1H ENDOR investigations on gamma-irradiated betaine calcium chloride dihydrate in the incommensurate phase

The electron nuclear double resonance (ENDOR) technique is applied to study the influence of the incommensurate modulation on the proton hyperfine structure (hfs) tensors of the N(CH₃)₃ radical in betaine calcium chloride dihydrate (BCCD). The temperature dependence of the edge singularities due to the increasing of the incommensurate (IC) modulation amplitude could be observed in the¹H ENDOR of the CH₃ groups for the first time. The rotation pattern of the edge singularities observed in the¹H spectra in the IC phase of BCCD can be explained by a libration around the C₁-N axis with a tilt angle of about ±7°.     

Observation of potassium hyperfine interactions in x-irradiated KH2AsO4 through the method of electron spin echo envelope modulation

Hyperfine interaction frequencies of ¹H and ³⁹K nuclei near the AsO^4-₄ radical in X-ray irradiated KH₂AsO₄ (KDA) have been observed through the method of electron spin echo envelope modulation (ESEEM). This method enabled us to record nuclear hyperfine interaction (ENDOR-like) spectra around the ferroelectric phase transition of KDA for the first time. The ESEEM spectrum of ³⁹K exhibits a clear change when passing the ferroelectric phase transition temperature, but that of close protons does not. The result for close protons is in agreement with the symmetry breaking of the AsO^4-₄ site as observed via the EPR spectrum [5]. Finally, at 4.2 K the hyperfine interaction parameters of a ³⁹K nucleus near the AsO^4-₄ unit could be determined through the ESEEM method.     

Nonperturbative simulation of two-pulse electron spin-echo-envelope modulation spectra. Comparison with experimental single-crystal spectra

The direct method of calculating magnetic observables provided by the Liouville formalism is utilized for a nonperturbative numerical treatment of electron spin-echo-envelope modulation (ESEEM). Calculated time-domain signals are compared with experimental ones. The particular system studied is the C0₂ anion trapped in NaHC₂O₄·H₂O single crystals. The experimental ESEEM spectra show modulations due to both proton and sodium nuclei. The hyperfine and quadrupole tensors, determined independently by ENDOR spectroscopy, are used as input for calculating the ESEEM spectra.     

Characteristics of ESEEM and HYSCORE spectra of S>1/2 centers in orientationally disordered systems

One- and two-dimensional electron-spin echo envelope modulation (ESEEM) spectra of Kramers’ multiplets in orientationally disordered systems are simulated using a simple mathematical model. A fairly general high-field spin Hamiltonian is considered with a general g-tensor and arbitrary relative orientations between all tensors involving the electron-spin S, the nuclear spin I, and their interaction. The zero field splitting (ZFS) and the nuclear quadrupole interactions are, however, approximated by their respective secular part in a way that retains all orientation dependencies and it is assumed that the nuclear quadrupole interaction is smaller than the hyperfine interaction. These approximations yield an effective sublevel nuclear Hamiltonian for each EPR transition and are sufficient to account for the most important characteristics of the ESEEM spectra of high electronic multiplets in orientationally disordered systems. Moreover, they allow to obtain some analytical expressions that for I=1/2 illuminate important aspects of 2D hyperfine sublevel correlation (HYSCORE) experiments in S=3/2, 5/2 systems. The pulses are considered as ideal and selective with respect to the different EPR transitions. The contributions of the latter to the echo intensity are weighed according to their different nutation angles and equilibrium Boltzmann populations. For simple axial cases with I=1/2, analytical expressions, analogous to the S=1/2 case, were derived for: (i) the modulation depth, (ii) the lineshapes of the HYSCORE cross-correlation ridges, and (iii) ENDOR powder pattern. Experimental results obtained from Mn(D₂O)₆²⁺ and VO(D₂O)₅²⁺ in frozen solutions are presented, compared, and analyzed in light of the theoretical part.     

