Piezoelectrics, which can change mechanical stress to electricity and back again, are widely used in many fields, including computer hard drives, medical ultrasound, and sonar. Even so, understanding exactly they work is less widespread. A team of researchers at the National Institute of Standards and Technology (NIST), Gaithersburg, MD, in collaboration with Simon Fraser University, BC, Canada, believes they've learned why one of the main classes of these materials, known as relaxors, behaves in distinctly different ways from the rest and exhibits the largest piezoelectric effect.
They examined two of the most commonly used piezoelectric compounds—the ferroelectric PZT and the relaxor PMN—which look similar on a microscopic scale. Both are crystalline materials composed of cube-shaped unit cells containing one lead atom and three oxygen atoms. The essential difference is found at the centers of the cells. In PZT, these are randomly occupied by either one zirconium atom or one titanium atom, both having the same electric charge. But, in PMN, one finds either niobium or manganese with very different electric charges. The differently charged atoms produce strong electric fields that vary randomly from one unit cell to another in PMN and other relaxors, a situation absent in PZT.
Using neutron beams revealed new details about where the atoms in the unit cells were located. In PZT, the atoms sat more or less right where expected, but in the PMN, their locations deviated from expected positions. The neutron beams scattered off PMN crystals in a butterfly shape, which reveals the nanoscale structure that exists in PMN, but not in PZT.
They believe that this butterfly-shaped scattering might be a characteristic signature of relaxors. Additional tests the team performed showed that PMN-based relaxors are more than 100 percent more sensitive to mechanical stimulation compared to PZT, another first-time measurement.