Odd you should ask that question, since it's about one of the few
materials issues I happen to have worked on. PLZT (Lead zirconium
titanate, doped with a little lanthanum) is what's called a relaxor
ferroelectric. In simple ferroelectrics, the atoms displace a little
from the positions they take in the simple crystal found at high
temperatures. They settle in with the positively charged atoms shifted
a little one way and the negative ones shifted the other way, on
average. That makes a big electric field.
In relaxors the disordered positions of the atoms (e.g. where the
lanthanums happen to land) somehow cause that ferroelectric effect to
break into rather small regions with the displacements pointing
different directions. These small regions realign pretty easily when an
external field is applied. That means that the internal field responds
very much to an external field. The 'dielectric constant' describing
that response is huge- say 10,000, as compared to around 10 for common
materials.
The dimensions of the crystal along the direction of its internal
field and along the other directions aren't exactly the same, since the
crystal stretches a little as the atoms move to their displaced
positions. That means that when you change the directions with an
external field, the shape of the crystal changes- the 'piezoelectric
effect'. Likewise, when you squeeze the crystal, if it has already had
its displacements partly lined up, the net displacement changes so you
get an electrical voltage on it. PLZT is very piezoelectric when it's
partly aligned, so it's very useful for making sensors that give a
voltage in response to mechanical strain, or that move a little in
response to a voltage.
Mike W.
(published on 10/22/2007)
