Identification of the impurity phase in high-purity CeB₆ by convergent-beam electron diffraction

Abstract

The rare earth hexaborides are known for their tendency towards very high crystal perfection. They can be grown into large single crystals of very high purity by inert gas arc floating zone refinement. The authors have found that single-crystal cerium hexaboride grown in this manner contains a significant number of inclusions of an impurity phase that interrupts the otherwise single crystallinity of this prominent cathode material. An iterative approach is used to unequivocally determine the space group and the lattice parameters of the impurity phase based on geometries of convergent-beam electron diffraction (CBED) patterns and the symmetry elements that they possess in their intensity distributions. It is found that the impurity phase has a tetragonal unit cell with space group P4/mbm and lattice parameters a = b = 7.23 AE 0.03 and c = 4.09 AE 0.02 Å . These agree very well with those of a known material, CeB 4 . Confirmation that this is indeed the identity of the impurity phase is provided by quantitative CBED (QCBED) where the very close match between experimental and calculated CBED patterns has confirmed the atomic structure. Further confirmation is provided by a density functional theory calculation and also by high-angle annular dark-field scanning transmission electron microscopy.

Electronic and structural relations between solid CaB6 and the molecular dianion B6H6(2-): A computational study

“…The same structure is adopted by other MB6 compounds (M = Sr, Ba, and Rare Earths). 12 Among them, LaB6 is well-known by its application as anticathode electron emitter in electron microscopes. In addition the isostructural compounds KB6 and SiB6 have also been reported 13,14 with unit cell parameters quite close to those of the other compounds, so that the nature of the M atoms does not seem decisive in stabilizing the MB6 structure, represented in Figures 1 and 2.…”