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NH4CuCl3, a well-studied antiferromagnet best known for its gapless magnetization and unusual magnetization plateaus, is shown here to exhibit magnetoelectric behavior below the antiferromagnetic transition temperature of 1.3 K. This is based on magnetic field and temperature dependent capacitance measurements made at millikelvin temperatures, along with the appearance of spontaneous polarization and electric-field reversible pyroelectric current. The possession of both long-range magnetic and dielectric order, combined with magnetoelectric coupling, renders this material a rather unique Cu-based multiferroic and serves as a unique template for the design of such materials.

The microscopic mechanim leading to a low-temperature antiferroelectric (AFE) phase in H-bonded NH 4 H2PO 4 is studied by means of first-principles calculations in the framework of the Density Functional Theory. We compute the total energy for atomic displacements following the ferroelectric (FE) and AFE mode patterns and also constraining the atoms in different cases. We calculate Mulliken populations for atoms and orbitals to show the important charge transfers and instabilities of the system. The AFE phase appears as a consequence of the H ordering in the O─H─O bridges leading to polarizations of the H 2 PO 4 perpendicular to the tetragonal crystal axis and alternating in chains. It is found that the formation of N─H─O bridges with correlated charge transfers is essential in stabilizing this configuration against a FE phase polarized along the tetragonal axis, as e.g. in KH 2 PO 4 (KDP). We also study de closeness of FE with AFE configuration energies and other hypothetical phases, which may explain th...

An anomaly in the magnetic susceptibility has been observed at and below the paraelectric – ferroelectric transition in polycrystalline Pb(Fe1/2Nb1/2)O3. The results are indicative of magnetoelectric coupling of the form P2M2. The non-linear relation between the induced magnetization and the applied magnetic field demonstrates the presence of large superparamagnetic clusters embedded in a paramagnetic medium over the whole ferroelectric phase, i.e. far above the antiferromagnetic transition temperature T N .

We report high resolution 15N NMR measurements of the paraelectric to antiferroelectric phase transition (TN = 216 K) of the model hydrogen-bonded antiferroelectric NH4H2AsO4 (ADA). We specifically examined whether the NH4 + ions undergo a displacive or an order-disorder behavior at the phase transition. High resolution NMR measurements were made on 15N-enriched single crystals, using the CPMAS technique at a frequency of 50.6 MHz (11.4T). The temperature dependence of the isotropic chemical shift, δiso, and of the spin-lattice relaxation time, T1, shows clear anomalies at the phase transition. These results are interpreted as evidence for the coexistence of an order-disorder and displacive behavior at the NH4 + site.

The high NMR spectral resolution attained via magic angle spinning (MAS-NMR) on single crystals have enabled us to detect new aspects of mechanism of phase transitions in hydrogen-bonded ferroelectrics and antiferroelectrics. Results are summarized for anomalous changes in the temperature dependence of the isotropic part of the NMR chemical shifts for 13C and 17O in squaric acid (SQA), and for 31P from NH4H2PO4, KD2PO4 and RbH2PO4, and more recently on 15N in NH4H2AsO4 and NH4H2PO4. These data are interpreted as providing an unambiguous evidence of a displacive component, together with an order-disorder behaviour at the same time scale for both SQA and KDP-type lattices.

Using ab-initio electronic structure calculations, we show that Slater defects in KH 2 PO 4 (KDP) are stable only within chains, in accordance with the displacive model of the transition. In contrast, Takagi defects are unstable, with formation energy (53.5 meV) in fair agreement with previous phenomenological estimations. As for Slater defects, their correlated formation energy per formula unit (5.2 meV) agrees well with previous model assignments, while the isolated defect energy comes out ≈3 times larger. Slater defect chains are accompanied by phosphate rotations and a basal lattice contraction. Observations of the latter near T c could thus be explained.

Magic angle sample spinning (MAS)7O NMR studies of the paraelectric-antiferroelectric phase transition (at TC = 373 K) of the model hydrogen-bonded compound squaric acid (H2C4O4 ) reveal a significant displacive component in the microscopic mechanism of the transition. The high resolution (400% enhancement over conventional MAS) was obtained by utilizing single crystals. All four oxygens were clearly shown to be chemically different at T TC impli...

An extensive electronic structure analysis of squaric acid (H 2 C 4 O 4 , SQA) was performed by density functional techniques, including a serie of hydrogen-bonded systems to model the solid state compound. In addition, some magnetic properties were calculated by GIAO method. As result, we obtain a theoretical model able to characterize with accuracy the 13 C NMR data of this type of systems with an antiferroelectric phase transition. A closer connection between hydrogen bonding and aromaticity was found in SQA. Thus, there are two different hydrogen bonds in each molecule of the solid, associated to the two clearly distinct O=C-C=C-OH resonant chains. Consistently, the study of the proton transfer process in SQA shows two energetically different coordinates for the hydrogen movement. Moreover, the theoretical NMR results show that a hydrogen bond lengthening is not coherent with the experimental evidences for the phase transition in SQA. Current theoretical results support a new description of the phase ...

