Ferroelectricity and isotope effects in hydrogen-bonded KDP crystals

Quasi-one-dimensional hydrogen bonding in nanoconfined ice

The Bernal-Fowler ice rules stipulate that each water molecule in an ice crystal should form four hydrogen bonds. However, in extreme or constrained conditions, the arrangement of water molecules deviates from conventional ice rules, resulting in properties significantly different from bulk water. In this study, we employ machine learning-driven first-principles simulations to identify a new stabilization mechanism in nanoconfined ice phases. Instead of forming four hydrogen bonds, nanoconfined crystalline ice can form a quasi-one-dimensional hydrogen-bonded structure that exhibits only two hydrogen bonds per water molecule. These structures consist of strongly hydrogen-bonded linear chains of water molecules that zig-zag along one dimension, stabilized by van der Waals interactions that stack these chains along the other dimension. The unusual interplay of hydrogen bonding and van der Waals interactions in nanoconfined ice results in atypical proton behavior such as potential ferroelectric behavior, low dielectric response, and long-range proton dynamics.

Tuning ferroelectric phase transition temperature by enantiomer fraction

Tuning phase transition temperature is one of the central issues in phase transition materials. Herein, we report a case study of using enantiomer fraction engineering as a promising strategy to tune the Curie temperature (TC) and related properties of ferroelectrics. A series of metal-halide perovskite ferroelectrics (S-3AMP)x(R-3AMP)1-xPbBr4 was synthesized where 3AMP is the 3-(aminomethyl)piperidine divalent cation and enantiomer fraction x varies between 0 and 1 (0 and 1 = enantiomers; 0.5 = racemate). With the change of the enantiomer fraction, the TC, second-harmonic generation intensity, degree of circular polarization of photoluminescence, and photoluminescence intensity of the materials have been tuned. Particularly, when x = 0.70 - 1, a continuously linear tuning of the TC is achieved, showing a tunable temperature range of about 73 K. This strategy provides an effective means and insights for regulating the phase transition temperature and chiroptical properties of functional materials.

Thermal dependence of the hydrated proton and optimal proton transfer in the protonated water hexamer

Water is a key ingredient for life and plays a central role as solvent in many biochemical reactions. However, the intrinsically quantum nature of the hydrogen nucleus, revealing itself in a large variety of physical manifestations, including proton transfer, gives rise to unexpected phenomena whose description is still elusive. Here we study, by a combination of state-of-the-art quantum Monte Carlo methods and path-integral molecular dynamics, the structure and hydrogen-bond dynamics of the protonated water hexamer, the fundamental unit for the hydrated proton. We report a remarkably low thermal expansion of the hydrogen bond from zero temperature up to 300 K, owing to the presence of short-Zundel configurations, characterised by proton delocalisation and favoured by the synergy of nuclear quantum effects and thermal activation. The hydrogen bond strength progressively weakens above 300 K, when localised Eigen-like configurations become relevant. Our analysis, supported by the instanton statistics of shuttling protons, reveals that the near-room-temperature range from 250 K to 300 K is optimal for proton transfer in the protonated water hexamer.

When Quantum Fluctuations Meet Structural Instabilities: The Isotope- and Pressure-Induced Phase Transition in the Quantum Paraelectric NaOH

Anhydrous sodium hydroxide, a common and structurally simple compound, shows spectacular isotope effects: NaOD undergoes a first-order transition, which is absent in NaOH. By combining ab initio electronic structure calculations with Feynman path integrals, we show that NaOH is an unusual example of a quantum paraelectric: zero-point quantum fluctuations stretch the weak hydrogen bonds (HBs) into a region where they are unstable and break. By strengthening the HBs via isotope substitution or applied pressure, the system can be driven to a broken-symmetry antiferroelectric phase. In passing, we provide a simple quantitative criterion for HB breaking in layered crystals and show that nuclear quantum effects are crucial in paraelectric to ferroelectric transitions in hydrogen-bonded hydroxides.

