Tutton Salt Research Articles

Temperature evolution of the thermal expansion tensors of ammonium Tutton salts

The temperature dependencies of the lattice parameters and thermal expansion tensors of ammonium Tutton salts (ND4)2 [MII(D2 O)6] (SeO4)2, with MII= Co, Cu, Zn, were studied between 5 and 380 K by time-of-flight neutron powder diffraction. The three compounds exhibit similarly pronounced negative thermal expansions of the monoclinic cell along the c-direction, a behavior which is markedly different from other isotypic Tutton salts. A swap of one N...O hydrogen bond in the ND4 hydrogen-bonding network is identified as the dominant structural feature responsible for the variation of thermal expansion properties within the ammonium Tutton salt family.

Variable temperature inelastic neutron scattering study of chromium(II) Tutton salt: manifestation of the [formula omitted] Jahn–Teller effect

Inelastic neutron scattering (INS) data are presented for the salt (ND 4) 2Cr(OD 2) 6(SO 4) 2, which enable the zero-field-splitting of the 5 A g ( C i ) ground term of the [Cr(OD 2) 6] 2+ cation to be defined in the temperature range 1.5–297 K. Above ∼100 K, the energies of the INS transitions are strongly temperature-dependent, as are the documented Cr–O bond lengths. The experimental data are interpreted using a 5E⊗e Jahn–Teller Hamiltonian perturbed by low symmetry strain. Good agreement is obtained using a parameter set analogous to that employed to describe the electronic and molecular structure of the [Cu(OD 2) 6] 2+ cation in the isostructural copper(II) salt.

Exchange Interactions between Dissimilar JT Copper Ions in K2Zn0.92Cu0.08(SO4)2 � 6D2O and (ND4)2Mg0.92Cu0.08(SO4)2 � 6D2O Crystals

A study of exchange and vibronic interactions in pairs of dissimilar (D-S) Jahn-Teller ions in Cu2+ doped zinc potassium (I) and magnesium ammonium (II) Tutton salts is reported. An interpretation of EPR spectra is done on the basis of the predicted differences in the angular dependences of resonance lines originating from isolated centres and pairs of similar and dissimilar ions. An assignment of D-S pairs is confirmed by the so-called reverse shift manifested as non-coincidence of the centres of pair spectra with those of appropriate isolated ion spectra. It is shown that for the spin–spin interaction parameter K < (g1z — g2z) βB (K is the sum of isotropic (J) and anisotropic (D) contributions) the reverse shift is always present in the spectrum of exchange pairs of D-S ions. The gz values for such pairs are determined by gz pair = g1, 2z ± ¼(g1z — g2z) Δ22, where Δ2 = (Kxx + Kyy)/[2(g1z — g2z) βB]. The reverse shift for the compounds studied takes values of 0.6–1.0 mT. The values of J(I) = 0.0034 cm—1; D(I) = 0.0072 cm—1 for the potassium and J(II) = 0.009 cm—1; D(II) = 0.0068 cm—1 for the ammonium compounds are determined. A possible existence of vibronic correlation is discussed.

Infrared Hole-Burning of Crystalline Ammonium Copper Tutton Salt: Tunneling Kinetics and Jahn−Teller Distortions

Irradiation of the N−D stretching band in crystalline deuterium-doped ammonium copper Tutton salt rotates the ammonium ion and changes the conformation of molecules about the copper. The crystal salt exists in at least two different crystal forms, each form containing a particular Jahn−Teller distorted copper conformation. We consider only the “A” form crystal. In this crystal, the minority conformation (B) appears with the majority conformation (A) as the temperature is raised. The spectral holes and antiholes produced by the changes in conformation occur in patterns that depend on the exact frequency of irradiation. Each pattern appears and eventually decays with different kinetics over a range of times. The variation of the hole widths, measured at different temperatures, reveals the presence of various conformations in the crystal lattice. This inhomogeneity is related to the statistical distribution of conformations A and B weighted by their thermal populations. The kinetics of the decay of the holes...

Elaboration of a new anode material for all-solid state Zn/MnO 2 protonic cells

A new composite consisting of a mixture of zinc and hydrated ammonium zinc sulfate, Tutton salt, has been elaborated and studied as anode material for all-solid state protonic cells. The results obtained point out the ability of (NH 4) 2Zn(SO 4) 2·6H 2O double salt to concomitantly exchange protons with the solid proton conducting electrolyte and accommodate Zn 2+ cations issued from oxidation. Such features have been evinced by thermal and crystallographic characterizations. The anode composition has been optimized through a kinetic study on a three-electrode type-cell, showing that a 35 wt.% salt-based composite displays the minimum polarization. Zn/MnO 2 cells prepared using such a composite anode achieve relatively high specific capacity and energy of, more than 30 Ah kg −1 and 40 Wh kg −1, respectively.

