Tailoring the local lattice distortion of Nd3+ by codoping of Y3+ through first principles calculation for tuning the spectroscopic properties

Abstract

Trivalent rare earth ions have rich and unique energy levels arising from 4f inner shell configurations, which can be widely used in lasers, display, and bioimaging, etc. As the f-f transition of the rare earth ions is parity-forbidden, it is necessary to dope the active ions in the host with an appropriate coordination structure to relax the forbidden. In this work, a first principles calculation based on the density functional theory was applied to investigate the local structure symmetries of Nd3+ ions in Nd3+,Y3+:SrF2 crystal. The computational results show that the clusters of [mNd3+-nY3+] (m + n = 2, 3, 4, 5 and 6) would be formed when Y3+ is codoped in Nd3+:SrF2 crystal. The formation energy of the [mNd3+-nY3+] cluster decreases when the value of m + n increases, or the value of n increases with m + n fixed, then the lattice structure becomes more stable. Furthermore, the first coordination shell of Nd3+ was cubic when the n ≤ 1, and it would transform to be the lower symmetric square antiprism structure with n ≥ 2. The forbidden of the electric dipole transition was thus relaxed due to breaking of the symmetry. The experimental results show the absorption cross section of Nd3+ was increased from 0.45 ×10−20 cm2 to 5.2×10−20 cm2, and the emission intensity was also enhanced by about 20 times through codoping Y3+ ions, which agreed well with the calculated results. It suggests that the tailoring of the local lattice distortion of the active ions may open interesting possibilities in the design of rare earth doped materials.