A straightforward method using the sign of the piezoelectric coefficient to identify the ferroelectric switching mechanism.

Some organic ferroelectrics have two possible switching modes: molecular reorientation and proton transfer. Typical examples include 2,5-dihydroxybenzoic acid (DHBA) and Hdabco-ReO[Formula: see text] (dabco = diazabicyclo[2.2.2]octane). The direction and amplitude of the expected polarization depends on the switching mode. Herein a straightforward method to identify the ferroelectric switching mechanism is demonstrated. First, the relationship between the polarization vectors corresponding to the two modes is illustrated using the Berry phase. Second, the theoretical background for the sign of the piezoelectric coefficient is used to decide which mode occurs. Finally, comparing the theoretically calculated piezoelectric coefficients to the experimental results confirms the switching mode of each compound.

Competition of polar and antipolar states hidden behind a variety of polarization switching modes in hydrogen-bonded molecular chains.

Switchable π-electron systems are very powerful fragments to emphasize ferroelectric or antiferroelectric polarizations up to record-high levels among organic molecular crystals. According to the Cambridge Structural Database, many azole crystals such as imidazoles and tetrazoles contain polar and bistable hydrogen-bonded molecular sequences suitable for ferroelectricity or antiferroelectricity. Indeed, polarization hysteresis experiments on the 5-phenyl-1H-tetrazole (PHTZ) family combined with single crystal structural analysis have revealed one ferroelectric, two antiferroelectrics, and two hybrid-like dielectrics. Here, the rich variations for the interrelation between the hydrogen-bonding states and the polarization switching modes are interpreted by density functional theory (DFT) calculations with an excellent consistency. Large switchable polarizations are theoretically confirmed, and, as expected, the largest contribution is the switchable π-electron systems. By mapping the energy levels of polar/antipolar states, the disordered hydrogen bonds always appear when the ground state is accompanied by a nearly degenerate state. The straightforward case is the hybrid-like dielectric caused by the competition between the polar and antipolar states. However, contrastive behaviors are observed when the switchable dipoles are involved in competition between the different antipolar arrangement. For example, the PHTZ crystal exhibits typical antiferroelectric switching regardless of the hydrogen disorder, whereas polarization switching is silent in the imidazole derivatives. The latter is explained by the switching field increase with depth of the ground state relative to the energy level of the polar state.

Large polarization and record-high performance of energy storage induced by a phase change in organic molecular crystals.

Dielectrics that undergo electric-field-induced phase changes are promising for use as high-power electrical energy storage materials and transducers. We demonstrate the stepwise on/off switching of large polarization in a series of dielectrics by flipping their antipolar or canted electric dipoles via proton transfer and inducing simultaneous geometric changes in their π-conjugation system. Among antiferroelectric organic molecular crystals, the largest-magnitude polarization jump was obtained as 18 µC cm-2 through revisited measurements of squaric acid (SQA) crystals with improved dielectric strength. The second-best polarization jump of 15.1 µC cm-2 was achieved with a newly discovered antiferroelectric, furan-3,4-dicarboxylic acid. The field-induced dielectric phase changes show rich variations in their mechanisms. The quadruple polarization hysteresis loop observed for a 3-(4-chlorophenyl)propiolic acid crystal was caused by a two-step phase transition with moderate polarization jumps. The ferroelectric 2-phenylmalondialdehyde single crystal having canted dipoles behaved as an amphoteric dielectric, exhibiting a single or double polarization hysteresis loop depending on the direction of the external field. The magnitude of a series of observed polarizations was consistently reproduced within the simplest sublattice model by the density functional theory calculations of dipole moments flipping over a hydrogen-bonded chain or sheet (sublattice) irrespective of compounds. This finding guarantees a tool that will deepen our understanding of the microscopic phase-change mechanisms and accelerate the materials design and exploration for improving energy-storage performance. The excellent energy-storage performance of SQA was demonstrated by both a high recoverable energy-storage density W r of 3.3 J cm-3 and a nearly ideal efficiency (90%). Because of the low crystal density, the corresponding energy density per mass (1.75 J g-1) exceeded those derived from the highest W r values (∼8-11 J cm-3) reported for several bulk antiferroelectric ceramics , without modification to relaxor forms.