Infrared Spectral Hole-Burning and Spectral Diffusion ofVibrational N-D Bands in a Disordered Tutton Salt

We have studied infrared spectral hole burning of the N-D stretching bands ofNH3D+ in salt [(NH4)2Co0.25Ni0.75(H2O)6(SO4)2] at 10 K. We have observed that thespectral line broadening occurred on a logarithmic timescale,which is consistent with the theoreticalprediction of a standard two-level system. The temperature of 10 K is alsobeyond the temperature limit in previous studies on spectral diffusion.

Dephasing relaxation of the electron spin echo of the vibronic Cu(H(2)O)(6) complexes in Tutton salt crystals at low temperatures.

Two-pulse electron spin echo (ESE) measurements of the phase relaxation (phase memory time T(M)) were performed in a series of Tutton salt crystals M(I)(2)M(II)(SO(4))(2).6X(2)O (M(I)=NH(4), K, Cs; M(II)=Zn, Mg; X=H, D) weakly doped with Cu(2+) ions (c approximately equal to 10(18) ions/cm(3)) in temperature range 4-60 K where ESE signals were detectable. The ESE decay was strongly modulated with proton (or deuteron) frequencies and described by the decay function V(2tau)=V(0)exp(-btau-mtau(2)) with the mtau(2) term being temperature independent and negligible above 20 K. Various mechanisms leading to the tau- or tau(2)-type ESE decay are reviewed. The m and b coefficients for nuclear spectral diffusion (NSD), electron spectral diffusion (SD), and instantaneous diffusion (ID) were calculated in terms of existing theories and the resulting rigid lattice T(0)(M) times were found to be close one to another within the crystal family with average values: 17.5 micros (NSD protons), 200 micros (NSD deuterons), 8 micros (SD), and 5 micros (ID). The ID dominates but the calculated effective T(M)(0) is longer than the experimental T(M)(0)=2 micros. This is due to a nonuniform distribution of the Cu(2+) ions with a various degree of the disorder in the studied crystals. The acceleration of the dephasing rate 1/T(M) with temperature is due to the mechanisms producing exp(-btau) decay. They are reviewed and two of them were found to be operative in Tutton salt crystals: (a) Excitations to the vibronic levels of energy Delta leading to the temperature dependence 1/T(M)=B exp(-Delta/kT), with the vibronic levels produced by strong Jahn-Teller effect, and (b) spin-lattice relaxation processes being effective above 50 K. Based on the Delta values being on the order of 100 cm(-1), the scheme of vibronic levels in the Tutton salts is presented, and the independence of the Delta on temperature proves that the adiabatic potential surface shape of Jahn-Teller active Cu(H(2)O)(6) complexes is not affected by temperature below 65 K.

Effects of ‘random deformations’ in the EPR spectra of Jahn–Teller Cu(D 2O) 6 complexes in the mixed KND 4Zn(SO 4) 2[rad]6D 2O Tutton salt

The EPR study of high quality mixed crystal K(ND 4)Zn(SO 4) 2 6D 2O shows that the doped 63Cu(D 2O) 6 complexes exist simultaneously in two configurations, differing by the directions of Jahn–Teller spontaneous deformation. This observation directly confirms the early supposition that the elongation direction of the doped copper complexes is determined by the indirectly interacting monovalent cations. The high temperature EPR spectra have a ‘powder-like’ appearance and, at lower temperatures, a significant deformation of the resonant lines is observed. These effects are explained as a result of random deformations, introduced by the mixing of ND 4 + and K + ions on the electronic structure and dynamics of the Jahn–Teller hexaaqua copper complexes. Internal deformations change the complex rhombicity and parameters of the Jahn–Teller adiabatic potential leading to their random spread. It is shown that in a low symmetry case, the statistic, symmetrical distributions of these parameters may lead to the asymmetric distribution of g- and A-values, resulting in the asymmetric distribution of the EPR resonant fields. The proposed approach is confirmed by the presentation of theoretical spectra calculated for the assumptions of randomly distributed EPR spectra parameters. These spectra reproduce well experimentally observed spectra in K(ND 4)Zn(SO 4) 2 6D 2O crystals.

