Using a neutron beam as a probe, researchers working at the National
Institute of Standards and Technology (NIST) have begun to reveal the crystal
structure of a compound essential to technologies ranging from sonar to computer
memory. Their recent work* provides long-sought insight into just how a widely
used material of modern technology actually works.
The compound is a "piezoelectric," a material capable of changing
one kind of energy into another—mechanical to electrical, or vice versa.
Long employed in sonar systems to detect sound waves, more recently piezoelectrics
have been applied in devices that require minuscule changes in position, such
as the head that reads data from your computer's hard drive.
For decades, the industry standard piezoelectric has been PZT, a compound that
contains titanium,zirconium, lead and oxygen. Crystals of PZT change a tiny
fraction of a percent in size when a sound wave strikes them, and thisshape
change creates an electrical impulse. Decades ago, it was discovered that PZT
performs at its best when the titanium and zirconium appear in approximately
equal proportions, but no one really understood why.
"The theories frequently concern what happens at the transition line between
having a surplus of zirconium and one of titanium," says Peter Gehring
of the NIST Center for Neutron Research (NCNR). "Some theories suggest
that right near the transition zone, the atoms take on a special configuration
that allows certain atoms to move more freely than they can otherwise. But because
it's been hard to grow a crystal of PZT large enough to analyze, we couldn't
completely test these ideas."
A breakthrough came when chemists at Canada's Simon Fraser University managed
to grow single crystals of a few millimeters in size and sent them to the NCNR
for examination with neutron scattering—a technique for determining the
positions of individual atoms in a complex crystal structure by observing the
patterns made by neutrons bouncing off it. The team, which also included researchers
from the University of Oxford, the University of Tokyo, and the University of
Warwick, was able to definitively rule out one of the proposed structures of
Instead, they found that each PZT crystal element likely assumes one of two
possible forms that coexist within the larger crystal array. These forms are
dictated by chemical composition, and they may influence how well the material
performs on a large scale. Their findings also suggest that the change in behavior
seen at the transition happens gradually, rather than at some sharply delineated
proportion of zirconium to titanium.
Gehring says the results could be a step toward bettering PZT. "Determining
the structure might give us the perspective necessary to design a piezoelectric
material from first principles, instead of just playing around and seeing what
works," he says. "That's what you need if you're ever going to build
a better mousetrap."
* D. Phelan, X. Long, Y. Xie, Z.-G. Ye, A.M. Glazer, H. Yokota, P.A. Thomas
and P.M. Gehring. Single crystal study of competing rhombohedral and monoclinic
order in lead zirconate titanate. Physical Review Letters, Nov. 8, 2010, DOI: