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