Determination of charge-compensated C3v (Ⅱ) centers for Er3+ ions in CdF2 and CaF2 crystals

A unified theoretical method is established to determine the charge-compensated C3 v(Ⅱ) centers of Er3+ions in CdF2and CaF2crystals by simulating the electron paramagnetic resonance(EPR) parameters and Stark energy levels.The potential(Er3+-F~--O42-) and(Er3+-F7~--O2-) structures for the O3 v(Ⅱ) centers of Er3+ions in CdF2and CaF2crystals are checked by diagonalizing 364×364 complete energy matrices in the scheme of superposition model.Our studies indicate that the C3 v(Ⅱ) centers of Er3+ions in CdF2and CaF2may be ascribed to the local(Er3+-F~--O42-) structure,where the upper ligand ion F~-undergoes an off-center displacement by ΔZ≈0.3 A for CdF2and ΔZ≈0.29 A for the CaF2along the C3axis.Meanwhile,a local compressed distortion of the(ErFO4)6-cluster is expected to be ΔR≈0.07 A for CdF2:Er3+and ΔR≈0.079 A for CaF2:Er3+.The considerable g-factor anisotropy for Er3+ions in each of both crystals is explained reasonably by the obtained local parameters.Furthermore,our studies show that a stronger covalent effect exists in the C3 v(Ⅱ) center for Er3+in CaF2or CaF2,which may be due to the stronger electrostatic interaction and closer distance between the central Er3+ion and ligand O2-with the(Er3+-F~--O42-) structure.