Evolution of Vibrational Properties in Lanthanum Hexaboride Nanocrystals

Lanthanum hexaboride (LaB6) is known for its hardness, mechanical strength, thermionic emission, and strong plasmonic properties. However, given the lack of colloidal synthetic methods to access this material, very little is understood about its physical properties on the nanoscale. Recently, a new moderate-temperature synthetic technique was developed to directly synthesize LaB6 nanoparticles [Mattox et al. Chem. Mater. 2015, 27, 6620]. We report the influence of nanoparticle size on the structural and vibrational properties of LaB6 using a combination of Raman and Fourier transform infrared spectroscopies. Our studies indicate that the size of the lanthanum salt anion has a larger influence on LaB6 vibrational energies than particle size. Surprisingly, our work finds that the LaB6 lattice readily expands to accommodate larger ions and contracts with their removal, while ligand incorporation significantly amplifies and shifts the Raman stretching modes.