X-band and Q-band electron paramagnetic resonance (EPR) spectra of Cu(2+) in BaF(2) crystal were recorded in the temperature range of 4.2-200 K. Spin-Hamiltonian parameters of single Cu(2+) complexes and of Cu(2+)-Cu(2+) 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 Cu(2+) ions are involved in local mode motions with energy of 82 cm(-1). Phase relaxation (ESE dephasing) of single Cu(2+) 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 Cu(2+)-Cu(2+) dimers, which disturb dipolar coupling between Cu(2+) 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 Cu(2+) as delta(s)=0.132 nm. The dynamical effects observed in EPR spectra and in electron spin relaxation both for single Cu(2+) ions and Cu(2+)-Cu(2+) pairs are discussed as due to jumps between six off-center positions in the crystal unit cell and jumps between various dimer configurations.

Temperature cw-EPR and pulsed EPR electron spin echo experiments were performed for a low concentration of Cu2+ ions in cubic SrF2 crystals. The well resolved EPR spectrum at low temperatures (below 30 K) with parameters g = 2.493, g = 2.083, A = 121, A = 8.7, A (19 F) = 135, A (19 F) = 33.0 (A -values in 10-4 cm-1 ) is transformed continuously into a single broad line above 225 K on heating, due to the g -factor shift and EPR line broadening. These data along with the angular variation EPR data are described in terms of a pseudo-Jahn-Teller effect of (T2g +A2u ) (a1g +eg +t1u ) type producing six off-centre positions of the Cu2+ ion in the fluorine cube. Above 30 K a two-step averaging g -factor process occurs and is governed by vibronic dynamics between potential wells of the off-centre positions. This dynamics governs the electron spin relaxation in the whole temperature range. The electron spin-lattice relaxation rate 1/T 1 grows rapidly by six orders of magnitude in the temperature range 30-100 K and is determined by the Orbach-type process with excitations to two excited vibronic levels of energy 83 and 174 cm-1 . For higher temperatures the relaxation is dominated by overbarrier jumps leading to the isotropic EPR spectrum above 225 K. The phase memory time TM has the rigid lattice value 3.5 µs determined by nuclear spectral diffusion and its temperature variation is governed by the vibronic dynamics indicating that the excitations between vibronic levels produce a dephasing of the electron spin precessional motion.

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Pressure and temperature variations of the spin-Hamiltonian parameters and electron paramagnetic resonance (EPR) linewidths of non-central Jahn–Teller [CuF4F4]6− complexes in SrF2 crystal were studied by continuous-wave EPR. It was found that the static spin-Hamiltonian parameters, found at T = 85 K and at normal pressure (g∥ = 2.491, g⊥ = 2.083, a∥ = 360, a⊥ = 26, Ax′′ = 96, Ay′′ = 99, Az′′ = 403 and βexp = 17°), are slightly changed with hydrostatic pressure and, at T = 85 K and P = 550 MPa, become equal to g∥ = 2.489, g⊥ = 2.083, a∥ = 348, a⊥ = 27, Ax′′ = 99, Ay′′ = 102, Az′′ = 406 and βexp = 20° (a and A values in megahertz, x′′-, y′′-and z′′-axes are eigenvectors of the super-hyperfine tensor A, βexp is the experimental value of the angle between the C4 symmetry axis of the complex and the x′′-axis). With increasing temperature the well-resolved EPR spectrum of the complex is transformed continuously into a single broad line both at normal pressure and at a hydrostatic pressure of 550 MPa. But in the first case the coalescence point corresponds to 220–230 K while in the second case it is 195–205 K. Treatment using the linear combination of atomic orbitals representation of molecular orbitals (LCAO MO) model was performed to establish some relations between variations of the spin-Hamiltonian parameters and pressure-induced changes in the molecular structure of the complex. To get some additional information about the molecular structure of the complex and variations of its structural parameters with pressure, treatment using the rigid-ion model was performed. Experimental and theoretical results are discussed in the framework of the Jahn–Teller model of the complex.

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