Effects of Size and Structural Defects on the Vibrational Properties of Lanthanum Hexaboride Nanocrystals

Lanthanum hexaboride (LaB₆) is notable for its thermionic emission and mechanical strength and is being explored for its potential applications in IR-absorbing photovoltaic cells and thermally insulating window coatings. Previous studies have not investigated how the properties of LaB₆ change on the nanoscale. Despite interest in the tunable plasmonic properties of nanocrystalline LaB₆, studies have been limited due to challenges in the synthesis of phase-pure, size-controlled, high-purity nanocrystals without high temperatures or pressures. Here, we report, for the first time, the ability to control particle size and boron content through reaction temperature and heating ramp rate, which allows the effects of size and defects on the vibrational modes of the nanocrystals to be studied independently. Understanding these effects is important to develop methods to fully control the properties of nanocrystalline LaB₆, such as IR absorbance. In contrast to previous studies on stoichiometric LaB₆ nanocrystals, we report here that boron content and lanthanum vacancies have a greater influence on their vibrational properties than their particle size.