Transformations in Crystalline Ammonium Nickel Tutton Salt Induced by Infrared Hole Burning

The title compound doped with a small amount of deuterium shows infrared bands due to the N−D and O−D stretching vibrations. When one of the N−D bands is irradiated with an infrared laser, holes and anti-holes are produced. The pattern of these holes/anti-holes depends on the irradiation time. A ten-minute irradiation produces holes/anti-holes in the N−D bands only. A ten-second irradiation produces holes/anti-holes in the O−D bands as well. The features produced by the ten-second irradiation decay during the longer irradiation. Thus, the burning and decay kinetics of the Tutton salts involve processes that occur on at least two time scales.

Low-frequency vibrational spectra of crystals of tutton salts

IR absorption spectra and polarized Raman spectra of crystals of Tutton salts K2M(SO4)26H2O and (NH4)2M(SO4)2·6H2O, where M=Co, Ni, Zn, have been obtained by experiment at 93 K and at room temperature. The frequencies and forms of normal modes of the [Zn(H2O)6]2+ octahedral complex have been calculated. The observed lines are assigned to the internal modes of the [M(H2O)6]2+ complex and external modes of the crystal lattice in accordance with the results of the calculations and factor-group analysis.

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

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 4.2–300K. 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 −0.026cm−1 at 4–20K to −0.047cm−1 above 110K was discovered. In (II) JS–S=−0.044cm−1 up to 110K and increases to −0.051cm−1 at 130K.The existence of the pair vibronic interactions was discovered in (I). The energy of the ferrodistortive correlations of 25cm−1 was found in (I) with energy interval between the two lowest Jahn–Teller configurations of the isolated ion equal to 72cm−1. The correlations in (II) are suggested to be antiferrodistortive.

Bonding effects and the crystal structures of (NH4)2[Cu(H2O)6](SO4)2 and its H2 18O substituted form at 9.5 K

The crystal structures of the Tutton salts (NH4)2[Cu(H2O)6](SO4)2, diammonium hexaaquacopper disulfate, formed with normal water and isotopically substituted H2(18)O, have been determined by X-ray diffraction at 9.5 K and are very similar, with Cu-O(7) the longest of the Cu-O bonds of the Jahn-Teller distorted octahedral [Cu(H2O)6]2+ complex. It is known that structural differences accompany deuteration of (NH4)2[Cu(H2O)6](SO4)2, the most dramatic of which is a switch to Cu-O(8) as the longest such bond. The present result suggests that the structural differences are associated with hydrogen-bonding effects rather than with increased mass of the water ligands affecting the Jahn-Teller coupling. The Jahn-Teller distortions and hydrogen-bonding contacts in the compounds are compared with those reported for other Tutton salts at ambient and high pressure.

Structure and some physical properties of K2Ni(SO4)2 ⋅ 6H2O crystals

The data obtained from the analysis of the solubility phase diagrams have been used in growth of K2Ni(SO4)2 ⋅ 6H2O crystals. The X-ray diffraction studies of the crystals grown showed that, at room temperature, the crystals belong to the diffraction class 2/m. The crystal structure is similar to the structures of the Tutton salts. The study of the absorption spectra and the differential scanning calorimetry (DSC) revealed a pronounced anomaly in the vicinity of 45–46°C, which seems to be associated with the structural changes in the crystal.

Structural and EPR study of the dependence on deuteration of the Jahn-Teller distortion in ammonium hexaaquacopper(II) sulfate, (NH4).

The variation of the EPR spectra with degree of deuteration of the partially deuterated Tutton salt ammonium hexaaquacopper(II) sulfate, (NH4)2[Cu(H2O)6](SO4)2, has been measured at 293 K. The measurements indicate that the structure changes quite abruptly from that of the pure hydrogenous salt to that of the fully deuterated salt at approximately 50% deuteration. The structure of a crystal in which approximately 42% of the hydrogen atoms were replaced by deuterium was elucidated at 15 K by single-crystal time-of-flight neutron diffraction. The hexaaquacopper(II) complex exhibits an orthorhombically distorted, tetragonally elongated octahedral coordination geometry (Cu-O bond distances of 2.281(1), 2.007(1), and 1.975(1) A). The structure is very similar to that reported for the undeuterated salt at 9.6 K, and markedly different from that of the fully deuterated compound at 15 K, which has similar Cu-O bond lengths but with the directions of the long and intermediate bonds interchanged. There is no evidence for disorder or partial switching of the Cu-O bond directions. This is consistent with the temperature dependence of the EPR spectrum of the approximately 42% deuterated compound, which indicates a thermal equilibrium between the two structural forms close to room temperature similar to that reported for the undeuterated compound, but complete reversion to the low-temperature phase on cooling to 5 K. The possible influence of deuteration upon the hydrogen-bonding distances and the bearing of this upon the structural modifications of the compound are discussed.

