Orthorhombic charge density wave on the tetragonal lattice of EuAl4.

EuAl4 possesses the BaAl4 crystal structure type with tetragonal symmetry I4/mmm. It undergoes a charge density wave (CDW) transition at T CDW = 145 K and features four consecutive antiferromagnetic phase transitions below 16 K. Here we use single-crystal X-ray diffraction to determine the 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(00σ)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 Al atoms as the location of the CDW. The tem-per-ature 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 EuAl4 leads to the suggestion of monoclinic instead of orthorhombic symmetry for the third AFM state.

Boranes: The Boron Subhydride B104.67H3 with a Distorted ß-Boron Crystal Structure.

A single crystal of the boron subhydride B104.67(4)H3 was serendipitously obtained while attempting to synthesize ß-boron. An accurate crystal structure analysis revealed a distorted ß-boron framework with the noncentrosymmetric space group R3m. We have found one interstitial site occupied by boron. The site related by inversion remains empty. The distortions of the framework result in ideal environments for the interstitial boron atom, and for the three hydrogen atoms at bridging positions between icosahedral B12 groups, they result in ideal B-H distances of 1.33 Å. B104.67(4)H3 is a borane with the lowest amount of hydrogen recorded to date, and it is the first compound with a noncentrosymmetrically distorted ß-boron framework.