Off-centre dynamic Jahn-Teller effect studied by electron spin relaxation of Cu²⁺ions in SrF₂crystal

Abstract

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.

Theoretical Studies on the Local Structure and Spin Hamiltonian Parameters for Single Cu2+Ion in BaF2

“…In fact, for other fluorite-type crystals doped with d 9 ions, the large off-centre shift (≈ 3.10 Å/2−∆Z ≈ 1.35 Å for the studied system) was also found, e.g. Cu 2+ ion SrCl 2 (≈ 1.35 Å) [2], Ni + ion in CaF 2 (≈ 0.99 Å) [6] and SrF 2 (≈ 1.08 Å) [7]. Therefore, the local structure parameter ∆Z of the impurity Cu 2+ center in BaF 2 obtained in this work can be regarded as reasonable.…”

“…Many paramagnetic ions (such as Cu 2+ [1][2][3][4], Ag 2+ [5], Ni + [6,7], Cr 2+ [8], Mn 2+ [9], Fe 3+ [10,11], Ce 3+ [12], Ti 3+ [13]) have been introduced into cubic fluorite-type crystals to study the local structure of the host lattice and the electronic properties of paramagnetic impurities in these crystals by means of electron paramagnetic resonance (EPR) technique, which would be useful to understand the properties (such as optical and luminescent properties) of crystals containing ions. Based on the EPR experiments and theoretical studies, it is found that the local structures around many paramagnetic centers in these crystals are of tetragonal symmetry or orthorhombic symmetry [8].…”

Electron Spin Relaxation in Pseudo-Jahn–Teller Low-Symmetry Cu(II) Complexes in Diaqua(L-Aspartate)Zn(II)·H2O Crystals

“…8). The dephasing of the precessional spin motion due to this mechanism has not been considered theoretically in detail, but it was suggested (48) and observed by us for Cu 2+ ions in different crystals (5,20,39). The possible excitations and relaxation back within the same potential well (Orbach-Aminov-type process) do not operate, since then w should be on the order of the vibrational frequency 10 13 s −1 .…”

Electron spin relaxation of exchange coupled pairs of transition metal ions in solids. pairs and single ions in crystals

“…However, the difference in ionic radii of host and guest ions and the difference in their electronic structure can produce a local distortion and host lattice relaxation around the impurity ion. Cu 2þ ions in SrF 2 [15,16] and in BaF 2 [8] are off-center shifted due to Jahn-Teller or pseudo-Jahn-Teller effect [17]. Ti 2þ ion ð3d 2 ; S ¼ 1Þ in the cubic environment has orbitally non-degenerate ground state, thus it is not the Jahn-Teller active ion and off-center shift is not expected.…”