The temperature dependence of the pressure switching of Jahn–Teller deformation in the deuterated ammonium copper Tutton salt

The pressure switch of the Jahn–Teller deformation direction in (ND4)2Cu(SO4)2·6D2O was investigated in the temperature range 130–320 K. Below 295 K, the new, pressure-induced phase, is stable under ambient pressure. Switching back is observed on heating to above 297 K. In the range 150–295 K a strong temperature dependence of the switching pressure (from 24 to 450 MPa) is observed. Below 150 K, the switching process is slow and a coexistence of two phases is observed. We conclude that the switch of the Cu(D2O)6 complex deformation direction is the Jahn–Teller response to the changes in the hydrogen bond system.

Cooperative Jahn−Teller Interactions in Dynamic Copper(II) Complexes. Temperature Dependence of the Crystal Structure and EPR Spectrum of Deuterated Ammonium Copper(II) Sulfate Hexahydrate

The crystal structure of the deuterated Tutton salt (ND4)2[Cu(D2O)6](SO4)2 determined by X-ray diffraction at several temperatures between 100 and 321 K is reported. The intermediate and longest Cu−O bond lengths of the Jahn−Teller distorted octahedral Cu(D2O)62+ ion progressively converge as the temperature is raised above ∼200 K, and this is accompanied by a partial rotation of the ammonium and sulfate groups. The g-values derived from the EPR spectrum of the compound exhibit similar behavior, as do those of ∼1% Cu2+ doped into the isomorphous zinc(II) compound. The temperature dependence of the g-values of Cu2+-doped (ND4)2[Zn(D2O)6](SO4)2 may be interpreted satisfactorily by a model which assumes a Boltzmann thermal distribution between two energy states which differ solely in the orientation of the Cu(D2O)62+ ion in the lattice. However, such a model does not satisfactorily explain the behavior of pure (ND4)2[Cu(D2O)6](SO4)2, and it is suggested that this is due to cooperative interactions. A new model, in which the probable energy state of each complex is estimated after taking into account the likely orientations of its neighbors in the lattice, is described. Application of this model suggests that the thermal behavior is dominated by the cooperative interactions between complexes, these being transmitted via the hydrogen-bonding network. Comparisons are drawn with the cooperative interactions observed in more strongly coupled Jahn−Teller systems and in compounds for which the structural changes are associated with equilibria between two spin states.

The host lattice influence on the Jahn-Teller effect of theCu(H2O)62+ complex studied by EPR in K2Zn(SO4)2.6H2Oand (NH4)2Zn(SO4)2.6H2O Tutton salt crystals

The EPR investigations of Cu2+ doped K2Zn(SO4)2.6(H2O) and (NH4)2Zn(SO4)2.6(H2O) Tutton salts in a temperature range of 4.2-300 K are reported. The analysis of the temperature dependences of the g-factors points to significant differences between Jahn-Teller Cu(H2O)62+ complexes in the ammonium and potassium compounds. In the ammonium salts the energy difference 12, between the ground and excited configurations, is found to be temperature dependent. This is explained by the lattice contribution to the adiabatic potential. Various mechanisms of such an influence are proposed. For the discussed case it is suggested that thermal librations of the ammonium groups will change the strength of the hydrogen bonds between the sulphate groups and water octahedra and in this way influence the Jahn-Teller complex.

The crystal structure of Tutton's salt type K2[Co(H2O)6](SO4)2. A combined X-ray and neutron study

Abstract The crystal structure of dipotassium hexaaquacobalt(II) sulfate, K2[Co(H2O)6](SO4)2, has been determined at room temperature by single crystal X-ray diffraction. The deep purple crystals grown from aqueous solution are monoclinic, space group P21/a, with unit cell dimensions a = 9.057(1) Å, b = 12.211(2) Å, c = 6.155(1) Å, β = 104.82(1)°, Z = 2, D c = 2.207 Mg/m3. The hydrogen atom positions were obtained from a Rietveld refinement on neutron powder data measured on a deuterated sample. Fixing these positions in the X-ray structure refinement on 1162 unique reflections (I &gt; 2σ) resulted in R(F) = 0.0188, wR(F 2) = 0.0504, GOF = 1.14. All structural details compare well with corresponding results reported for other members of the Tutton's salts series.