Large Phase-Transition Temperature Enhancement Achieved in a Layered Lead Iodide Hybrid Crystal by H/F Substitution

Organic-inorganic hybrid metal halides with structural flexibility and solution processability have been widely investigated for different application scenarios. However, the effective construction of phase-transition materials with a high phase-transition temperature (Ttr) for potential practical applications remains a great challenge, and reports on the regulation of Ttr with significant enhancement have been rare. In this manuscript, we have realized a large Ttr increase of 148 K in a layered hybrid lead iodide crystal (4-FTMBA)4Pb3I10 (4-FTMBA = 4-fluoro-N,N,N-trimethylbenzenaminium) by the H/F substitution strategy. Compared to the parent (TMBA)4Pb3I10 (TMBA = N,N,N-trimethylbenzenaminium), H/F substitution preserves the structural framework and crystal symmetry in (4-FTMBA)4Pb3I10. The introduction of heavier fluorine will significantly increase the motion barrier for the order-disorder transition, resulting in the remarkably improved Ttr. Temperature-dependent crystal structures, Raman spectra, and dielectric analyses well support the phase-transition behavior. In addition, evident thermochromism with a tunable direct band gap in (4-FTMBA)4Pb3I10 has been observed using UV-vis spectra. To the best of our knowledge, the achieved Ttr enhancement of 148 K by H/F substitution is the highest among the organic-inorganic hybrid lead halide phase-transition materials. This finding would greatly inspire the rational design of functional materials with high performance.

High-T c Single-Component Organosilicon Ferroelectric Crystal Obtained by H/F Substitution

Organic single-component ferroelectrics are highly desirable for their low molecular mass, light weight, low processing temperature, and excellent film-forming properties. Organosilicon materials with a strong film-forming ability, weather resistance, nontoxicity, odorlessness, and physiological inertia are very suitable for device applications related to the human body. However, the discovery of high-T _c organic single-component ferroelectrics has been very scarce, and the organosilicon ones even less so. Here, we used a chemical design strategy of H/F substitution to successfully synthesize a single-component organosilicon ferroelectric tetrakis(4-fluorophenylethynyl)silane (TFPES). Systematic characterizations and theory calculations revealed that, compared with the parent nonferroelectric tetrakis(phenylethynyl)silane, fluorination caused slight modifications of the lattice environment and intermolecular interactions, inducing a 4/mmmFmm2-type ferroelectric phase transition at a high T _c of 475 K in TFPES. To our knowledge, this T _c should be the highest among the reported organic single-component ferroelectrics, providing a wide operating temperature range for ferroelectrics. Moreover, fluorination also brought about a significant improvement in the piezoelectric performance. Combined with excellent film properties, the discovery of TFPES provides an efficient path for designing ferroelectrics suitable for biomedical and flexible electronic devices.

Highest-Tc single-component homochiral organic ferroelectrics

Organic single-component ferroelectrics with low molecular mass have drawn great attention for application in organic electronics. However, the discovery of high-T_c single-component organic ferroelectrics has been very scarce. Herein, we report a pair of homochiral single-component organic ferroelectrics (R)-10-camphorsulfonylimine and (S)-10-camphorsulfonylimine under the guidance of ferroelectric chiral chemistry. They crystallize in the chiral–polar space group P2₁, and their mirror image relations have been identified using vibrational circular dichroism spectra. They both exhibit 422F2 multiaxial ferroelectricity with T_c as high as 429 K. Besides, they possess superior acoustic impedance characteristics with a value of 2.45 × 10⁶ kg s⁻¹ m⁻², lower than that of PVDF. To our knowledge, enantiomeric (R and S)-10-camphorsulfonylimine show the highest T_c among the known organic single-component ferroelectrics and low acoustic impedance well matching with that of bodily tissues. This work promotes the development of high-performance organic single-component ferroelectrics and is of great inspiration to explore their application in next-generation flexible smart devices.

A pair of enantiomeric organic ferroelectrics (R and S)-10-camphorsulfonylimine show the highest T_c among the known single-component organic ferroelectrics.

Thermal and Nuclear Quantum Effects at the Antiferroelectric to Paraelectric Phase Transition in KOH and KOD Crystals

Crystalline KOH undergoes an antiferroelectric (AFE) proton ordering phase transition at low temperatures, which results in a monoclinic bilayer structure held together by a network of weak hydrogen bonds (HBs). The Curie temperature shifts up when the compound is deuterated, an effect that classical MD is not able to catch. For deeper insights into the transition mechanism, we carry out ab initio MD simulations of KOH and KOD crystals by including quantum effects on the nuclei through Feynman path integrals. The geometric isotope effect and the evolution of the lattice parameters with temperature agree with the experimental data, while the purely classical description is not appropriate. Our results show that deuteration strengthens the HBs in the low-T AFE ordered phase. The transition is characterized by the flipping of OH/OD groups along a bending mode. Above the transition, the system is driven into a dynamical disordered paraelectric phase.

