The figure illustrates point defects in NiAl. NiAl is a compound having the B2, or CsCl, structure. It is an important structural material used in high-temperature applications (e.g., Boeing jet turbines). In the perfect lattice, the Ni atoms are surrounded by Al atoms, and vice versa, as shown schematically in the figure. The figure shows an indium impurity atom representing a PAC probe as well as three types of intrinsic point defects. The defects shown are an Al-vacancy, a Ni-antisite atom, and three Ni-vacancies. The Ni-vacancy shown next to the probe can be detected because it exerts a torque on the quadrupole moment of the probe's nucleus, thereby modifying the precessional motion of its spin.
While defects of opposite charge, such as vacancies on the two sublattices, are normally found to bind strongly together in ionic compounds, interactions among defects in intermetallic compounds are much smaller owing to screening of the defects' charges by the conduction electrons. Therefore defects such as those shown in the figure are expected to move more or less independently of each other.
Through PAC measurements, we have determined a number of things. As the temperature of a solid is increased to high temperature, it becomes favorable for point defects to form and grow in concentration. In an ordered compound like NiAl, defects have to form as a grouping of elementary defects. Our PAC measurements show that the grouping that occurs in NiAl is a so-called triple defect, consisting of two Ni-vacancies and one Ni-antisite atom. The formation energy of the triple defect was determined from recent measurements by Bin Bai at high temperature to be about 1.75(10) electron-volts. The binding energy between an indium probe atom and a Ni-vacancy (pictured above as a bound state) was also measured, and found to be 0.22(3) electron-volts. Finally, the jump frequency of a Ni-vacancy next to a probe atom was determined to reach about 10 MHz at 1500 K, as determined from nuclear relaxation caused by vacancy's motion.
What's important about the PAC method is that one can resolve the different
defects. This removes uncertainty in the interpretation of measurement
and brings one as close to "seeing" defects inside metals as possible.
References for further reading