Deciphering the Polymorphism of CaSi 2 : The Influence of Heat and Composition.

The Zintl phase CaSi 2 is a layered compound with stacking variants known as 1P, 3R, and 6R. We extend the series by the 21R polytype formed by rapid cooling of the melt. The crystal structure of 21R-CaSi 2 (space group R 3̅ m ) was derived from HRTEM images, and the atomic positions were optimized by using the FPLO code ( a = 3.868 Å, c = 107.276 Å). We explore polytype transformations by powder X-ray diffraction (PXRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), electron backscattering diffraction (EBSD), and thermal analysis. While 6R-CaSi 2 is thermodynamically stable at ambient conditions, nanosized impurities of silicon stabilize 3R-CaSi 2 as a bulk phase.