M.A. Augustyniak-Jabłokow

Raman spin-lattice relaxation, Debye temperature and disorder effects studied with electron spin echo of Cu2+ in Tutton salt crystals

Spin-lattice relaxation time T1 was determined by the electron spin echo (ESE) method in the temperature range 4-60 K in a series of Tutton salt crystals MI2MII(SO4)26X2O (MI = NH4, K; MII = Zn, Mg; X = H, D) weakly doped (≤1018 ions cm-3) with the 63Cu2+ isotope. The ESE signal was undetectable at higher temperatures. The relaxation rate increases over the six decades in the studied temperature range with T1 equal to 1 s at 4 K and 0.5 µs at 50 K. Various possible relaxation mechanisms are discussed with the conclusion that the relaxation is governed by two-phonon Raman processes without a noticeable contribution from the reorientations of Cu(H2O)6 octahedra between Jahn-Teller distorted configurations. Deuteration of the crystal has no effect in spin-lattice relaxation. For a few crystals, having the largest Cu2+ concentration among the studied crystals, a strong and linear in temperature contribution to the relaxation rate was found below 15 K. Possible explanations are discussed with the final conclusion that this effect is due to a non-uniform Cu2+ distribution in the host lattice producing effective relaxation via pairs and triads of the Cu2+ ions. From the T1(T) dependence the Debye temperature ΘD was determined for the all crystals studied. This varies from ΘD = 166 K for K2Zn(SO4)26H2O to ΘD = 238 K for (NH4)2Mg(SO4)26H2O. The ΘD values are discussed and used for calculation of the sound velocity which was found to be similar in all crystals and equal to ν = 4150(±150) m s-1.

Electron spin relaxation of vibronic Cu(H2O)6 complexes in K2Zn(SO4)2·6H2O single crystals

The low temperature (6 K) EPR spectrum of Cu2+ in K2Zn(SO4)26H2O is characteristic for the ground state a|x2-y2-b|z2 with essentially the |x2-y2 state and 4.5% admixture of the |z2 state due to the zero-point motions. Temperature variations of the g and A parameters are characteristic for two-well vibronic dynamics and the vibronic averaging of the g-factor is well described with the simple Silver-Getz model above 60 K with energy difference δ12 = 68 cm-1 between the wells. However, this model does not reproduce the A(T) dependence, which seems to be influenced by temperature induced changes in the unpaired electron delocalization onto ligands. Electron spin relaxation was measured at low temperatures up to 55 K where the electron spin echo signal was detectable. At low temperatures, in the static Jahn-Teller limit, the Cu(H2O)6 complexes are strongly localized in the deepest potential well and the slow vibronic dynamics is overdominated by two-phonon Raman processes in electron spin-lattice relaxation. The relaxation rate is described by 1/T1 = aT9I8(ΘD/T) with transport integral I8 and the Debye temperature ΘD = 170 K. Electron spin echo decay is strongly modulated by dipolar coupling with 1H and 39K nuclei and the decay function is V(2τ) = V0exp (-aτ-mτ2). The quadratic term dominates in the static Jahn-Teller limit (below 18 K) and describes the decay produced by the nuclear spectral diffusion. For higher temperatures the exp (-aτ) term dominates and describes an effect of the intrawell excitations with phase relaxation rate 1/TM = a + bexp (-Δ/kT) with Δ = 67 cm-1 being the energy of the first excited vibronic level. The excitations produce a clear broadening of the Fourier transform electron spin echo (FT-ESE) spectra (at 18 K), where peaks from potassium and hydrogen nuclei have been identified.

Electron spin relaxation of the Jahn–Teller Cu(H2O)6 complex in Cs2Zn(SO4)2·6H2O crystals

Abstract The electron spin–lattice and phase relaxation times were measured by electron spin echo (ESE) method in the temperature range 10–50xa0K, where Jahn–Teller dynamics of the Cu(H2O)6 complexes is too slow to dominate in the EPR spectra of Cu2+ in Cs2Zn(SO4)2·6H2O. The spin–lattice relaxation is governed by ordinary the two-phonon Raman processes which overdominate the effects expected from the Jahn–Teller dynamics. It allowed us to determine the Debye temperature of Cs2Zn(SO4)2·6H2O crystals as Θ D =220 K . The ESE amplitude decay is governed by nuclear spectral diffusion below 23xa0K with phase memory time T M =5 μs . At this temperature a broadening of the lines in FT-ESE spectrum (pseudo-ENDOR spectrum) appears, and then a continuous acceleration of the phase relaxation on heating is observed. This is due to the excitations to the vibronic level of energy 71xa0cm−1 in the deepest well of the potential energy surface.

