What is Curie temperature Tc?

At temperatures below the Curie temperature TC , the lattice structure of the PZT crystallites becomes distorted and asymmetric. This brings about the formation of dipoles and the rhombohedral and tetragonal crystallite phases, which are of interest for piezo technology. The ceramic exhibits spontaneous polarization. Above the Curie temperature the piezoceramic material loses its piezoelectric properties.

Curie temperature Tc data table

View Piezo Ceramics Material Data table with Curie Temperature, Tc

What is the difference between monocrystalline and polycrystalline structure?
When to chose soft or hard PZT?