Molecular structure and dynamics of off-center Cu2+ ions and strongly coupled Cu2+-Cu2+ pairs in BaF2 crystals: Electron paramagnetic resonance and electron spin relaxation studies

摘要

X-band and Q-band electron paramagnetic resonance (EPR) spectra of Cu2+ in BaF2 crystal were recorded in the temperature range of 4.2-200 K. Spin-Hamiltonian parameters of single Cu2+ complexes and of Cu2+-Cu2+ pairs were derived and discussed. A special attention was paid to the dimeric species. Their molecular ground state configuration was found as having antiferromagnetic intradimer coupling with the singlet-triplet splitting J=-35 cm(-1). The zero-field splitting being D=0.0365 cm(-1) at 4.2 K increases with temperature as an effect of thermal population of excited dimer configurations. Electron spin echo (ESE) method was used for measurements of electron spin lattice and phase relaxation. The spin-lattice relaxation data show that except for coupling to the host lattice phonons the Cu2+ ions are involved in local mode motions with energy of 82 cm(-1). Phase relaxation (ESE dephasing) of single Cu2+ ions is due to spin diffusion at low temperatures. This relaxation is hampered for temperatures higher than 30 K due to the triplet state population of neighboring Cu2+-Cu2+ dimers, which disturb dipolar coupling between Cu2+ ions. For higher temperatures the relaxation is dominated by Raman T-1 processes. Fourier transform ESE spectrum displays dipolar Cu-F splitting which allowed determination of the off-center shift of Cu2+ as delta(s)=0.132 nm. The dynamical effects observed in EPR spectra and in electron spin relaxation both for single Cu2+ ions and Cu2+-Cu2+ pairs are discussed as due to jumps between six off-center positions in the crystal unit cell and jumps between various dimer configurations. (C) 2007 American Institute of Physics.