Electron paramagnetic resonance and electron nuclear double resonance of the AsO4 4- centre in X-irradiated KH2AsO4

Electron paramagnetic resonance studies of the AsO₄ ^4- centre in X-irradiated crystals of KH₂AsO₄ in the ferroelectric phase at 77 and 4·2 K are reported. The symmetry of the spin-hamiltonian has been found to be orthorhombic, and the g tensor and the A tensor describing the interaction of the unpaired electron with the arsenic nucleus (I = 3/2) have been obtained. Domain splitting has been observed in he spectra recorded in the ab plane of the crystal. By studying these spectra in the presence of an applied electric field, it has been possible to plot the hysteresis curve of ferroelectric KH₂AsO₄. Electron-nuclear double resonance (ENDOR) of protons surrounding the AsO₄ ^4- units has been studied at 4·2 K. Two sets of ENDOR lines have been found arising from the protons in the two equilibrium positions (labelled ‘ close ’ and ‘ far ’) along the hydrogen bonds linking the AsO₄ tetrahedra. The angular variation of the ENDOR lines from both ‘ close ’ and ‘ far ’ protons has been plotted in the crystal symmetry planes. The observed ENDOR frequencies have been fitted to those calculated from the numerical diagonalization of the Hamiltonian. The superhyperfine parameters for the ‘ close ’ and ‘ far ’ protons thus obtained are found to be quite anisotropic. The ENDOR results are shown to explain all details of the partially-resolved proton superhyperfine structure at room temperature as well as at low temperatures. An isotropic contact hyperfine coupling of -2·875 MHz of the unpaired electron to the proton in the ‘ far ’ position of the hydrogen bond has been determined, providing direct evidence for covalency in the hydrogen bonding in KH₂AsO₄ crystals.     

ESR, ENDOR, and ESEEM investigations on UV-irradiated 2,4,6-tri-tert-butyl phenol crystals

Electron spin resonance (ESR), electron nuclear double resonance (ENDOR), and electron spin echo envelope modulation (ESEEM) measurements were carried out for UV-irradiated 2,4,6-tri-tert-butyl phenol in the polycrystalline state. The radical produced in the crystal was detected by ESR and identified to be the corresponding phenoxyl radical, which is well characterized in the chemical oxidations in solutions. ENDOR and ESEEM spectra were unambiguously analyzed in terms of the hyperfine coupling constants determined from well-resolved ESR in solutions. Radical pairs in the crystals were also ascertained, and together with the single-crystal study the analysis disclosed zero-field splitting parameters in the triplet states. ESEEM time decays gave relaxation timesT ₁ = 5.94 andT ₂ = 1.12 μs at room temperature. These appropriate values permit an easy detection of the spin echoes, and therefore this radical matrix can be used as a useful standard for pulsed ESR investigations.     

Editorial

This review contains up-to-date information about experimental and theoretical investigations of phase transitions and specific features of properties of mixed crystals involving the incipient ferroelectric KTaO₃ with the off-center Li⁺, Nb⁵⁺, Na⁺ impurities. Main attention is paid to the results of the study of the mixed systems by radiospectroscopy methods (ESR and NMR).

Theoretical criteria and experimental evidence of ferroelectric and ferroelastic phase transitions as well as of dipole and quadrupolar glass states induced by the off-center impurities are discussed. It is shown that at high enough concentrations of the off-center impurities phases with long-range order appear, but at low concentrations dipole glass states occur. At intermediate concentrations of the off-center impurities new phases appear in which the coexistence of long-range properties and those of dipole glasses is possible. These phases could be named ferroelectric or ferroelastic glasses to stress their similarity with the ferroglass phases which were discovered earlier in mixed magnetic systems. The usefulness of radiospectroscopy methods for the study of phase transitions induced by off-center impurities in mixed systems, which can be considered as model disordered systems, is underlined.

In conclusion, some questions on the physics of phase transitions in disordered systems which should be solved in the future are discussed.     

