Lithium Conduction in NASICON-Related Phases

Much work has been undertaken in order to modify the sodium content of related phases. As the octahedral site is filled in NaZr2(P04)3 it is clear that any increase in the extra-framework cation concentration requires population of additional sites. Considerable effort has been made to understand the distribution of these extra sodium cations. 

The structural differences between these four polymorphs are subtle but are strongly coupled with the ionic conductivity of this compound and so merit detailed discussion. First we shall examine the polymorph with the highest symmetry and hence the simplest description the a-LiZr2(P04)3 polymorph that is formed at 1200 °C and stable at temperatures above ca 30—60 °C. This structure of this phase has been the subject of some dispute due to the difficulties of locating the lithium cations in the structure. Some preliminary studies concluded that lithium would be found in the same environment that sodium occupied in the NASICON structure of NaZr2(P04)3. ° However, this elongated octahedral coordination would require the stabilisation of Li by coordination to six oxide anions at a distance, 2.53 that is too large for the relatively small lithium cation. 

As the temperature is increased the population of this site is decreased further, and a study of the difference Fourier map showed that there is significant concentration of lithium cations on another distorted four-coordinate position within the structure. This second lithium position is more strongly enclosed by the oxide anions and the mean lithium oxide 

Similar structural features are found in the two polymorphs of LiZr2(P04)3 that are accessed if the sample is prepared by heating at 900 °C. Such a preparation leads to the formation of P-LiZr2(P04)3 with an orthorhombic structure containing a complex distribution of lithium. As in the a- and a -LiZr2(P04)3 polymorphs the zirconium phosphate framework is fully ordered. However, in P-LiZr2(P04)3 the lithium cations partially occupy two crystallographically distinct sites that are 

The P to P structural transition that occurs at 300 °C contains the same key features seen for the analogous a to a transition when the sample is prepared at 1200 °C. The p phase has lower symmetry than its higher temperature parent phase the transition P to P involves a change in space group symmetry from orthorhombic Pbna to monoclinic These 

---

The NASICON structure is capable of accommodating considerable compositional variation and a large number of related compounds have been studied in order to try an improve the lithium ion conducting properties. There have been two distinct approaches to this. One approach has been to try and reduce the temperature of the structural transition and reduce the barrier to ion mobility and so access a compound that shows fast lithium conductivity under ambient conditions. An alternative strategy is to adjust the number of mobile cations and vacant sites in order to increase the conductivity of the cations in the higher temperature, disordered phase. Both approaches have had considerable success in both illuminating the mechanism for ion motion in the structure and in changing the physical properties towards those of a useful fast ion conductor. 

---