Orthorhombic charge density wave on the tetragonal lattice of EuAl4

Abstract

PREPRINT: IUCrJ A Journal of the International Union of Crystallography EuAl 4 possesses the BaAl 4 crystal structure type with tetragonal symmetry I4/mmm. It undergoes a charge-density-wave (CDW) transition at T CDW = 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 EuAl 4 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 F mmm(00σ)s00, where F mmm 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 T CDW = 145 K of a magnitude similar to canonical CDW systems. The present discovery of orthorhombic symmetry for the CDW state of EuAl 4 leads to the suggestion of monoclinic instead of orthorhombic symmetry for the third AFM state.

Thermodynamic insights into the intricate magnetic phase diagram of EuAl$_{4}$

“…Previous investigations of EuAl 4 have observed a charge density wave (CDW) below about 140 K [16,19,22,23,27]. Figure 4 presents thermodynamic signatures of this transition.…”

“…[23]. More recent analysis of single crystal data by Ramakrishnan et al suggest that an orthorhombic CDW modulation is the most likely [22]. Despite broken tetragonal symmetry below T CDW no peak splitting was observed by either diffraction study although, an in-plane shear mode should be allowed.…”

“…Shimomura et al state that phases I and II appear to be tetragonal [23] because they did not observe peak splitting indicating in-plane distortions. Despite this, a new diffraction report reveals that the CDW itself breaks tetragonal symmetry [22] at temperatures well above the magnetic transitions. Their data [22] is best modeled by an orthorhombic structure that would allow a B 2g strain.…”

“…Despite this, a new diffraction report reveals that the CDW itself breaks tetragonal symmetry [22] at temperatures well above the magnetic transitions. Their data [22] is best modeled by an orthorhombic structure that would allow a B 2g strain. Critically, they also do not see a resolvable splitting of diffraction peaks indicating that the distortion of the unit cell is limited.…”

“…Five transitions are observed in EuAl 4 below room temperature (panel (a)). Below T CDW , around 140 K, a charge density wave (CDW) modulation appears [19][20][21][22]. SrAl 4 also hosts a CDW below 243 K [18].…”