When driving a piezo with a large vibration amplitude, self-heating can be a problem, and N6 material with low dielectric loss is advantageous in this regard. N6 material, which has a high Qm, provides a large vibration amplitude even when driven at the resonance frequency. On the other hand, the g-constant is the ability to convert stress into an electric field, and N21 has a high g33, making it suitable for sensing applications. Both materials have high coupling coefficients and piezoelectric  constants d, and in addition, these materials have high Curie temperatures, stable temperature characteristics, and are extremely easy to use.