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.     

Determination of signs of hyperfine coupling constants for an S = ½ system through E.P.R., ENDOR and double ENDOR

A systematic method of obtaining relative signs of hyperfine coupling constants is described. It applies to systems consisting of (a) a set of one or more nuclei coupled fairly strongly to the electron spin, and possessing a two-fold (or higher) axis of symmetry, together with (b) a set of weakly coupled nuclei defining superhyperfine transitions. ENDOR measurements for several E.P.R. hyperfine transitions, with the field oriented along the symmetry axis, give relative signs of hyperfine components for this direction. Signs for the other directions can then be obtained through ENDOR measurements on a single hyperfine transition at various field orientations. Additional double ENDOR measurements may be necessary for very weakly coupled nuclei. This method can complement double ENDOR studies in favourable cases. It is illustrated by the determination of signs of coupling constants of protons and of ⁷⁵As in the AsO₄ ^4- radical in KH₂AsO₄.     

Hydrogen-bonded ferroelectrics: Evidence for Slater configurations from E.P.R. studies of X-irradiated KH2AsO4 and NH4H2AsO4

The temperature dependence of the powder E.P.R. spectrum of the AsO₄ ^4- centre in x-irradiated samples of KH₂AsO₄ and NH₄H₂AsO₄ yields results which can be understood as arising from exchange processes taking place between the six configurations in the Slater model for this type of hydrogen-bonded ferroelectric and antiferroelectric. Knowing that the ⁷⁵As hyperfine parameters have rhombic symmetry in the compounds studied, it is possible to interpret certain specific features in the spectra as being associated with the Slater configurations of higher energy.     

Mise en evidence par rpe de transformations radicalaires a la transition de phase para-ferroelectrique du cristal de KH2AsO4 irradie

Reversible transformations of free radicals have been observed by EPR at the para-ferroelectric phase transition in X or gamma irradiated crystals of potassium dihydrogen arsenate. In particular, the existence of a transformation of a radical of type AsO^2-₃ into a radical of type AsO^4-₄ suggests that these two radical families result from a common cause during irradiation, namely the capture of an electron on the original AsO^3-₄ tetrahedron.     

23Na nuclear magnetic resonance study of the phase transition in ammonium rochelle salt,NaNH4C4H4O6 • 4H2O

The satellite NMR spectrum of ²³Na in ammonium Rochelle salt (NaNH₄C₄H₄O₆ ? 4H₂O) is investigated near the transition temperature. Each line in the paraelectric phase splits discontinuously into four lines at the transition temperature; this fact is compatible with the first-order phase transition and reveals the existence of the superstructure in the ferroelectric 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.     

The roles of “ammonium” and “hydrogen-bond” protons in single crystals of the superionic conductor NH4HSeO4 by H NMR relaxation

The variations with temperature of the line-shape, spin-lattice relaxation time, T₁, and spin-spin relaxation time, T₂, of the ¹H nuclei in NH₄HSeO₄ single crystals were investigated, and with these ¹H NMR results we were able to distinguish the crystals’ “ammonium” and “hydrogen-bond” protons. The line width of the signal due to the ammonium protons abruptly narrows near the temperature of the superionic phase transition, T_SI, which indicates that they play an important role in this phase transition. The ¹H T₁ for NH₄⁺ and HSeO₄⁻ in NH₄HSeO₄ do not change significantly near the ferroelectric phase transition of T_C1 (=250 K) and the incommensurate phase transition of T_i (=261 K), whereas they change near the temperature of the superionic phase transition T_SI (=400 K). Our results indicate that the main contribution to the low-temperature phase transition below T_SI is that of the molecular motion of ammonium and hydrogen-bond protons, and the main contribution to the conductivity at high temperatures above T_SI is the breaking of the O-H?O bonds and the formation of new H- bonds in HSeO₄⁻. In addition, we compare these results with those for the NH₄HSO₄ and (NH₄)₃H(SO₄)₂ single crystals, which have similar hydrogen-bonded structure.     

Spectroscopic and magnetic properties of Fe3Fe4(AsO4)6

The infrared (IR) and ⁵⁷Fe-Mössbauer spectra of Fe₃^IIFe₄^III(AsO₄)₆ were recorded and analyzed on the basis of its structural characteristics. The IR spectrum presents a high complexity, showing an important number of bands and splittings, as a consequence of the presence of three structurally independent AsO₄³⁻ groups. The analysis of the four quadrupole signals shown by the Mössbauer spectrum allowed to attain a detailed insight into the cation distribution over the available crystallographic sites. The alternating current susceptibility measurements indicate a paramagnetic to ferrimagnetic transition in the material at about 59 K.     