Dopant activation process in Mg-implanted GaN studied by monoenergetic positron beam.

A process for activating Mg and its relationship with vacancy-type defects in Mg-implanted GaN were studied by positron annihilation spectroscopy. Mg+ ions were implanted with an energy of 10 keV, and the Mg concentration in the subsurface region (≤ 50 nm) was on the order of 1019 cm-3. After the Mg-implantation, N+ ions were implanted to provide a 300-nm-deep box profile with a N concentration of 6 × 1018 cm-3. From capacitance-voltage measurements, the sequential implantation of N was found to enhance the activation of Mg. For N-implanted GaN before annealing, the major defect species were determined to Ga-vacancy related defects such as divacancy. After annealing below 1000 °C, the clustering of vacancies was observed. Above 1200 °C annealing, however, the size of the vacancies started to decrease, which was due to recombinations of vacancy clusters and excess N atoms in the damaged region. The suppression of vacancy clustering by sequential N-implantation in Mg-implanted GaN was attributed to the origin of the enhancement of the Mg activation.

Effects of ultra-high-pressure annealing on characteristics of vacancies in Mg-implanted GaN studied using a monoenergetic positron beam.

Vacancy-type defects in Mg-implanted GaN were probed by using a monoenergetic positron beam. Mg ions were implanted into GaN to obtain 0.3-µm-deep box profiles with Mg concentrations of 1 × 1019 cm-3. The major defect species in an as-implanted sample was determined to be Ga-vacancy related defects such as a complex between Ga and N vacancies. The sample was annealed under a nitrogen pressure of 1 GPa in a temperature range of 1000-1480 °C without a protective capping layer. Compared with the results for Mg-implanted GaN annealed with an AlN capping layer, the defect concentration was decreased by the cap-less annealing, suggesting that the surface of the sample was an effective sink for vacancies migrating toward the surface. Depth distributions of Mg after annealing above 1300 °C were influenced by the presence of residual vacancies at this temperature. Hydrogen atoms were unintentionally incorporated into the sample during annealing, and their diffusion properties were also affected by both vacancies and Mg.

Metaelectric multiphase transitions in a highly polarizable molecular crystal.

Metaelectric transition, i.e. an abrupt increase in polarization with an electric field is just a phase change phenomenon in dielectrics and attracts increasing interest for practical applications such as electrical energy storage and highly deformable transducers. Here we demonstrate that both field-induced metaelectric transitions and temperature-induced phase transitions occur successively on a crystal of highly polarizable bis-(1H-benzimidazol-2-yl)-methane (BI2C) molecules. In each molecule, two switchable polar subunits are covalently linked with each other. By changing the NH hydrogen location, the low- and high-dipole states of each molecule can be interconverted, turning on and off the polarization of hydrogen-bonded molecular ribbons. In the low-temperature phase III, the tetragonal crystal lattice comprises orthogonally crossed arrays of polar ribbons made up of a ladder-like hydrogen-bond network of fully polarized molecules. The single-step metaelectric transition from this phase III corresponds to the forced alignment of antiparallel dipoles typical of antiferroelectrics. By the transition to the intermediate-temperature phase II, the polarity is turned off for half of the ribbons so that the nonpolar and polar ribbons are orthogonal to each other. Considering also the ferroelastic-like crystal twinning, the doubled steps of metaelectric transitions observed in the phase II can be explained by the additional switching at different critical fields, by which the nonpolar ribbons undergo "metadielectric" molecular transformation restoring the strong polarization. This mechanism inevitably brings about exotic phase change phenomena transforming the multi-domain state of a homogeneous phase into an inhomogeneous (phase mixture) state.

