## 5 Perspectives of Aurivillius Materials as Lead Free Piezoelectrics  
Aurivillius type structure materials have a number of characteristics that condition their applications in different fields. In terms of piezoelectric activity at room temperature, they are quite far from other lead free piezoelectric compositions. While the highest \(d_{33}\) coefficients reach values no higher than 40 pC/N (even in textured ceramics), other families of compositions reach very much higher values [90]. Thus, within the solid solution BaTiO\({}_{3}\)-BaZrO\({}_{3}\)-CaTiO\({}_{3}\) (BCZT), the composition Ba(Zr\({}_{0.15}\)Ti\({}_{0.85}\))O\({}_{3}\)-x(Ba\({}_{0.85}\)Ca\({}_{0.2}\))TiO\({}_{3}\) has shown values up to \(d_{33}\) = 620 pC/N [91]. For the (Bi\({}_{0.5}\)Na\({}_{0.5}\))TiO\({}_{3}\)-BaTiO\({}_{3}\) (BNT-BT) system, values of \(d_{33}\) = 180 pC/N have been achieved for the (Bi\({}_{0.5}\)Na\({}_{0.5}\))\({}_{0.94}\)Ba\({}_{0.06}\)TiO\({}_{3}\) compositions. With respect to the families of niobates with (K\({}_{0.5}\)Na\({}_{0.5}\))NbO\({}_{3}\) (KNN) based composition, a high value of \(d_{33}\) = 490 pC/N was recently reported for a 0.96(K\({}_{0.48}\)Na\({}_{0.52}\))(Nb\({}_{0.97}\)Sb\({}_{0.03}\))O\({}_{3}\)-0.04 Bi\({}_{0.5}\)(Na\({}_{0.82}\)K\({}_{0.180.5}\)ZrO\({}_{3}\)) solid solution [92].  
It seems obvious that the piezoelectric activity for Aurivillius compounds at room temperature is not competitive with other piezoelectric lead-free materials in terms of applications where high piezoelectric coefficients are essential. The d\({}_{33}\) values are one order of magnitude lower in some cases. However, the high coercive fields for Aurivillius ferroelectrics could make them good candidates for actuators applications, because the depoling by the application of an electrical field is difficult, even at room temperature.  
There are other properties for Aurivillius compounds that allow to have their own potential niches as lead-free piezoelectric applications. The most characteristic property for Aurivillius compounds is their high ferro-paraelectric transition temperature, which in some cases can be higher than 900 \({}^{\circ}\)C (as for Bi\({}_{3}\)TiNbO\({}_{9}\)). It is in these applications (at temperatures higher than 500 \({}^{\circ}\)C) where this family of materials can be competitive as lead-free piezoelectrics. They cover several fields as aerospace and aircraft industries, nuclear power plants, wind power and geothermal power. In all of these cases, sensors working at high temperature can be useful for in situ inspection and monitoring [93]. In these cases, other materials must be compared with Aurivillius compounds, as quartz, Langasite and Oxyporate. In all these cases, these materials can be used only as single crystals, due to the difficultiesof obtaining ceramics with their compositions. Besides the difficulties of working with single crystals, their piezoelectric activity is lower than for Aurivillius compounds. Besides, Aurivillius compounds can have potential applications in no piezoelectric fields, as photocatalytic or multiferroic materials, as explained before.  
[PERSON] is indebted to MINECO for the \"Ramon y Cajal\" contract (Ref. RYC-2013-14436), which is co-financed with European Social Fund.  
The authors declare no conflict of interest.