Discovery and Crystallographic Challenges of Diamond-Like Semiconductors with Attractive Physicochemical Properties

Diamond-like semiconductors (DLSs) adopt crystal structures that can be considered as superstructures of diamond, either the cubic or hexagonal form. Although diamond-like structures are seemingly simple, these materials exhibit a number of crystallographic challenges and opportunities. Solving complex problems such as delineating cation patterns, differentiating polymorphs, discerning isoelectronic elements, and locating dopants/elemental substituents within ternary I-III-VI2 and quaternary I2-II-IV-VI4 DLSs is important to garner a fundamental understanding of structure-property relationships to improve these materials for potential applications. The attractive electrical, optical and magnetic properties of DLSs can be tuned through compositional changes, which often result in subtle structural variations that are sometimes only detectable through careful analysis of high-quality diffraction data. The assortment of structure types that have been observed among ternary and quaternary DLSs can be categorized by the closest-packed arrangement of the anions. Within each kind of closest-packed anion array, cations can display several unique ordering patterns within the tetrahedral holes, as found in the hexagonally derived wurtzstannite (Pmn21), wurtzkesterite (Pn) and lithium cobalt (II) silicate (Pna21) structure types. X-ray diffraction patterns of such materials can be strikingly similar and are difficult to distinguish in many instances. Concomitant with structural changes, marked effects on the observed physicochemical properties can be observed in polymorphs within a single composition. In the cases where the ions making up the formulae are isoelectronic, or nearly isoelectronic, challenges arise in discriminating between the ions using X-ray diffraction. Furthermore, the properties of these materials can be altered by partially replacing one ion in the lattice with another through a process of doping/substitution. Understanding the structural implications of these alterations on the resulting properties can benefit the future design of DLSs