Crystal Lattice/crystaline structure

[astro_kat]

Hi, I've got a question, I recently got interested in piezoelectricity. I almost no nothing about it tho. From the wiki artcile I gathered that when a force acts upon a piezoelectric material and deforms its shape, an electrical potential is created. I know that it has many applications in electronics, medicine, ect... but how does it work? I was thinking about a couple of crystal compounds from the wiki, and i thought perhaps a lewis-dot-structure might help.

Am I on the right track in saying that an outide force causes a distortion which makes the individual molecules' intermolecular forces interact?

 

[Gokul43201]

The Lewis dot structure is not really useful for systems of a large number of interacting atoms (such as in a crystal). But, in this case, a very simplistic argument can be extracted from a single molecular structure, that provides some insight into the physics behind peizoelectricity. I'll first make the simplistic argument, and then try to develop a better picture of the real phenomenon.

Consider a single H₂O molecule with two slightly positively charged H-atoms bonded to a slightly negatively charged O-atom, with a bond angle of about 105 degrees. Because of the non-linear geometry, the molecule has a natural dipole moment pointing along the axis of rotational symmetry. If you were to somewhow take a single H₂O molecule and flatten it so it became linear, then it would no longer have a dipole moment. Alternatively, if you compressed the molecule from the sides, making the bond angle smaller, you will increase the dipole moment. In other words, forcing the molecule into an unnatural state of stress changes its dipole moment. From your recollection of electrostatics, you know that a electric dipole produces an electrostatic field, so changing the dipole moment will change the E-field within the molecule. This is the simplistic, single molecule picture: you apply a stress, you change the dipole moment, and thus change the electric field.

 

[Gokul43201]

In crystals lacking certain symmetry elements, it is possible for a unit cell to have a net dipole moment. Typically, in a polycrystalline material, dipole moments tend to line up within tiny regions known as Weiss Domains (similar to the magnetic domains in a ferromagnetic material) which, in the absence of an applied field, are oriented randomly, resulting in no net polarization. When a stress is applied to the materials, domains that are oriented in the right direction become more polar (or tend to grow) at the cost of domains that are oriented at right angles to them. This mechanism is somewhat like the one explained in the above post. As a result, the crystal gains a net polarization.

This net polarization is equivalent to the creation of a net dipolar surface charge on the opposite surfaces of the crystal, normal to the direction of applied stress. Or, without having to think about the surface charge, you can still see that now an electric field is set up inside the crystal, as a result of the non-zero polarization.