Credit: Alan Stonebraker

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 $GaAs$ derives from $Si$ by placing different atoms in the fcc and tetrahedral positions, and the $LiZnAs$ structure (describing several small-band-gap thermoelectrics) derives from $NaCl$ by filling half of the tetrahedral voids. (Bottom) The $CuInSe2$ structure of semiconductors used in photovoltaic cells is derived from $GaAs$ by replacing $Ga$ with an ordered arrangement of $Cu$ and $In$ and the structure of $MnCu2Sn$ (important for half metallic ferromagnets and superconductors) is derived from the $LiZnAs$ structure by filling the remainder of the tetrahedral voids. Finally, $LiMnAs$ (Jungwirth et al.) can be derived from either $GaAs$ or $LiZnAs$. At every step in the derivative pathway, the added chemical degrees of freedom offer more means to tune the electronic properties.