Two-dimensional ENDOR-ESEEM correlation spectroscopy

A two-dimensional (2D) experiment that correlates electron-nuclear double resonance (ENDOR) and electron spin-echo envelope modulation (ESEEM) frequencies, useful for unraveling and assigning ENDOR and ESEEM spectra from different paramagnetic centers with overlapping EPR spectra, is presented. The pulse sequence employed is similar to the Davies ENDOR experiment with the exception that the two-pulse echo detection is replaced by a stimulated echo detection in order to enhance the resolution in the ESEEM dimension. The two-dimensional data set is acquired by measuring the ENDOR spectrum as a function of the time interval T between the last two microwave pulses of the stimulated echo detection scheme. This produces a series of ENDOR spectra with amplitudes that are modulated with T. Fourier transformation (FT) with respect to T then generates a 2D spectrum with cross peaks connecting spectral lines of the ESEEM and ENDOR spectra that belong to the same paramagnetic center. Projections along the vertical and horizontal axes give the three-pulse FT-ESEEM and ENDOR spectra, respectively. The feasibility of the experiment was tested by simulating 2D ENDOR-ESEEM correlation spectra of a system consisting of an electron spin (S = (1/2)) coupled to two nuclei (I(1) = I(2) = (1/2)), taking into account experimental conditions such as pulse durations and off-resonance irradiation frequencies. The experiment is demonstrated on a single crystal of Cu(2+) doped l-histidine (Cu-His), containing two symmetrically related Cu(2+) sites that at an arbitrary orientation exhibit overlapping ESEEM and ENDOR spectra. While the ESEEM spectrum is relatively simple and arises primarily from one weakly coupled (14)N, the ENDOR spectrum is very crowded due to contributions from two nonequivalent nitrogens, two chlorides, and a relatively large number of protons. The simple ESEEM projection of the 2D ENDOR-ESEEM correlation spectrum is then used to disentangle the ENDOR spectrum and resolve two sets of lines corresponding to the different sites. Copyright 2000 Academic Press.     

Infrared spectra of paraelectric and ferroelectric CsH2PO4

ir spectra are reported for polycrystalline CsH₂PO₄ above and below the transformation temperature, 153 K. Three absorption bands at 1760 cm^-1 2350 cm^-1 and 2800 cm^-1 are assigned to -OHO- vibrations. Whilst the 2350 cm^-1 band remains essentially unaltered, the 1760 cm^-1 band splits into multiplets and the 2800 cm^-1 band shifts to lower frequency on cooling below 153 K. A broad band centred at 3240 cm^-1 is found to exist in the ferroelectric phase and is associated with the -OHO- mode involving the H(2) proton ordered below the transition.     

Nuclear magnetic resonance study of Na (DxH1−x)3(SeO3)2 crystals

A comparison of the quadrupole perturbed deuteron magnetic resonance spectra of the α,β, and γ phase of partially deuterated sodium trihydrogen selenite crystals shows that not only in the monoclinic γ but also in the triclinic β phase the deutrons are ordered whereas they are disordered in the α phase. Spin-lattice relaxation time measurements show the existence of slow hindered rotations of the HSeO₃ and H₂SeO₃ groups at high temperatures as well as the existence of an overdamped ferroelectric soft mode which condenses at the paraelectric to ferroelectric transition.     

Untersuchungen zur Struktur des Triglycinsulfats mit Hilfe der hochauflösenden magnetischen Kernresonanzspektroskopie im Festkörper

Investigations of the Structure of Triglycinsulfate by High Resolution Magnetic Resonance in Solids Measurements were performed in the ferroelectric and paraelectric phase of TGS monocrystals with the aid of multiple pulse experiments for high resolution magnetic proton resonance in solids. The contributions to the spectra, which are to be expected theoretically from the different proton groups (NH₃, CH₂, OH), were calculated and compared with the corresponding experimental results. Splittings are discussed, which occur as a consequence of the ¹⁴N? ¹H-interaction and indicate changes at the phase transition point. The anisotropy of the effective chemical shift tensor caused by the rotation of NH₃-groups and the anisotropy of the chemical shift tensor of OH-groups were determined.     

