Deciphering the Polymorphism of CaSi₂: The Influence of Heat and Composition

The Zintl phase CaSi₂ 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-CaSi₂ (space group R3̅m) 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-CaSi₂ is thermodynamically stable at ambient conditions, nanosized impurities of silicon stabilize 3R-CaSi₂ as a bulk phase.