Blue LaB6+δ, previously reported to be LaB9, was investigated in arc-melted samples of LaB9 bulk stoichiometry. The samples were structurally analyzed by high-resolution transmission electron microscopy and studied analytically by using back-scattered electron imaging, quantitative characteristic X-ray analysis, nuclear magnetic resonance, and other techniques. The blue boride had a composition of LaB6.13±0.03 due to random La vacancies in the LaB6 structure, and was interpreted as belonging to the non-stoichiometric LaB6 phase region. A color change from purple to blue occurred, with decreasing lanthanum content, at the approximate composition of LaB6.07.

Analytical and Structural Analysis of the Lanthanum-Deficient Lanthanum Hexaboride. McKelvy, M.J., Eyring, L., Storms, E.K.: Journal of Physical Chemistry, 1984, 88[9], 1785-90