Figure 1: The principle of “derivative structures” in solid-state chemistry, with examples relevant to electronic materials. (Top) The familiar salt and diamond structures (the latter describing silicon) are themselves derived from the simpler face-centered cubic (fcc) structure. (Middle) The III-V crystal structure of $\text{GaAs}$ derives from $\text{Si}$ by placing different atoms in the fcc and tetrahedral positions, and the $\text{LiZnAs}$ structure (describing several small-band-gap thermoelectrics) derives from $\text{NaCl}$ by filling half of the tetrahedral voids. (Bottom) The ${\text{CuInSe}}_2$ structure of semiconductors used in photovoltaic cells is derived from $\text{GaAs}$ by replacing $\text{Ga}$ with an ordered arrangement of $\text{Cu}$ and $\text{In}$ and the structure of ${\text{MnCu}}_2\text{Sn}$ (important for half metallic ferromagnets and superconductors) is derived from the $\text{LiZnAs}$ structure by filling the remainder of the tetrahedral voids. Finally, $\text{LiMnAs}$ (Jungwirth et al.) can be derived from either $\text{GaAs}$ or $\text{LiZnAs}$. At every step in the derivative pathway, the added chemical degrees of freedom offer more means to tune the electronic properties.