Electronic structure, magnetic and optical properties of the Ba7(BO3)4−xF2+3x crystal

Abstract

Large size Ba7(BO3)4−xF2+3x (x=0.49) single crystals of optical quality, up to 20mm in size, were grown. Crystals exhibit a wide transparent spectral range from 0.215 to 6.6μm with the indirect energy band gaps of 4.97 and 5.09eV at 300 and 80K, respectively. X-ray irradiation can induce strong absorption in crystals in the UV to near IR regions, mainly from four types of paramagnetic centers: two hole-type centers where oxygen near some defect is ionized (OX1- or O1-) and two electron-type centers where fluorine vacancies capture electrons (e6- and e4-). The electron spin resonance analysis and first-principles studies identified the detailed optical transitions in these centers. The induced absorption can be removed by illumination with photon energy 2.5<hν<4.5eV or after heating to 450K. The electronic structure calculations reveal that in Ba7(BO3)4−xF2+3x both the top of valence bands and the bottom of conduction bands are composed of B 2p and O 2p orbitals. The linear and nonlinear optical properties are dominantly determined by the electron transition within (BO3)3− groups. Nonlinear susceptibility and birefringence are predicted to be d31=0.14pm/V and 0.016 at x=0.25 and decrease to 0.01 and 0.011, respectively, as x grows to 0.75, due to the increase of F content in Ba7(BO3)4−xF2+3x.