I-centered vs F-centered orthorhombic symmetry and negative thermal expansion of the charge density wave of EuAl2Ga2

TL;DR

This study investigates the distinct orthorhombic symmetries of charge density waves in EuAl2Ga2 and EuAl4, revealing how Ga substitution influences their structural and thermal properties, including negative thermal expansion.

Contribution

It identifies the incommensurate CDW symmetry in EuAl2Ga2 as Immm(00g)s00 and compares it with EuAl4, highlighting the effects of Ga substitution on CDW symmetry and thermal behavior.

Findings

EuAl2Ga2 exhibits incommensurate CDW with orthorhombic symmetry Immm(00g)s00.

Both EuAl2Ga2 and EuAl4 CDWs involve Al1 layers, not Eu or Al2/Ga atoms.

EuAl2Ga2 shows negative thermal expansion in its CDW state.

Abstract

Together with EuGa4 and EuAl4, EuAl2Ga2 belongs to the BaAl4 structure type with space group symmetry I4/mmm. EuAl2Ga2 develops an incommensurate charge density wave (CDW) at temperatures below TCDW = 51 K. On the basis of temperature dependent single-crystal X-ray diffraction (SXRD) data, the incommensurately modulated CDW crystal structure of EuAl2Ga2 is determined to possess orthorhombic superspace symmetry Immm(00g)s00. This symmetry is different from the orthorhombic Fmmm based symmetry of the CDW state of EuAl4. Nevertheless, both symmetries Immm(00g)s00 and Fmmm(00g)s00 lead to the same conclusion, that the CDW is supported by the layers of Al1 type atoms, while the Eu and Al2 or Ga atoms are not directly involved in CDW formation. The different symmetries of the CDW states of EuAl4 and EuAl2Ga2, as well as the observation of negative thermal expansion in the CDW state of EuAl2Ga2 might be explained by the effects of Ga substitution in the latter compound.