Proton spin-lattice relaxation by critical polarization fluctuations in KH2PO4

The observed anomalous decrease in the proton spin-lattice relaxation timeT ₁ on approaching the Curie point in a rather pure KH₂PO₄ single crystal is explained by magnetic dipolar coupling to the ferroelectric mode. The isolated “non-interacting” O?H...O proton flipping time is estimated from theT ₁ data as τ=0.66·10^?12 sec for the paraelectric phase and τ=2.24·10^?12 sec for the ferroelectric phase, in good agreement with the results obtained from other methods.     

Statistical mechanics of cation ordering and lattice dynamics of PbZr<Subscript>x</Subscript>Ti<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>3</Subscript> solid solutions

An effective Hamiltonian for Zr-Ti cation ordering in PbZr_xTi_1?xO₃ solid solutions is written out. To determine the parameters of the effective Hamiltonian, a nonempirical calculation is performed within an ionic-crystal model taking into account the deformation and dipole and quadrupole polarizabilities of ions. The thermodynamic properties of cation ordering are studied using the Monte Carlo method. The calculated phase transition temperatures (180 and 250 K for the concentrations x=1/3 and 1/2, respectively) are much lower than the melting temperature of the compound under study. At such temperatures, the ordering kinetics is frozen and, in reality, the phase transition to the ordered phase does not occur, in agreement with experimental observations. Within the same ionic-crystal model, we calculated the high-frequency permittivity, Born dynamic charges, and the lattice vibration spectrum for a completely disordered phase and certain ordered phases. It is shown that soft vibration modes, including ferroelectric ones, exist in the lattice vibration spectrum of both the completely disordered and the ordered phases.     

Non-Kramers ENDOR and ESEEM of the S = 2 Ferrous Ion of [Fe(II)EDTA]

We report here the first non-Kramers (NK) ESEEM and ENDOR study of a mononuclear NK center, presenting extensive parallel-mode ESEEM and ENDOR measurements on the S_t = 2 ferrous center of [Fe(II)ethylenediamine-N,N,N′,N′-tetraacetato]²⁻; [Fe(II)EDTA)]²⁻. The results disclose an anomalous equivalence of the experimental patterns produced by the two techniques. A simple theoretical treatment of the frequency-domain patterns expected for NK-ESEEM and NK-ENDOR rationalizes this correspondence and further suggests that the very observation of NK-ENDOR is the result of an unprecedentedly large hyperfine enhancement effect. The mixed nitrogen–carboxylato oxygen coordination of [Fe(II)EDTA]²⁻ models that of the protein-bound diiron centers, although with a higher coordination number. Analysis of the NK-ESEEM measurements yields the quadrupole parameters for the ¹⁴N ligands of [Fe(II)EDTA]²⁻, K = 1.16(1) MHz, 0 ≤ η ≤ 0.05, and the analysis indicates that the electronic zero-field splitting tetragonal axis lies along the N–N direction.     

Luminescence of Mn ions in ordered and disordered LiAl5O8

The phosphorescence and excitation spectra of Mn ions in the ordered and disordered phases of LiAl₅O₈ have been measured. In both phases Mn²⁺ ions substitute for Al³⁺ ions in two different tetrahedral sites of the LiAl₅O₈ lattice. In both sites in the ordered phase, sharp zero-phonon transitions have been observed in the low temperature phosphorescence and excitation spectra - these transitions were considerably broadened in the disordered phase due to crystal field inhomogeneity in that phase. The deviation from neutrality caused by the Mn²⁺ ions in the ordered phase is largely compensated by Mn⁴⁺ ions occupying octahedral Al³⁺ sites. On disordering, a large proportion of Mn⁴⁺ is reduced to Mn²⁺, while the remainder takes up a site with a higher proportion of Li⁺ ions as next nearest neighbours. This leads to an increase in the ionicity of the Mn⁴⁺ site in the disordered phase and hence to a larger value of the Racah parameter B.     

