What is the difference between monocrystalline and polycrystalline structure?

The piezoelectric effect of natural monocrystalline materials such as quartz, tourmaline and Rochelle salt is relatively small. Polycrystalline ferroelectric ceramics such as lead zirconate titanate (PZT) exhibit larger displacements or induce larger electric voltages. PZT piezo ceramic materials are available in many variations and are most widely used for actuator or sensor applications.