The current interest in the wide bandgap II-VI semiconductor compounds can be traced back to the initial developments in semiconductor optoelectronic device physics that occurred in the early 1960s. The II-VI semiconductors were the object of intense research in both industrial and university laboratories for many years. The motivation for their exploration was the expectation that, possessing direct bandgaps from infrared to ultraviolet, the wide bandgap II-VI compound semiconductors could be the basis for a variety of efficient light-emitting devices spanning the entire range of the visible spectrum.

During the past thirty years or so, development of the narrower gap III-V compound semiconductors, such as gallium arsenide and related III-V alloys, has progressed quite rapidly. A striking example of the current maturity reached by the III-V semiconductor materials is the infrared semiconductor laser that provides the optical source for fiber communication links and compact-disk players. Despite the fact that the direct bandgap II-VI semiconductors offered the most promise for realizing diode lasers and efficient light-emitting-diode (LED) displays over the green and blue portions of the visible spectrum, major obstacles soon emerged with these materials, broadly defined in terms of the structural and electronic quality of the material. As a result of these persistent problems, by the late 1970s the II-VI semiconductors were largely relegated to academic research among a small community of workers, primarily in university research laboratories.

Over the past 30 years or so, some fascinating new developments have taken place in a field that is now called “nonlinear science.” These developments have been of such magnitude that historian Alan Beyerchen concluded “There is every reason to believe that the westernized world is in the early stages of an intellectual transformation of major proportions, perhaps as significant as the emergence of the modern worldview in the fifteenth through seventeenth centuries.”

These new developments are rooted in work conducted by mathematician Jules Henri Poincaré around the turn of the century. He carried out some calculations regarding planetary orbits and demonstrated the possibility of erratic or “chaotic” dynamical behavior. What is particularly interesting about such behavior is that it is exhibited by deterministic systems, that is, systems that have no stochastic (noisy) character of any kind. Little was done with Poincaré's findings until the 1960s when new work in nonlinear science, based on numerical models, led to the discovery of amazingly complex behavior that can be exhibited even by very simple deterministic systems. Since that time, the discipline has grown enormously and has been applied in practically every area of science and engineering, as well as in other diverse areas such as the financial market, political science, economics, and even Adlerian psychology.

Just as in ordinary metals, defects in intermetallic compounds fall into three basic categories: point defects (vacancies, substitutional and interstitial atoms), line defects (dislocations), and planar defects (stacking faults, interfaces, grain boundaries). Also like ordinary metals, many important physical and chemical properties of intermetallic compounds are governed by the presence of these defects and the effects on them from temperature, composition, chemical environment, elastic stress state, and so on.

What ultimately distinguishes an intermetallic compound from ordinary metals is its superlattice crystal structure. A two-dimensional analogue of the actual three-dimensional superlattice structure is shown in Figure 1a where the superlattice (unit cell marked by full lines) is made up of two identical sublat-tices (unit cells marked by dotted lines). A property of the sublattice is that it is exclusively occupied by one atom species, and accordingly sublattices are named after the atoms that occupy them, for example, the A and B sublattices in Figure 1a. In three dimensions, a super-lattice may consist of several sublattices. For example, the L12 superlattice of Ni3Al consists of four interpenetrating cubic sublattices, one occupied by Al atoms (Al sublattice), the other three by Ni atoms (Ni sublattices). When the sub-lattices are occupied exclusively by their designated atoms, the crystal is said to be fully ordered. The crystal will be partially ordered if a certain fraction of the sublattice sites is taken up by atoms that would otherwise sit at other sublattices; this fraction is used to describe the degree of long-range order.

During the past decade, perturbed-angular-correlation (PAC) spectroscopy has emerged as an important technique in several areas of materials science. PAC spectroscopy is used to measure the effects of local fields at well-defined lattice sites in a crystal. These measurements can provide unique information about the structures, kinetics, and energetics associated with point defects, the mechanisms of phase transitions, and the strengths and symmetries of chemical bonds of atoms on surfaces and at interfaces. In what follows, I describe the PAC technique in the context of several examples of these applications and I comment on the historical evolution of this spectroscopy.

Hyperfine Interactions

The field of hyperfine interactions encompasses spectroscopic techniques that use the electric and magnetic nuclear moments to measure the specific extranuclear environment of the nuclear moments. Examples of these techniques are nuclear-magnetic-resonance (NMR), nuclear-quadrupole-resonance (NQR), electron-paramagnetic-resonance (EPA), muon-spin-rotation (MSR), Mössbauer-effect (ME), and perturbed-angular-correlation (PAC) spectroscopies. Each of these techniques is well-suited to certain types of measurements but useless for others. For example, NMR spectroscopy can be particularly useful for measuring chemical shifts associated with nuclei of atoms that are major constituents of crystalline solids. However, nuclear electricquadrupole interactions associated with some NMR-active nuclei are not only difficult to measure but they often obscure much of the chemical-shift information by producing spectral-line broadening. PAC spectroscopy can be used to accurately measure the nuclear electric-quadrupole interactions at the sites of nuclei of atoms that are trace dopants in crystals. But the PAC technique is insensitive to the effects of local charge distributions that produce the NMR chemical shifts. ME spectroscopy can be used to measure both of these effects as well as nuclear magnetic-dipole interactions. However, ME measurements are often only practical on crystals that have one of several elements as a major constituent, that is, either Fe or Sn. In addition, the ME sensitivity depends on temperature, and the NMR and EPR sensitivities also depend on temperature. However, the PAC measurement is independent of temperature, which can be a great advantage for studying phenomena such as phase transitions.