The temperature dependent dielectric and magnetic relaxations in the quantum nanomagnet Mn12-Acetate and its partially and fully deuterated analogs have been studied. One magnetic mode and four dielectric modes have been identified in the temperature interval from 300 K to 2.8 K and in the frequency range from 20 Hz to 1 MHz. The relaxation times of all modes follow an Arrhenius law. Comparison of the magnetic susceptibility mode to the NMR data shows that the magnetic susceptibility mode can be related to the magnetization reorientation fluctuations, while three slowest dielectric modes can be attributed to the motion of H2O and the forth mode to that of the CH3 groups.

Despite the general belief that hydrogen-bonded ferro- and antiferroelectrics undergo a pure order/disorder transition at the structural instability, new NMR data and a new theoretical concept yield convincing evidence that a pronounced displacive component is present in these systems. The experiments and their interpretation are presented.

Using squaric acid (H2C4O4) as an example, it is shown that the temperature dependence of the isotropic chemical shift (δISO), as measured by magic angle spinning (MAS) NMR, can serve as a probe of the displacive vs. order-disorder character of a phase transition. It is also found that the utilization of single crystals in the MAS measurements leads to significantly narrower peaks than using powders. In the case of MAS on squaric acid. single crystals exhibit a four-fold narrowing of the 13C peaks, as compared to those from powders. This additional gain in resolution has made it possible to measure a sharp anomaly in the δISO at the paraelectric-antiferroelectric phase transition of squaric acid (Tc ∼ 373 K). This anomaly is interpreted as a direct evidence that the transition mechanism involves a displacive component for the hydrogens as T- < Tc, suggesting that MAS/NMR can indeed serve as a new complement to the current methodologies for investigating phase transitions.

The temperature dependence of lineshapes of EPR signals of AsO4 4 center has been utilized as a probe of the slow dynamics of (NH4H2PO4)x-(RbH2PO4)1–x proton glass system over the whole composition range x. The EPR signals exhibit anomalous line-broadening as a function of x as well as temperature. A theoretical procedure was developed based on the formalism of modified Bloch equations, that enabled us to evaluate the broadening effect of motional dynamics, and, consequently, lattice strain. A comparison of the experimental and theoretical lineshapes revealed that strain broadening, and not exchange effects, makes a dominant contribution to the broadening of EPR signals in the proton glass.

A quantitative understanding of the Electron Spin Resonance spectra of the AsO4- 4 center in RbH2AsO4 (RDA) and RbD2AsO4 (DRDA)—needed to investigate ESR features in the mixed crystal Rb1–x(NH4)xH2AsO4—is presented. The methodology developed for RDA and DRDA is alluded to provide a quantitative understanding of the mixed crystal as well.

Ab-initio molecular orbital cluster calculations with Gaussian-90 at Pittsburgh Supercomputing Center were carried out in order to understand the mechanism of dipole moment development and, hence, ferroelectricity in KH2PO4. Our specific goal was to evaluate the effect of the movement of H and P atoms on the magnitude and sign of the dipole moment. The equilibrium geometry was taken from earlier neutron diffraction studies. We designed two sets of clusters, H2PO4, and H4PO4, to perform the electronic structure calculations with bases set of 6–31G*, for paraelectric and ferroelectric phases. The calculations yield electric charge density distribution in the systems. It is seen that the dominant contribution to the development of dipole moment arises from the charge redistribution along the P-O-H bonds. The calculated results show clearly that the z-direction polarization develops as the H atoms move closer to the O atoms. Then, the polarization drives the P atom to move away from the center of the...

A methodology is presented for carrying out ab-initio molecular orbital calculation of the potential energy surface as a function of position of the H atom in the O-H—O bonds of KH2PO4 (KDP). The calculations employ the Hartree-Fock Self-Consistent-Field (SCF) approach utilizing the Linear Combination of Atomic Orbital for Molecular Orbital (LCAOMO) procedure with basis in Gaussian form. This approach seems to yield a reliable representation of double-minimum potential well for the O-H—O bond in KDP, as judged from the agreement between the predicted (0.3604 Ŕ) and experimental (0.3674 Ŕ) values for the separation between the two equilibrium positions of the H atom along the O—O direction. The calculated value of the barrier height (0.08 eV) helps clarify some earlier spectroscopic data on the activation energy for the motion of the H-atom in the O-H—O bond in KDP. The same methodology should be applicable for investigating the possible double-minimum wells for the heavier atoms in the KDP-type o...

Research in ferroelectricity has expanded to new and more complex systems and new experimental techniques are being applied. A brief review is given of recent information obtained. In particular, it is now becoming possible to probe on different length scales, from the local order to the long range order. This information should open the way for the development of microscopic models of ferroelectric transitions and a better understanding of the relationship between electronic structure and lattice dynamics.