Continuous changes of the Jahn-Teller deformation of Cu(H2O)6 complex in ferroelastic Cs2Cu(ZrF6)2.6H2O crystal

Abstract Q-band EPR spectra of Cs 2 Cu(ZrF 6 ) 2 ·6H 2 O measured in temperature range 4.2–350 K and supported by the X-ray crystal structure analysis, show a gradual transformation of an adiabatic potential surface of the Jahn–Teller (JT) Cu(H 2 O) 6 2+ complex leading to the switch of its elongation direction. This effect is joined with a ferroelastic phase transition at 320 K. It is shown that gradual movement of minima of the adiabatic potential surface along the φ coordinate for ρ ≅ ρ 0 , describing this phenomenon, is a result of a competition between a small lattice deformation at T > T c and the new deformation appearing at T c and increasing with decreasing temperature.

The pair exchange interactions and Jahn–Teller correlations of Cu(D2O)62+ centres in some Tutton salt crystals

Abstract The pair exchange and vibronic interactions in the K2Zn0.92Cu0.08(SO4)2·6D2O (I) and (ND4)2Mg0.92Cu0.08(SO4)2·6D2O (II) single crystals were studied by the X-band EPR in the temperature range 4.2–300xa0K. The pair of the similar (S–S) and dissimilar (D–S) Cu(D2O)62+ ions were recognised and characterised. All interactions were found to be ferromagnetic. The S–S exchange is about ten times stronger than D–S ones. The change of JS–S> in (I) from −0.026xa0cm−1 at 4–20xa0K to −0.047xa0cm−1 above 110xa0K was discovered. In (II) J S – S =−0.044 cm −1 up to 110xa0K and increases to −0.051xa0cm−1 at 130xa0K. The existence of the pair vibronic interactions was discovered in (I). The energy of the ferrodistortive correlations of 25xa0cm−1 was found in (I) with energy interval between the two lowest Jahn–Teller configurations of the isolated ion equal to 72xa0cm−1. The correlations in (II) are suggested to be antiferrodistortive.

Supertransferred hyperfine interactions in layer LaSrGa0.995Cu0.005O4

Abstract The EPR allowing a direct observation of supertransferred hyperfine fields on the nuclei of the next-nearest cations to copper ion in the structure of LaSrGa0.995Cu0.005O4 isomorphous to the superconducting cuprates prove the far delocalisation of the spin density. The theoretical analysis of the value and mechanisms of this process by the method of configurational interaction leads to a good agreement with the experiment. It is shown that the cascade processes involving simultaneous electron transfers from the oxygen to the copper atom and from the neighbouring cation to the same oxygen, in the fragment considered bring a substantial contribution into polarisation of 3s-shell of gallium ion. A comparison of the supertransferred hyperfine fields and g-tensor values in the studied, diluted LaSrGa0.995Cu0.005O4 crystal and in the La2CuO4 cuprate confirms the validity of the local centre approach in the analysis of cuprate properties.

The plasticity of the Cu(H2O) 6 2+ Jahn-Teller complex affected by lattice strains and cooperative interactions

The EPR spectra evolution of Cs2Zn1−xCux(ZrF6)2 · 6H2O (x=0.01, 0.6, 0.8, and 1.0) in the temperature range 4.2–330 K and the x-ray structure analysis of the compound with x=1.0 in the range 150–327 K show that the Jahn-Teller (JT) complex Cu(H2O)6 coordination sphere undergoes a plastic deformation. The observed effect is due to the combined influence of small lattice strains existing in the paraphase and a new one appearing as a result of a ferroelastic phase transition and increasing with decreasing temperature below Tc. It is proved that both cooperative interactions between JT complexes and ferroelastic strain stabilize a certain JT configuration. The problem of instability of a JT configuration compressed at T ∼ 265 K is discussed.

EPR and X-Ray Studies of the Vibronic Effects in Cs2Cu(ZrF6)2.6(H2O) Crystal

EPR and x-Ray studies of Cs2Cu(ZrF6)26(H2O) are reported with special attention paid to the structural phase transitions as a function of temperature and pressure.