Concentration effects on the local structures and electronic properties of Erx BaY2- x F8: a first-principles study

Er3+ doped barium yttrium fluoride (BaY2F8) crystal has gained long-term attention due to its great potential in laser and medical device applications. However, the local structures of Er3+ doped BaY2F8 system (Er:BYF) remain uncertain, and the effect of doping concentration on structures and properties is unknown. Therefore, in this study, the first-principles study of the structural evolution of ErxBaY2-xF8 (x = 0.125, 0.25) crystals was carried out. By means of density functional theory and particle swarm optimization algorithm, the stable structures of Er:BYF crystals with two different concentrations are shown as standard monoclinic structures with P2 symmetry for the first time. The impurity Er3+ ions successfully enter the main lattice, replacing the Y3+ ions, and forming a [ErF8](5-) polyhedron with C-2 point group symmetry. By calculating the electronic properties, the band gap values of the two structures are significantly reduced compared with that of pure BaY2F8 crystal. However, the conduction band does not break through the Fermi level, and the crystals still maintain the insulation characteristic. According to the calculation of the electron local density function, we conclude that Er-F and Y-F in Er:BYF are connected by ionic bonds. These results fill a theoretical gap in the study of Er:BYF crystals and provide inspiration for structural evolution and material design at different doping concentrations.