What is the physical mechanism behind piezoelectricity in PZT? (no maths please) What exactly does applying a mechanical force to a PZT ceramic do to its crystal structure that results in a potential difference being generated across the crystal?
I can't work out whether the applied force results in:


*

*A change in the dipole moment of the crystal base unit

*Rotation of the ferroelectric domains in the crystal

*Some other effect


Thanks for your help!
 A: You can think of a piezoelectric material as made up of crystals with an asymmetric charge (ion) configuration. In other words, they have an asymmetric / non-centre-symmetric unit cell as on the right image below (from Wikipedia):

More technically, there is often a specific temperature limit, below which the crystal spontaneously will become asymmetric and above which it will be symmetric. This limit is called the Curie temperature $T_c$, or the Curie point, after the discoverers. It is the same Curie temperature as you hear about in the topic of magnetism.
When "squeezed" to slight compression, these charges are pushed further out of equilibrium and separated more from their oppositely charged counterpart. The fact that the unit cell is asymmetric gives the possibility of some ions being pushed further out of equilibrium than others. The neutral crystal is still neutral but now has a slightly higher concentration of ions pushed towards one side. They correspond to a larger net charge in one side of the material, thus a net potential difference across it. 
A good illustration, that visualized why squeezing would cause and strengthen asymmetry is this (from here)

where the the centres of charge symmetry are shown to be unequally displaced when the structure is "squeezed" whereas they would be coinciding with no mechanical "squeeze".
Some interesting reads are: https://www.comsol.com/blogs/piezoelectric-materials-crystal-orientation-poling-direction/ and http://www.cyberphysics.co.uk/topics/medical/Ultrasound/piezoElectric.html
