Preparation and Optical Absorption Properties of Ternary Rare Earth Boride LaPr_1-B₆ Submicron Powders

As multifunctional materials, rare-earth hexaborides (RB₆) display many interesting physical properties such as optical absorption, magnetic and thermionic emission. With the wide application of rare earth hexaboride and the continuous extension of its research fields, researchers have studied the synthesis of multi-rare earth hexaboride nano-powders and their thermal emission and light absorption properties. In the present work, ternary Single-phase LaPr_1-B₆ submicron powders are successfully synthesized using a solid-state reaction, in which lanthanum chloride (LaCl₃) and praseodymium chloride (PrCl₃) are used as rare-earth source and NaBH₄ as the boron source under continuous vacuum conditions. The reaction temperature is 1150 °C and the holding time is 2 h. The Pr doping effects on crystal structure, grain morphology, and optical absorption properties were investigated using X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM) and ultraviolet-vis absorption measurements. The XRD patterns show that the diffraction peaks are sharp and well-defined, and no other extra impurity peaks were detected, indicating the characteristics of well-crystallized materials. It is found that all the LaPr_1-B₆ solid powders are of single phase. The SEM results demonstrate that the cubicshaped ternary LaPr_1-B₆ submicron crystals with a size of 50~200 nm are obtained. The TEM images reveal the cubic single-crystalline nature, and the FFT patterns implies the lattice fringe d = 0.416 nm which agrees well with the (100) crystal plane. The elements mapping results indicates that the Pr atoms occupied the lattice sites of LaB₆. The optical absorption results show that the absorption valley of LaB₆ are located at 591 nm. With the increase of Pr doping contents from 0.2 to 0.8, the absorption valley moves from 596.3 nm to 612.9 nm, indicating the characteristics of visible light high transparency. The first-principle calculation results manifest that the move of the absorption valley of LaB₆ in the visible region after doping Pr is related to the decrease of kinetic energy of electrons near the Fermi plane. X-ray photoelectron spectroscopy (XPS) analysis shows that the La and Pr exist in the type of La³⁺ and Pr³⁺ in LaPr_1-B₆. Therefore, it exists as an efficient optical absorption material. The LaPr_1-B₆ should open up a new route to extend the optical applications of rare-earth hexaborides.