Dynamics of proton, ion, molecule, and crystal lattice in functional molecular assemblies

Dynamic molecular processes, such as short- or long-range proton (H⁺) and ion (M⁺) motions, and molecular rotations in electrical conducting and magnetic molecular assemblies enable the fabrication of electron-H⁺ (or M⁺) coupling systems, while crystal lattice dynamics and molecular conformation changes in hydrogen-bonded molecular crystals have been utilised in external stimuli responsive reversible gas-induced gate opening and molecular adsorption/desorption behavior. These dynamics of the polar structural units are responsible for the dielectric measurements. The H⁺ dynamics are formed from ferroelectrics and H⁺ conductors, while the dynamic M⁺ motions of Li⁺ and Na⁺ involve ionic conductors and coupling to the conduction electrons. In n-type organic semiconductors, the crystal lattices are modulated by replacing M⁺ cations, with cations such as Li⁺, Na⁺, K⁺, Rb⁺, and Cs⁺. The use of polar rotator or inversion structures such as alkyl amides, m-fluoroanilinium cations, and bowl-shaped trithiasumanene π-cores enables the formation of ferroelectric molecular assemblies. The host-guest molecular systems of ESIPT fluorescent chromic molecules showed interesting molecular sensing properties using various bases, where the dynamic transformation of the crystal lattice and the molecular conformational change were coupled to each other.

Bromine-Substitution-Induced High-Tc Two-Dimensional Bilayered Perovskite Photoferroelectric

High-Curie-temperature (T_c) ferroelectrics have exhibited broad applications in optoelectronic devices. Recently, two-dimensional multilayered perovskite ferroelectrics with excellent photoelectric attributes are attracting increasing interest as new systems of photoferroelectrics. However, the effective tuning of the T_c value of a multilayered perovskite photoferroelectric system still remains a huge challenge. Here, by a halogen substitution strategy to introduce bromine atoms on n-propylamine cations, the hybrid perovskite photoferroelectric (3-bromopropylaminium)₂(formamidinium)Pb₂Br₇ (BFPB) with a high T_c value (348.5 K) was obtained. It is notable that BFPB adopts a two-dimensional bilayered inorganic framework, with tight linking to the organic cation by C-Br···Br-Pb halogen···halogen interactions and N-H···Br hydrogen bonds. Intriguingly, in comparison with the prototypical compound (n-propylaminium)₂(formamidinium)Pb₂Br₇, a remarkable augmentation of 85.2 K in the resulting T_c value of BFPB is clearly observed, which further broadens the temperature range of its application. In combination with the remarkable ferroelectric and semiconducting attributes, the reversible bulk photovoltaic effect was realized in single crystals of BFPB. This finding can not only enhance the hybrid perovskite ferroelectric family but also further promote the photoelectric application of ferroelectrics.

Proton strings and rings in atypical nucleation of ferroelectricity in ice

Ordinary ice has a proton-disordered phase which is kinetically metastable, unable to reach, spontaneously, the ferroelectric (FE) ground state at low temperature where a residual Pauling entropy persists. Upon light doping with KOH at low temperature, the transition to FE ice takes place, but its microscopic mechanism still needs clarification. We introduce a lattice model based on dipolar interactions plus a competing, frustrating term that enforces the ice rule (IR). In the absence of IR-breaking defects, standard Monte Carlo (MC) simulation leaves this ice model stuck in a state of disordered proton ring configurations with the correct Pauling entropy. A replica exchange accelerated MC sampling strategy succeeds, without open path moves, interfaces, or off-lattice configurations, in equilibrating this defect-free ice, reaching its low-temperature FE order through a well-defined first-order phase transition. When proton vacancies mimicking the KOH impurities are planted into the IR-conserving lattice, they enable standard MC simulation to work, revealing the kinetics of evolution of ice from proton disorder to partial FE order below the transition temperature. Replacing ordinary nucleation, each impurity opens up a proton ring generating a linear string, an actual FE hydrogen bond wire that expands with time. Reminiscent of those described for spin ice, these impurity-induced strings are proposed to exist in doped water ice too, where IRs are even stronger. The emerging mechanism yields a dependence of the long-time FE order fraction upon dopant concentration, and upon quenching temperature, that compares favorably with that known in real-life KOH doped ice.

