The effect of disorder in the local lattice distortions on the EPR and optical spectroscopy parameters for a new Cr 3+ defect center in Cr 3+:Mg 2+:LiNbO 3

Abstract

The relationships between the crystal structure parameters, the zero-field splitting parameter D, the g-factor components ( g ∥, g ⊥), and the splitting δ( 2E) of the 2E(3d 3) state for the double-doped Cr 3+:Mg 2+:LiNbO 3 system are investigated using the superposition model and a complete diagonalization procedure within the 3d 3 configuration. To study the effect of disorder in the local lattice distortions on the experimentally measured parameters, two possible local structure models are considered. Model I assumes variation of the position of O 2− ions around the Cr 3+ defect ion, whereas model II assumes the displacement of Cr 3+ ion from the original Nb 5+ position towards a more centered position in the oxygen octahedron. The detailed analysis suggests that model I describes reasonably the properties of the new Cr 3+ defect center in the Cr 3+:Mg 2+:LiNbO 3 system. The very small value of D, the isotropic g-factor, and the large value of δ( 2E) observed in this system have been successfully interpreted. The calculations indicate that the three O 2− ions above the Cr 3+ rotate by 1.87° towards the [1 1 1]-axis, while the three O 2− ions below the Cr 3+ rotate by 0.31° towards the [1 1 1]-axis. Hence, the disorder effects play an important role in explaining the spectroscopic properties of Cr 3+ ions in Cr 3+:Mg 2+:LiNbO 3.