**5. Structural factors involved in ionic conductivity: Correlation of structure-electrical properties**

Ionic conductors have been intensively researched since the discovery of properties of ionic superconductors [37] whose conductivity is sufficiently high to consider applications as solid electrolytes in batteries [38, 39] in storage devices of energy and sensors [40]. On the other hand, materials with low ionic conductivity remain interesting to elucidate certain mechanisms of cation transport. In these materials, the charge carriers are cations.

The open framework is an essential factor that governs the mobility of cations within a crystal lattice [6, 41, 42]. Among these structures, there are:


**23**

*Cobalt Phosphates and Applications*

hedral groups (SiO4

energetically equivalent.

La1.2Sr1.8Mn2−xTxO7 with T = Fe, Co, Cr [48].

control of the microstructure.

**6. Conclusion**

triangular faces.

materials.

*DOI: http://dx.doi.org/10.5772/intechopen.86215*

<sup>3</sup><sup>−</sup>, PO4

• Structures with isolated tetrahedral groups: these structures consist of tetra-

Other factors than the open framework can also promote ionic mobility [40]:

• Site occupation: the partial occupations of the ionic sites (occupancy rate

ions. These independent tetrahedra facilitate the movement of cations [43–45].

lower than 1) favor the displacement of the mobile ion from one site to another

• Coordination polyhedra: the cation environment can play an important role in its mobility. Indeed, mobile ions can cross rectangular faces more easily than

• Ion size: to promote conduction, congestion must be minimized, so the use of

• Structural defects: substitution or doping of one or more elements with other(s) having different degrees of oxidation is responsible for the creation of cationic vacancies at the origin of conduction properties in certain

Taking into account the structural factors influencing the conductivity mentioned above, several studies have been devoted to improving the electrical properties of such materials by acting on other factors. This is the case of the total or partial substitution of the mobile species such as in NASICON Na1+xZr2<sup>−</sup> xMgx/2(PO4)3 (0 < x ≤ 2) [46] and in SKELETON phosphates (3D) A3M2(PO4)3: A = Li, Na, Ag, K, and M = Cr, Fe [47]. The doping of materials by one or more chemical elements can also promote the mobility of cations like the oxides

On the other hand, work on a series of materials is being processed in order to show the effect of microstructure optimization (grain size) on conductivity [49]. Moreover, it has been demonstrated in previous studies, such as for LAMOX ceramics (La2−xRxMo2−yWyO9 with R = Nd, Gd, Y) [50, 51] and for β-Xenophyllitetype Na4Co7(AsO4)6 [21] and Ag4Co7(AsO4)6 [26], that the electrical properties are related to the relative density of sample (100 porosity), which requires a rigorous

In this chapter, synthesis methods of cobalt phosphates and metallo-cobalt phosphates in the crystalline form have been described: single crystals and/or polycrystalline powders. The structural studies of the studied compounds show structural diversity with open anionic frameworks showing tunnels (3D) and inter-sheet space (2D). However, it shows that the electrical property is related to the structural characteristics of the material. In order to correlate structure and physical properties especially electrical properties of metallo-cobalt phosphates, structural factors influencing the ionic conductivity have been treated. Based on the structural characteristics, the electrical properties of the crystalline materials can be modeled theoretically, especially in the case of purely ionic conductors. In fact, it is possible to determine the value of the activation energy which corresponds to the minimum energy that must be supplied to an ion to move from one site to another site in the

small cations is recommended, to facilitate their movement.

<sup>3</sup><sup>−</sup>, etc.) connected to each other solely by alkaline

#### *Cobalt Phosphates and Applications DOI: http://dx.doi.org/10.5772/intechopen.86215*

*Cobalt Compounds and Applications*

group Pn21a. Projections of the three-dimensional framework of this material (**Figure 17**) show that PO4 monophosphates are bound to CoO6 octahedra, on the one hand by edge sharing and on the other hand by sharing vertices, while diphos-

**5. Structural factors involved in ionic conductivity: Correlation of** 

Ionic conductors have been intensively researched since the discovery of properties of ionic superconductors [37] whose conductivity is sufficiently high to consider applications as solid electrolytes in batteries [38, 39] in storage devices of energy and sensors [40]. On the other hand, materials with low ionic conductivity remain interesting to elucidate certain mechanisms of cation transport. In these

The open framework is an essential factor that governs the mobility of cations

• Three-dimensional frameworks with windows or channels: this type of material has an ionic conduction influenced by the size of the bottlenecks separating two adjacent available sites. The existence of wide-sectioned channels between the cationic sites promotes the passage of cations. According to Hong, for fast ionic conduction, the minimum sections of the windows must be greater than or equal to twice the sum of the radii of the cation and the nearest anion [40].

• Layered structures: in this case, mobile ions move in parallel planes, located in the interlayer space. The conduction in this case is probably two-dimensional [27].

within a crystal lattice [6, 41, 42]. Among these structures, there are:

phates join four CoO10 units by pooling vertices.

*Projections of Na4Co3(PO4)2P2O7 structure in (a) a, (b) b, and (c) c directions.*

**structure-electrical properties**

materials, the charge carriers are cations.

**22**

**Figure 17.**

• Structures with isolated tetrahedral groups: these structures consist of tetrahedral groups (SiO4 <sup>3</sup><sup>−</sup>, PO4 <sup>3</sup><sup>−</sup>, etc.) connected to each other solely by alkaline ions. These independent tetrahedra facilitate the movement of cations [43–45].

Other factors than the open framework can also promote ionic mobility [40]:


Taking into account the structural factors influencing the conductivity mentioned above, several studies have been devoted to improving the electrical properties of such materials by acting on other factors. This is the case of the total or partial substitution of the mobile species such as in NASICON Na1+xZr2<sup>−</sup> xMgx/2(PO4)3 (0 < x ≤ 2) [46] and in SKELETON phosphates (3D) A3M2(PO4)3: A = Li, Na, Ag, K, and M = Cr, Fe [47]. The doping of materials by one or more chemical elements can also promote the mobility of cations like the oxides La1.2Sr1.8Mn2−xTxO7 with T = Fe, Co, Cr [48].

On the other hand, work on a series of materials is being processed in order to show the effect of microstructure optimization (grain size) on conductivity [49]. Moreover, it has been demonstrated in previous studies, such as for LAMOX ceramics (La2−xRxMo2−yWyO9 with R = Nd, Gd, Y) [50, 51] and for β-Xenophyllitetype Na4Co7(AsO4)6 [21] and Ag4Co7(AsO4)6 [26], that the electrical properties are related to the relative density of sample (100 porosity), which requires a rigorous control of the microstructure.
