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EuAl4 possesses the BaAl4 crystal structure type with tetragonal symmetryI4/mmm. It undergoes a charge-density-wave (CDW) transition at TCDW = 145 K andit features four consecutive antiferromagnetic phase transitions below 16 K.Here, we use single-crystal x-ray diffraction to determine incommensuratelymodulated crystal structure of EuAl4 in its CDW state. The CDW is shown to beincommensurate with modulation wave vector q = (0, 0, 0.1781(3)) at 70 K. Thesymmetry of the incommensurately modulated crystal structure is orthorhombicwith superspace group Fmmm(00{\sigma})s00, where Fmmm is a subgroup of I4/mmmof index 2. Both the lattice and the atomic coordinates of the basic structureremain tetragonal. Symmetry breaking is entirely due to the modulation wave,where atoms Eu and Al1 have displacements exclusively along a, while thefourfold rotation would require equal displacement amplitudes along a and b.The calculated band structure of the basic structure and interatomic distancesin the modulated crystal structure both indicate the aluminum atoms as locationof the CDW. The temperature dependence of the specific heat reveals an anomalyat TCDW = 145 K of a magnitude similar to canonical CDW systems. The presentdiscovery of orthorhombic symmetry for the CDW state of EuAl4 leads to thesuggestion of monoclinic instead of orthorhombic symmetry for the third AFMstate.

Orthorhombic charge density wave on the tetragonal lattice of EuAl4