Effect of electric field on the dielectric permittivity of betaine phosphite crystals in the paraelectric phase

Temperature dependences of the dielectric permittivity of betaine phosphite crystals are studied both without and under application of an electric bias. It is shown that, in view of the fact that the high-temperature improper ferroelastic (antiferrodistorsive) phase transition at T _c1=355 K is nearly tricritical, the nonlinear temperature dependence of inverse dielectric permittivity in the paraelectric phase and the effect of the field on the dielectric permittivity can be described within a phenomenological model containing two coupled (polar and nonpolar) order parameters with a negative coupling coefficient. An analysis of the model revealed that, in the case where two phase transitions, a nonpolar and a ferroelectric one, can occur in the crystal, all of its dielectric properties, including the polarization response in a field, can be described by one dimensionless parameter a. For the crystal under study, we have a=?2.5. This value of the parameter corresponds to a second-order ferroelectric transition far from the tricritical point, at which a=?1. It is shown that the polarization response in the paraelectric phase in an electric field calculated within this model differs radically from that in the ferroelectric phase-transition model for which the Curie-Weiss law holds in the paraelectric phase.     

Ferroelectric phase transition in PbHPO4 studied by IR-spectroscopy of internal modes

The spectrum of the internal modes in PbHPO₄ from 300 to 1100 cm^-1 is measured in reflection between 77 K and 370 K, and mode frequencies are determined by Kramers Kronig analysis. The spectrum is the ferroelectric phase is in agreement with space group Pc. The temperature dependence of the modes can be attributed to proton disorder. There is no indication of PO₄³-distortion in the phase transition and local ferroelectric order still exists in the paraelectric phase.     

Nonlinear dielectric properties of planar structures based on ferroelectric betaine phosphite films

Ferroelectric films of partly deuterated betaine phosphite are grown on NdGaO₃(001) substrates with an interdigitated system of electrodes on their surfaces by evaporation at room temperature. These films have a high capacitance in the ferroelectric phase transition range. The dielectric nonlinearity of the grown structures is studied in small-signal and strong-signal response modes and in the intermediate region between these two modes by measuring the capacitance in a dc bias field, dielectric hysteresis loops, and the Fourier spectra of an output signal in the Sawyer-Tower circuit. In the phase transition range, the capacitance control ratio at a bias voltage U _bias = 40 V is K ? 7. The dielectric nonlinearity of the structures in the paraelectric phase is described by the Landau theory of second-order phase transitions. The additional contribution to the nonlinearity in the ferroelectric phase is related to the motion of domain walls and manifests itself when the input signal amplitude is higher than U _st ～ 0.7–1.0 V. The relaxation times of domain walls are determined from an analysis of the frequency dependences of the dielectric hysteresis.     

Quantitative characterization of the Mn2+ complexes of ADP and ATPγS by W-band ENDOR

W-band (95 GHz) pulsed electron nuclear double resonance (ENDOR) measurements were carried out to determine quantitatively the first coordination shell of Mn²⁺ with ADP and ATPγS. The intensity of the ENDOR effect was used for counting the number of equivalent phosphate oxygens and water ligands. Titration curves for determining the binding constant of Mn²⁺. ADP were obtained using the intensity of the X-band EPR spectrum and the³¹P ENDOR effect. Both curves gave the same binding constant showing that phosphate ligand counting is plausible, provided that an appropriate reference is available. The comparison of the³¹P ENDOR effect of the 1:1 ADP and ATPγS complexes shows that two phosphates are coordinated in both; while in ADP they are equivalent, in ATPγS they are slightly different. The reference system for water ligand counting was Mn(H₂O) ₆ ²⁺ in a H₂O-D₂O mixture. The results show a smaller error for the²H ENDOR effect, compared to the¹H ENDOR effect. Unlike the³¹P ENDOR effect, the¹H ENDOR effect dependence on [ADP] in the titration experiments showed that it is sensitive to variations in the zero-field splitting, which in turn alters the contributions of transitions other than the ‖?1/2>?‖1/2>. This results in a larger error in the determination of the number of water ligands.