Spectra and relaxation of electronic excitations in CsCdBr_3:Yb~(3+) and CsCdBr_3:Nd~(3+) monocrystals

摘要

Experimental and theoretical studies of the optical and EPR spectra, and the spin-lattice relaxation in CsCdBr_3 crystals containing 0.1, 0.3 and 1.0 mol.% of impurity Yb~(3+) ions have been fulfilled. In the optical excitation spectrum, a broad charge transfer band has been detected in the wave-number range of 25000-35000 cm~(-1). The hybridization of the excited electron configuration corresponding to the charge transfer from the ligand (Br~-) 4p closed shells on the ground 4f~(13) configuration of the ytterbium ion is considered to interpret the anomalously large crystal field splitting of the ~2F_(5/2) multiplet. The EPR spectra with comparable intensities of the axial single-ion and symmetric dimer centres in the sample containing 0.3 mol.% of impurity Yb~(3+) ions were observed. Spin-lattice relaxation rates and linewidths in the EPR spectra of the single-ion and dimer centres were measured in the temperature range of 1.5-35 K at the frequency of 9.5 GHz at different directions of the applied magnetic field relative to the crystal symmetry axis. Experimental results are analyzed in the framework of the microscopic theory of the electron-phonon interaction with taking into account peculiarities of the phonon spectrum of the impurity CsCdBr_3 lattice. High-frequency EPR spectra of dimer centres in CsCdBr_3 crystals containing 0.2 and 0.5 mol.% of impurity Nd~(3+) ions were taken in the range of 205-250 GHz at 4.2 K in the magnetic fields up to 0.8 T parallel to the crystal symmetry axis. The crystal field splitting between the first excited and the ground Kramers doublets, and magnetic splitting factors of these doublets were determined. An estimate of the isotropic ferromagnetic exchange constant A = (2.3 ± 0.3) 10~(-3) cm~(-1) in symmetric dimer centres formed by impurity Nd~(3+) ions was obtained from the zero-field splitting of the EPR signals.