1H one- and two-dimensional five-pulse ESEEM investigations on a radiation center in betaine phosphite single crystals

1D and 2D five-pulse ESEEM experiments on a PO ₃ ^2? center in γ-irradiated betaine phosphite are presented to study the protons bound to the radical in more detail. The ESEEM results are in accordance with previous ENDOR investigations. From the 2D fivepulse ESEEM experiments the¹H ENDOR assignment in the ferroelectric ordered state of betaine phosphite could be completed. Additionally, the occurrence of ordered proton states in the disordered paraelectric high-temperature phase and of disordered proton states in the ordered ferroelectric low-temperature phase as well could be observed due to the enhanced resolution of the 2D method.     

H nuclear magnetic resonance study of the ferroelastic and superionic phase transitions of (NH4)4LiH3(SO4)4 single crystals

We investigated the temperature dependences of the line shape, spin-lattice relaxation time, T₁, and spin-spin relaxation time, T₂, of the ¹H nuclei in (NH₄)₄LiH₃(SO₄)₄ single crystals. On the basis of the data obtained, we were able to distinguish the “ammonium” and “hydrogen-bond” protons in the crystals. For both the ammonium and hydrogen-bond protons in (NH₄)₄LiH₃(SO₄)₄, the curves of T₁ and T₂ versus temperature changed significantly near the ferroelastic and superionic phase transitions at T_C (=232 K) and T_S (=405 K), respectively. In particular, near T_S, the ¹H signal due to the hydrogen-bond protons abruptly narrowed and the T₂ value for these protons abruptly increased, indicating that these protons play an important role in this superionic phase transition. The marked increase in the T₂ of the hydrogen-bond protons above T_S indicates that the breaking of O-H?O bonds and the formation of new H-bonds with HSO₄^- contribute significantly to the high-temperature conductivity of (NH₄)₄LiH₃(SO₄)₄ crystals.     

Observation of a chemical reaction reversibly triggered by a structural phase transition

Two distinct radical species, (AsO₄)^4- and (AsO₃)^2-, created by irradiation in crystals of ND₄D₂AsO₄, undergo (taking into account the deuterons) the following chemical reaction: AsO₃D^- + OD^- ? AsO₄D^2-₂The effect occurs at the antiferroelectric-paraelectric phase transition of the compound. Its essential feature is that the reaction is completely reversible i.e. that it can be triggered by the phase transition, the direction of the reaction depending on the direction of passage of the Curie point. An analogy is presented with a biological process, namely the second step of the cooperative binding of oxygen on haemoglobin.     

Nuclear magnetic resonance in [N(CH3)4]2CoCl4 single crystals: transferred hyperfine interaction and spin-lattice relaxation rate

The molecular susceptibility and paramagnetic shift of [N(CH₃)₄]₂CoCl₄ single crystals were measured, and from these experimental results we obtained the transferred hyperfine interaction, H_hf, due to the transfer of spin density from Co²⁺ ions to [N(CH₃)₄]⁺ ions. The transferred hyperfine interaction can be expressed as a linear equation, with H_hf increasing with increasing temperature. The remarkable change in H_hf near T_c5 (=192 K) corresponds to a phase transition. The proton spin-lattice relaxation times of [N(CH₃)₄]₂CoCl₄ single crystals were also investigated, and it was found that the relaxation process can be described by a single exponential function. The variation of the relaxation time with temperature undergoes a remarkable change near T_c5, confirming the presence of a phase transition at that temperature. From the above results, we conclude that the increase in H_hf with increasing temperature is large enough to allow the transfer of spin density between Co²⁺ ions and the nuclear spins of the nonmagnetic [N(CH₃)₄]⁺ ions in the lattice, and thus the increase in the relaxation time with temperature is attributed to an increase in the transferred hyperfine field.     

Etude ENDOR d'un radical HAsO3 - dans la phase anti-ferroélectrique du cristal d'arséniate d'ammonium NH4H2AsO4

L'identification par ENDOR d'un centre radicalaire HAsO₃ ^- créé par irradiation gamma d'un monocristal de NH₄H₂AsO₄ a été réalisée à 1,5 K. En particulier, les interactions hyperfines correspondant aux protons des trois liaisons hydrogène O-H … O reliant le radical à des tétraèdres AsO₄ ^3- sont déterminées. Un de ces protons est en position ‘proche’ par rapport à un oxygène du radical, en accord avec la formule HAsO₃ ^-, tandis que les deux autres protons sont en position ‘éloignée’. Les parties isotropes des interactions des protons ‘éloignés’ sont trouvées négatives, comme le veut un mécanisme de polarisation de spin à travers une liaison hydrogène partiellement covalente; les protons ‘éloignés’ sont trouvés être en configuration de Slater latérale, ce qui correspond au niveau du défaut à l'ordre antiferroélectrique des protons proposé par Nagamiya.     