Computational study of positron annihilation parameters for cation mono-vacancies and vacancy complexes in nitride semiconductor alloys.

We calculate positron annihilation parameters, namely the S and W parameters from the Doppler broadening spectroscopy and the positron lifetime [Formula: see text], for defect-free states as well as cation mono-vacancies and vacancy complexes in nitride semiconductor alloys Al0.5Ga0.5N, In0.5Ga0.5N and Al0.5In0.5N. The obtained distributions of these parameters differ from compound to compound. Especially, the S-W relation for In0.5Ga0.5N is very different from that for Al0.5Ga0.5N. For the cation mono-vacancies, introducing local structural parameters, their correlations with S, W and [Formula: see text] are investigated. The S and [Formula: see text] variations are well described with the size distributions of the vacancies while the W variation is related to the presence of localized d electrons. For the vacancy complexes as well as the cation mono-vacancies, multiple-linear-regression models to describe S, W and [Formula: see text] are successfully constructed using the local structural parameters as descriptors. The S-W and S-[Formula: see text] relations are also compared with those for AlN, GaN and InN.

First-principles studies of spin-orbital physics in pyrochlore oxides.

The pyrochlore oxides [Formula: see text]O7 exhibit a complex interplay between geometrical frustration, electronic correlations, and spin-orbit coupling (SOC), due to the lattice structure and active charge, spin, and orbital degrees of freedom. Understanding the properties of these materials is a theoretical challenge, because their intricate nature depends on material-specific details and quantum many-body effects. Here we review our recent studies based on first-principles calculations and quantum many-body theories for 4d and 5d pyrochlore oxides with B = Mo, Os, and Ir. In these studies, the SOC and local electron correlations are treated within the local density approximation (LDA) + U and LDA + dynamical mean-field theory formalisms. We also discuss the technical aspects of these calculations.

Single-component molecular conductor [Pt(dmdt)2]-a three-dimensional ambient-pressure molecular Dirac electron system.

The single-component molecular conductor [Pt(dmdt)2] is a sought-after ambient-pressure molecular Dirac electron system, which exhibits a high temperature-insensitive conductivity and temperature-dependent magnetic susceptibility nearly vanishing below 120 K. First-principles DFT calculations reveal that Dirac cones emerge along the a* direction, and form Dirac nodal lines.

Coexistence of normal and inverse deuterium isotope effects in a phase-transition sequence of organic ferroelectrics.

Supramolecular cocrystals of anilic acids with 2,2'-bipyridines exhibit successive phase transitions as well as unusual isotope effects. Ferroelectricity driven by a cooperative proton transfer along the supramolecular chains is accompanied by huge permittivity (a maximum of 13 000) at the Curie point, as well as a large spontaneous polarization (maximum 5 µC cm-2) and a low coercive field ranging from 0.5 to 10 kV cm-1. Deuterium substitutions over the hydrogen bonds smoothly raise the Curie point and simultaneously reduce other phase-transition temperatures by a few tens of degrees. The coexistence of opposite isotope effects reduces the temperature interval of the intermediate paraelectric phase from 84 to 10 K for the 5,5'-dimethyl-2,2'-bipyridinium bromanilate salt. The bipyridine molecules exhibit interplanar twisting, which represents the order parameter relevant to the high-temperature phase transitions. The normal and inverse temperature shifts are ascribed to the direct and indirect effects, respectively, of the lengthened hydrogen bonds, which adjusts the molecular conformation of the flexible bipyridine unit so as to minimally modify their adjacent intermolecular interactions.

Field-Induced Antipolar-Polar Structural Transformation and Giant Electrostriction in Organic Crystal.