Concepts in high-frequency EPR — Applications to bio-inorganic systems

In this contribution we describe a high-frequency high-field EPR facility which has been developed at the University of Nijmegen. We present the design of a heterodyne quasi-optical bridge based on a millimeter-wave vector network analyzer as source and detection system. The mm-waves are transported in free-space through Gaussian beam optic elements and through a corrugated guide inside the resonator insert. The Fabri-Perot (TEM00n) resonator is coupled through a metallic mesh and because of its bimodal property it can be operated using orthogonal detection leading to substantial improvement in sensitivity. In the first stage of the project, a multifrequency CW-facility is realized covering the 100–500 GHz range. In our initial explorative experiments we demonstrate the advantages of HF-EPR of high-spin systems: Due to the large microwave quantum, transitions which would be undetectable at X-band due to the large zerofield splitting can now be observed in good sensitivity. As a model for biological high-spin systems a sample of metmyoglobin was measured at D-band (130 GHz). Theg = 5.9 perpendicular line from theS = 5/2 ferric heme was detected and its line-width was compared to data previously obtained at Q-, X-, S- and L-band. As a model for biological integer spin systems theS = 1 signal of Ni(II) in nickel Tutton salt (Ni(NH4)2(SO4)2) was studied at 35 and 130 GHz.

Vibronic Behavior and Electron Spin Relaxation of Jahn−Teller Complex Cu(H2O)62+ in (NH4)2Mg(SO4)2·6H2O Single Crystal

Temperature dependences of the CW-EPR spectrum as well as the electron spin−lattice relaxation time T1 and phase memory time TM determined by electron spin echo were measured for Cu2+ ions in (NH4)2Mg(SO4)2·6H2O single crystal. The dependences are dominated by vibronic behavior of the Cu(H2O)62+ complex and connected to the dynamic Jahn−Teller effect producing reorientations between two lowest energy wells of the adiabatic potential surface. Below 70 K the static Jahn−Teller effect is observed, and the system is strongly localized, by the local strains, in the deepest potential well leaving higher wells not populated. Above this temperature the second well becomes progressively populated, and a rapid averaging of the gz and gy factors as well as corresponding hyperfine splittings appears. The Boltzmann population of these two wells is achieved at 160 K. Simultaneously with g factors averaging a continuous broadening of the hyperfine lines is observed with line shape transformed from Gaussian at 70 K to Lorentzian at 160 K. The averaging and broadening processes are thermally activated with energy barrier δ12 = 108 ± 3 cm-1 = 156 K = 1.26 kJ/mol being the energy difference between the two deepest potential wells. Electron spin relaxation was measured below 50 K where electron spin-echo signal was detectable. Spin−lattice relaxation is driven by the direct and Raman processes with relaxation rate 1/T1 = aT + bT5 as expected for dynamic Jahn−Teller systems. Spin−spin phase relaxation described by the phase memory time TM depends on temperature as 1/TM = a + bT + c exp(−Δ/kT) with Δ = 102 ± 2 cm-1. At low temperatures the higher energy well is not populated; thus, Δ can be assigned as the energy of the first excited vibronic level in the deepest well. The Δ and δ12 are temperature-independent, indicating that adiabatic potential surface is not affected by temperature. We suggest that the deviations of experimental data from theoretically predicted vibronic g-factors averaging observed for Cu2+ in many Tutton salt type crystals are not due to temperature variations of the local strains or barrier height but are due to the fact that Boltzmann population of the potential wells cannot exist at low temperatures. This effect is especially pronounced for Cu2+ ions in (NH4)2Mg(SO4)2·6H2O since the energy Δ of the first vibronic level is lower than the energy difference δ12 between adjacent wells. In such case the phonon-assisted tunneling jumps between the energy wells induced by two-phonon Raman processes via virtual state of energy δ12 become to be effective when kT ≥ δ12/2, and the Boltzmann population of the second well is achieved via direct phonon process when thermal phonons of energy kT ≥ δ12 are available.