SINGLE-CRYSTAL X-RAY-DIFFRACTION STUDY OF NDB6, EUB6, AND YBB6

The structure of three rare earth hexaborides, NdB6, EuB6 and YbB6, wa s studied by single-crystal X-ray diffraction. The refinements indicat ed boron vacancies for EuB6 and YbB6, but the site occupancies of NdB6 were strongly model dependent, showing either boron or metal vacancie s. No impurity atoms could be detected in the structures. Refinements of a model allowing anharmonic thermal motion of atoms gave small but significant fourth-order anharmonic thermal parameters for NdB6. Stati c displacements of both the rare earth and boron atoms were considered and tested as an alternative representation. It was concluded that th e observed features in the difference density maps were due to charge asphericity.