Anion-mediated negative thermal expansion in lanthanum hexaboride

Lanthanum hexaboride (LaB6) is well known for its thermionic emission, mechanical hardness, and intriguing optical properties. Though this material has been studied for decades, it is difficult to design LaB6 to meet application needs because little is understood about the mechanistic details of the synthesis. The ability to observe lattice formation during the reaction through in-situ x-ray diffraction is helping improve our knowledge. We report here the strong influence of anion size of the lanthanum precursor in the solid state reaction of LaX3 (X = Cl or I) and NaBH4. The Cl atom of the precursor remains within LaB6 post-synthesis and causes negative thermal expansion when the lattice is heated. Replacing Cl with the larger I atom has a larger impact on crystal growth; however, I does not remain within the lattice post-synthesis. These results suggest subtle new synthetic knobs may be available to optimize the synthesis of LaB6 that have previously gone unexplored. [Display omitted] •In-situ diffraction shows LaB6 lattice evolution during the reaction.•The anion of the lanthanum salt largely influences how crystal growth evolves.•Bridging-Cl remains in final LaB6 particles, but I and Cl both influence growth.•Bridging atoms counter-intuitively cause negative thermal expansion in LaB6.•Halogens within LaB6 are a new concept and opens doors to new research fields.