Red-emission over a wide range of wavelength at various temperatures from tetragonal BaCN2:Eu2+

A new polymorph of BaCN2 was obtained via a simple nitridation reaction of BaCO3 under an NH3 flow at 900 °C. The product was analyzed by single crystal X-ray diffraction and infrared spectroscopy, and found to have a tetragonal I4/mcm crystal structure (space group no. 140) with a = 0.60249(4) nm and c = 0.71924(5) nm. In this structure, each Ba2+ cation is situated in the square antiprism of N atoms of NCN2− anionic groups. Eu2+ doped BaCN2 can be excited by irradiation of blue and green light (from 400 to 550 nm), and generates an intense red emission peak at 660 nm with a quantum efficiency of 42% in response to 465 nm excitation at room temperature. The peak emission wavelength varies over an extremely wide range with temperature, from 640 nm at 500 K to 680 nm at 80 K, and this red-shift with decreasing temperature is attributed to a unit cell shrinkage that results in significant crystal field splitting of the 5d energy levels of the Eu2+ ions.