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).