$I$-centered versus F -centered orthorhombic symmetry and negative thermal expansion of the charge density wave of EuAl$_2$Ga$_2$

Together with EuGa$_4$ and EuAl$_4$, EuAl$_2$⁢Ga$_2$ belongs to the BaAl$_4$ structure type with space group 𝐼⁢4/mmm. EuAl2⁢Ga2 develops an incommensurate charge density wave (CDW) at temperatures below $𝑇_{CDW}$=51K. On the basis of temperature-dependent single-crystal x-ray diffraction data, the incommensurately modulated CDW crystal structure of EuAl2⁢Ga2 is determined to possess orthorhombic symmetry Immm(00γ)𝑠⁢00. This symmetry is different from the orthorhombic Fmmm-based symmetry of the CDW state of EuAl$_4$. Nevertheless, both symmetries, Immm(00γ)𝑠⁢00 and Fmmm(00γ)𝑠⁢00, lead to the same conclusion, that the CDW is supported by the layers of Al1 type atoms, while the Eu and Al$_2$ or Ga atoms are not directly involved in CDW formation. The different symmetries of the CDW states of EuAl4 and EuAl2⁢Ga2, as well as the observation of negative thermal expansion in the CDW state of EuAl$_2$⁢Ga$_2$, might be explained by the effects of Ga substitution in the latter compound.