The field-induced antipolar-polar structural transition in an organic antiferroelectric 2-trifluoromethylnaphthimidazole crystal is investigated by performing synchrotron X-ray diffraction. The polarities of all of the hydrogen-bonded chains become parallel with each other in the presence of an external electric field. The switching is accompanied by a giant electrostriction, which provides 1 order of magnitude larger strain than the piezoelectric strain of the organic ferroelectrics: croconic acid and poly(vinylidene fluoride); however, it is comparable to those of typical commercial piezoelectric ceramics. The crystal structure analysis with electric field shows that the origin of the observed giant electrostriction can be attributed to the shear strain that emerges from the polarity switching of the hydrogen-bonded chains. The antipolar-polar structural transition in antiferroelectrics could be employed for the development of high-performance electrostrictive organic materials.

Proton tautomerism for strong polarization switching.

Ferroelectrics based on proton tautomerism are promising in low-field and above-room-temperature operations. Here seven organic ferroelectric crystals are examined to search for efficient switching of strong spontaneous polarization on proton tautomerism. Solution-grown crystals exhibit strong pinning of ferroelectric domain walls, but excellent switching performance is awakened by depinning domain walls under thermal annealing and/or repetitive bipolar pulses with a high voltage. Compared with ferroelectric polymers such as polyvinylidefluoride, the optimized polarizations are comparable or stronger in magnitude whereas the coercive fields are two orders of magnitude weaker. The polarization of croconic acid, in particular, breaks its own record for organic systems in increasing from 21 to 30 µC cm-2 and now exceeds those of some commercial ferroelectric materials such as SrBi2Ta2O9 and BaTiO3. Optimization reduces the discrepancy of the spontaneous polarization with the results of the first-principles calculations to less than 15%. The cooperative roles of proton transfer and π-bond switching are discussed by employing the point-charge model and hydrogen-bond geometry.

Weyl Node and Spin Texture in Trigonal Tellurium and Selenium.

We study Weyl nodes in materials with broken inversion symmetry. We find based on first-principles calculations that trigonal Te and Se have multiple Weyl nodes near the Fermi level. The conduction bands have a spin splitting similar to the Rashba splitting around the H points, but unlike the Rashba splitting the spin directions are radial, forming a hedgehog spin texture around the H points, with a nonzero Pontryagin index for each spin-split conduction band. The Weyl semimetal phase, which has never been observed in real materials without inversion symmetry, is realized under pressure. The evolution of the spin texture by varying the pressure can be explained by the evolution of the Weyl nodes in k space.

Structure-property relationship of supramolecular ferroelectric [H-66dmbp][Hca] accompanied by high polarization, competing structural phases, and polymorphs.

Three polymorphic forms of 6,6'-dimethyl-2,2'-bipyridinium chloranilate crystals were characterized to understand the origin of polarization properties and the thermal stability of ferroelectricity. According to the temperature-dependent permittivity, differential scanning calorimetry, and X-ray diffraction, structural phase transitions were found in all polymorphs. Notably, the ferroelectric α-form crystal, which has the longest hydrogen bond (2.95 Å) among the organic acid/base-type supramolecular ferroelectrics, transformed from a polar structure (space group, P21) into an anti-polar structure (space group, P21/c) at 378 K. The non-ferroelectric ß- and γ-form crystals also exhibited structural rearrangements around hydrogen bonds. The hydrogen-bonded geometry and ferroelectric properties were compared with other supramolecular ferroelectrics. A positive relationship between the phase-transition temperature (TC ) and hydrogen-bond length () was observed, and was attributed to the potential barrier height for proton off-centering or order/disorder phenomena. The optimized spontaneous polarization (Ps ) agreed well with the results of the first-principles calculations, and could be amplified by separating the two equilibrium positions of protons with increasing . These data consistently demonstrated that stretching is a promising way to enhance the polarization performance and thermal stability of hydrogen-bonded organic ferroelectrics.

A single-component molecular superconductor.