Electron Paramagnetic Resonance (EPR) has been found to be a quick and easily accessible method for measuring the London penetration depth, λ, for the high-Tc superconductors. The method utilizes the broadening of the EPR signal of a spin-probe compound adsorbed to the surface of the superconductor. The broadening of the EPR signal is due to the formation of the magnetic flux lattice below Tc and is measured as the second moment, . is fitted to the Brandt equation for a simple triangular lattice: = 0.00371λ0[1-(T/Tc)4]−1/2. This method yields λ0 = 2700 ± 100 A with a Tc of 84 K for Bi2Sr2CaCu2 Ox and 2520 ± 100 A with a Tc of 119 K for Tl2Ba2Ca2Ca2Cu3Ox. The precision of ±100 A or better compares well with those of the more traditional techniques of μ+SR, neutron diffraction, and magnetic susceptibility.

A methodology is described for making electron nuclear double resonance (ENDOR) measurements on highly polar materials, such as the KH2PO4 -type of ferroelectrics (KH2PO4, KH2AsO4, RbH2AsO4, RbD2AsO4, CsH2AsO4 and CsD2AsO4) and the antiferroelectric NH4H2AsO4 in the vicinity of their phase transitions. Unlike EPR, the ENDOR lineshapes undergo drastic changes and yield direct information about the motional correlation time of a spin probe in the phase transition region. The ENDOR methodology enhances the spectral resolution and motional sensitivity of the EPR technique by approximately two orders of magnitude, and, thus, opens up another avenue for investigating molecular reorientations and, hence, the mechanism of ferroelectric transitions.

EPR techniques were used to study electron paramagnetic resonance spectra of Cu2+ and to determine microwave Josephson absorption in the Y-Ba-Cu-D superconductor. On the basis of the powder like EPR spectrum we have concluded that superconductivity is independent of the amount of Cu2+, which is present in the material and the valency of copper in HTS phase is Cu1+ or Cu3+. The broad Josephson oscillations spectrum with pronounced maximum at low magnetic fields was applied to determine Tc and Hc1.

It is shown that properly selected paramagnetic ions can serve as microscopic probes for investigating the effects of electric fields on the nucleation of ferroelectric domains. The technique is discussed for the SeO4 3- probe in KD2PO4.

Techniques are described for growing colored and single crystals of KH2PO4-type ferroelectric and antiferroelectrics. The color centers are identified by EPR and optical spectroscopy.

We succeeded in substituting paramagnetic Cr5+(S = 1/2) ions for the same sized As5+ in KH2AsO4, and for P5+ in KH2PO4 and their isomorphs. The high-temperature spectra are axial about the c-axis of the crystals and change over to tilted orthorhombic spectra for K band at T* ⋍ Tc + 110 K for KDA and DKDA and T* ⋍ Tc + 60 K for KDP and DKDP. We deduce the existence of clusters with lifetimes r longer than 1.7 × 10-8 sec below T*, γ depends mainly on T - Tc for deuterated and undeuterated crystals.

The nature of elementary excitations leading to cooperative phenomenon is not yet understood. A key feature of several recent theoretical models in the prediction that near the transition temperature Tc, thought still at T > Tc, some parts of the samples should exhibit the symmetry of the phase corresponding to T < Tc. These regions, called “clusters” are dynamic, in that their size increases very rapidly as T < Tc. Direct experimental evidence for the existence of such clusters is rather scanty. We shall present what we believe to be the first direct EPR evidence for such clusters. KD2PO4 was chosen as a model compound and electric field effects in the EPR spectra of the SeO4 3− radicals were used as microscopic probes. Details of the experimental procedure and a comparison of this method with NMR, Brillouin and neutron scattering and dielectric relaxation will be discussed.

Paramagnetic probes have provided valuable, quantitative information on the role of the anionic fragments (PO4 3− and AsO4 3−) in the KH2PO4-type of ferroelectrics and antiferroelectrics. However, parallel studies of the cations (K+, Rb+, Cs+ and NH4 +) have not yet been possible, the progress being hampered mainly by the lack of suitable paramagnetic probes which could accurately simulate the cationic behavior. Previously used probes are Fe3+, Cr3+, Cu2+ and Tl2+, all of which have an excess of positive charge compared with that of the substituted cations. This excess charge would necessarily lead to some structural deformation of the surroundings, and hence all these previously used ions might not be considered as faithful probes. We have now succeeded in stabilizing a variety of monovalent, paramagnetic ions in the KDP-type of ferroelectrics and antiferroelectrics. The structure and the dynamics of these probes have been studied via the temperature and angular dependence of the Zeeman, the hyp...

Generally, the EPR spectra of solids undergoing phase transitions show features characteristic of either the “fast motion” or of the “slow motion” limit. In the spectra of the AsO4 4− radical in polycrystalline KH2AsO4 and related ferroelectric compounds, however, EPR spectra characteristic of both regimes have been observed. These new features have been interpreted in terms of discrete jumps of the AsO4 4− molecules. Here we propose an alternative explanation: the new features correspond to those parts of the “high temperature” sample in which the phase transition has already set in on the EPR time scale. New evidence based on the CrO4 3− and SeO4 3− probes supporting this viewpoint will be discussed. The results provide new insight into the microscopic picture of the phase transition phenomenon.