Synthesis, and magnetic and optical properties of nanocrystalline alkaline-earth hexaborides

Cubic-shaped ultrafine alkaline-earth hexaborides (MB6, M = Ca, Ba, Sr) have been synthesized via a solid-state reaction of MO with NaBH4 at 1150 °C. Phase composition, grain morphology, microstructure, magnetic and optical absorption properties were investigated using XRD, FESEM, HRTEM, a SQUID magnetometer and optical measurements. Results show that all of the synthesized hexaborides are composed of a CsCl-type single phase and the average grain sizes of nanocrystalline CaB6, SrB6 and BaB6 are 150 nm, 20 nm and 30 nm at a reaction temperature of 1150 °C. The magnetic measurement results show that all of the synthesized samples show a weak ferromagnetic behavior at room temperature. This is the first time that we have found ferromagnetic properties in nanocrystalline alkaline-earth hexaborides. Moreover, the HRTEM results strongly supported that the magnetic moment of the alkaline-earth hexaborides originated from the intrinsic defects of the nanocrystals. The optical absorption results show strong light absorption in the visible light region and transparency for near infrared rays for nanocrystalline alkaline-earth hexaborides. This interesting optical property should have important effects for extending optical applications, such as in near infrared filtering or detectors.