Proton spin-lattice relaxation study of the phase transition in TlH2AsO4

Measurements of the temperature and frequency dependence of the proton laboratory and dipolar frame spin-lattice relaxation rates in powdered TlH₂AsO₄ demonstrate that the ⁷⁵As quadrupole resonance frequency changes from v_Q ≈ 38 MHz at T ?T_c to v_Q 〈 10 MHz at T 〉 T_c. The structural phase transition at T_c = 251 K is thus connected with a disordering of the protons in the O–H … O bonds surrounding the AsO₄ groups.     

Transferred hyperfine field of Rb2CoCl4 single crystals in the ferroelectric-incommensurate-normal phase by Rb NMR

The molecular susceptibility and paramagnetic shift of Rb₂CoCl₄ single crystals grown using the slow evaporation method were measured, and from these experimental results we obtained the transferred hyperfine interaction due to the transfer of spin density from Co²⁺ ions to Rb⁺ ions. The transferred hyperfine field was obtained for the ferroelectric, incommensurate, and normal phases. In the case of Rb(I), the transferred hyperfine interaction decreases with increasing temperature in the incommensurate phase, and increases with increasing temperature in the normal phase. The value of H_hf in the incommensurate and normal phases increases abruptly with increasing temperature in the case of Rb(II). These results indicate that the effects due to the transfer of spin density from Co²⁺ ions to the Rb(I) and Rb(II) ions are large above T_i. In particular, the effect due to the transfer of spin density to Rb(II) ions in the normal phase is very large; the variations with temperature of the transferred hyperfine interactions of the Rb(I) and Rb(II) nuclei are more or less continuous in T_c1 and T_i, and are not affected by the ferroelectric-incommensurate-normal phase transitions.     

EPR study of the symmetry breaking effect in ferroelectric cesium dihydrogen phosphate doped with Cr5+ ions

The (PO₄)^3? units in a CsH₂PO₄ (CDP) crystal were replaced in a small fraction of sites by (CrO₄)^3? groups and the EPR of the Cr⁵⁺ center was investigated. Splitting of the EPR line appears at T¹_c=245 K, 91 K higher that the ferroelectric transition temperature T_c=154 K. The electronic wave function of Cr⁵⁺ (3d¹) is identified as d_x2_?y2. The d_x2_?y2 function couples with the near protons and the reorientation of this unit in the two possible configurations occurs in the paraelectric phase and breaks the symmetry far above T_c. The observed correlation time 10^?9 sec and associated activation energy ΔU=0.215 eV are discussed.     

Study of commensurate and incommensurate phases in NaNO2 by 23Na NMR

The temperature dependence was determined of the ²³Na NMR spectrum of NaNO₂ with special emphasis on the small incommensurate phase and the surroundings of the phase transition points. Whereas far away from the transitions satellite lines of Gaussian shape with the natural line widths are observed near the transitions broad line distributions are detected showing distinct edge singularities. The results are related to the sinusoidally modulated and homogeneous order parameters of the incommensurate and ferroelectric phases.     

Critical anomaly in the electron spin-lattice relaxation at the ferroelectric phase transition of X-irradiated KDA

The spin-lattice relaxation of X-irradiated ferroelectric KDA has been investigated by means of the electron spin-echo method in the range between 2 and 200 K. In the vicinity of the phase transition point an anomalous increase of T₁ has been observed. This effect could not be detected for KDA-KDP mixed crystals with a high concentration of KDP. The anomaly of the spin-lattice relaxation at the phase transition is explained by the increased damping of the “hard” optical mode which governs the relaxation behaviour at this temperature region.     

Temperature defendence of the hyperfine interaction in (NH4)2ZrF6

The thermal evolution of the hyperfine quadrupole interaction in (NH₄)₂ZrF₆ has been investigated via TDPAC from 14 to 400 K. A phase transition is detected near 370 K. The low-temperature phase exhibits two sites equally populated for¹⁸¹Ta probes. The associated electric field gradients are described by both high asymmetry parameters and high interaction frequencies. On the other hand, the high-temperature phase corresponds to an unique axially symmetric and more intense electric field gradient.     

Proton Dynamics in Letovicite, (NH4)3H(SO4)2: A H and N NMR Spectroscopic Study

The improper ferroelastic phase letovicite (NH₄)₃H(SO₄)₂ has been studied by ¹H MAS NMR as well as by static ¹⁴N NMR experiments in the temperature range of 296–425 K. The ¹H MAS NMR resonance from ammonium protons can be well distinguished from that of acidic protons. A third resonance appears just below the phase transition temperature which is due to the acidic protons in the paraelastic phase. The lowering of the second moment M₂ for the ammonium protons takes place in the same temperature range as the formation of domain boundaries, while the signals of the acidic protons suffer a line narrowing in the area of T_c. The static ¹⁴N NMR spectra confirm the temperature of the motional changes of the ammonium tetrahedra. Two-dimensional ¹H NOESY spectra indicate a chemical exchange between ammonium protons and the acidic protons of the paraphase.