Polymorphic transformations of CaSi2 and CaGe2

Abstract

The control of CaSi2 and CaGe2 polymorphic transformations is important for their use as raw materials to prepare widely applicable 2D materials. CaSi2 and CaGe2 have several polymorphs, such as 2H, 3R, 4H, and 6R, having layered structures with different stacking of Ca and Si or Ge layers. In this study, CaSi2 and CaGe2 ingots were synthesized by melting and solidifying Ca with Si or with Ge, respectively, and their polymorphs and polymorphic transformations were investigated using X-ray diffraction and transmission electron microscopy. The most stable CaSi2 polymorph, 6R, was formed by solidifying at a 10 ​°C/min cooling rate, whereas the metastable 3R polymorph was formed with slow cooling of 0.167 ​°C/min because the formation of 3R–CaSi2 was promoted by tensile stress. After fluorine diffusion, 6R–CaSi2 was transformed into 3R–CaSi2, and 4H-CaGe2 and 6R–CaGe2 were transformed into 3R–CaGe2 with the largest unit cell volume/Z value (where Z is the number of formula units in a unit cell). Polymorphic transformations occurred owing to the occupation of F− ions in interstitial sites because the volume/Z values for the polymorphs of CaSi2 and CaGe2 are in the following order: 3R ​> ​4H ​> ​2H ​> ​6R. The c lattice constant of each polymorph remained constant after fluorine diffusion. Thus, the formation of the polymorphs of CaSi2 (and CaGe2) can be governed by controlling the unit cell volume of CaSi2 (and CaGe2).