The pressure dependence of the resistivities of a single-component molecular conductor, [Ni(hfdt)2] (hfdt = bis(trifluoromethyl)tetrathiafulvalenedithiolate) with semiconducting properties at ambient pressure was examined. The four-probe resistivity measurements were performed up to ∼10 GPa using a diamond anvil cell. The low-temperature insulating phase was suppressed above 7.5 GPa and the resistivity dropped, indicating the superconducting transition occurred around 7.5-8.7 GPa with a maximum Tc (onset temperature) of 5.5 K. The high-pressure crystal and electronic band structures were derived by the first-principle calculations at 6-11 GPa. The crystal was found to retain the semiconducting band structure up to 6 GPa. But the electron and hole Fermi surfaces appear at 8 GPa. These results of the calculations agree well with the observation that the pressure-induced superconducting phase of [Ni(hfdt)2] appeared just above the critical pressure where the low-temperature insulating phase was suppressed.

Polarization switching ability dependent on multidomain topology in a uniaxial organic ferroelectric.

The switching of electric polarization induced by electric fields, a fundamental functionality of ferroelectrics, is closely associated with the motions of the domain walls that separate regions with distinct polarization directions. Therefore, understanding domain-walls dynamics is of essential importance for advancing ferroelectric applications. In this Letter, we show that the topology of the multidomain structure can have an intrinsic impact on the degree of switchable polarization. Using a combination of polarization hysteresis measurements and piezoresponse force microscopy on a uniaxial organic ferroelectric, α-6,6'-dimethyl-2,2'-bipyridinium chloranilate, we found that the head-to-head (or tail-to-tail) charged domain walls are strongly pinned and thus impede the switching process; in contrast, if the charged domain walls are replaced with electrically neutral antiparallel domain walls, bulk polarization switching is achieved. Our findings suggest that manipulation of the multidomain topology can potentially control the switchable polarization.

Noncollinear magnetism and spin-orbit coupling in 5d pyrochlore oxide Cd2Os2O7.

We investigate the electronic and magnetic properties of the pyrochlore oxide Cd2Os2O7 using the density-functional theory plus on-site repulsion (U) method, and depict the ground-state phase diagram with respect to U. We conclude that the all-in-all-out noncollinear magnetic order is stable in a wide range of U. We also show that the easy-axis anisotropy arising from the spin-orbit coupling plays a significant role in stabilizing the all-in-all-out magnetic order. A pseudogap was observed near the transition between the antiferromagnetic metallic and insulating phases. Finally, we discuss possible origins of the peculiar low-temperature (T) properties observed in experiments.

Tuning the magnetic dimensionality by charge ordering in the molecular TMTTF salts.

We theoretically investigate the interplay between charge ordering and magnetic states in quasi-one-dimensional molecular conductors TMTTF(2)X, motivated by the observation of a complex variation of competing and/or coexisting phases. We show that the ferroelectric-type charge order increases two-dimensional antiferromagnetic spin correlation, whereas in the one-dimensional regime two different spin-Peierls states are stabilized. By using first-principles band calculations for the estimation for the transfer integrals and comparing our results with the experiments, we identify the controlling parameters in the experimental phase diagram to be not only the interchain transfer integrals but also the amplitude of the charge order.

Charge-transport in tin-iodide perovskite CH3NH3SnI3: origin of high conductivity.

The structural and electrical properties of a metal-halide cubic perovskite, CH(3)NH(3)SnI(3), have been examined. The band structure, obtained using first-principles calculation, reveals a well-defined band gap at the Fermi level. However, the temperature dependence of the single-crystal electrical conductivity shows metallic behavior down to low temperatures. The temperature dependence of the thermoelectric power is also metallic over the whole temperature range, and the large positive value indicates that charge transport occurs with a low concentration of hole carriers. The metallic properties of this as-grown crystal are thus suggested to result from spontaneous hole-doping in the crystallization process, rather than the semi-metal electronic structure. The present study shows that artificial hole doping indeed enhances the conductivity.

First-principles electronic-band calculations on organic conductors.

Predicting electronic-band structures is a key issue in understanding the properties of materials or in materials design. In this review article, application examples of first-principles calculations, which are not based on adjustable empirical parameters, to study electronic structures of organic conductors are described.