the d-shell. 

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. 

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