High-pressure sapphire capillary cell for synchrotron single-crystal X-ray diffraction measurements to 1500 bar

A new sapphire capillary pressure cell for single-crystal X-ray diffraction measurements at moderate pressures (200−1500 bar; 1 bar = 100 kPa) has been developed and optimized for use on beamline I19 at Diamond Light Source. The three-component cell permits optical centring of the crystal and in situ pressure modification to a precision of 1 bar. Compression of hexamethylenetetramine and its deuterated analogue to 1000 bar was performed, showcasing the accuracy and precision of the measurements, and highlighting evidence of a geometric isotope effect.

Record Enhancement of Phase Transition Temperature Realized by H/F Substitution

A high transition temperature (T_c ) is essential for the practical application of ferroelectrics as electronic devices under extreme thermal conditions in the aerospace, automotive, and energy industries. In recent decades, the isotope effect and strain engineering are found to effectively modulate T_c ; however, these strategies are limited to certain systems. Developing simple, universal, and practical methods to improve T_c has become an imminent challenge for expanding the applications of ferroelectrics. Here, by adopting a molecular design strategy involving H/F substitution on an organic-inorganic hybrid perovskite (1-azabicyclo[2.2.1]heptane)CdCl₃ at a T_c of 190 K, the successful synthesis of a multiaxial, ferroelectric hybrid perovskite (4-fluoro-1-azabicyclo[2.2.1]heptane)CdCl₃ is reported, which demonstrates a large spontaneous polarization of 11.2 µC cm^-2 (greater than that of polyvinylidene difluoride) and a T_c of 419 K (greater than that of BaTiO₃ ). This temperature enhancement (229 K) is the largest reported for molecular ferroelectrics, far exceeding the reported enhancements induced by the isotope effect and other techniques. This pioneering technique provides an effective and universal method for improving T_c in ferroelectrics and represents an important step toward the development of high-performance ferroelectric technology.

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.

Microscopic Difference of Hydrogen Double-minimum Potential Well Detected by Hydroxyl Group in Hydrogen-bonded System

We investigate the microscopic structure of hydrogen double-well potentials in a hydrogen-bonded ferroelectric system exposed to radioactive particles of hydrogen-ion beams. The hydrogen-bonded system is ubiquitous, forming the base of organic-inorganic materials and the double-helix structure of DNA inside biological materials. In order to determine the difference of microscopic environments, an atomic-scale level analysis of solid-state ¹H high-resolution nuclear magnetic resonance (NMR) spectra was performed. The hydrogen environments of inorganic systems represent the Morse potentials and wave function of the eigen state and eigen-state energy derived from the Schrödinger equation. The wave functions for the real space of the localized hydrogen derived from the approximated solutions in view of the atomic scale by using quantum mechanics are manifested by a difference in the charge-density distribution.

Three diphosphates, α-Li2Na2P2O7, Li8Pb3Ba(P2O7)4 and Li7Rb(P2O7)2: influences of co-substitution on the crystal structure

We reported the synthesis, structure, characterization and first-principles calculations of diphosphates, α-Li₂Na₂P₂O₇, Li₈Pb₃Ba(P₂O₇)₄ and Li₇Rb(P₂O₇)₂, and studied the influences of co-substitution on the crystal structure.

H/F-Substitution-Induced Homochirality for Designing High-Tc Molecular Perovskite Ferroelectrics

A ferroelectric with a high phase-transition temperature (T_c ) is an indispensable condition for practical applications. Over the past decades, both strain engineering and the isotope effect have been found to effectively improve the T_c within ferroelectric material systems. However, the former strategy seems to prefer working in inorganic ferroelectric thin films, while the latter is also limited to some certain systems, such as hydrogen-bonded ferroelectrics. It is noted that a mono-fluorinated molecule is geometrically very similar to its parent molecule and the substitution of H by an F atom can introduce a chiral center on the molecule to template or stabilize polar structures. Significantly, the barrier of rotation of the fluorinated organic molecules is raised, resulting in a remarkable increase in T_c . Herein, by applying the molecular design strategy of H/F substitution to the organic-inorganic perovskite ferroelectric (pyrrolidinium)CdCl₃ with a low T_c of 240 K, two high-T_c chiral perovskite ferroelectrics, (R)- and (S)-3-F-(pyrrolidinium)CdCl₃ are successfully synthesized, for which the T_c reaches 303 K. The significant enhancement of 63 K in T_c extends the ferroelectric working temperature range to room temperature. This finding provides a new effective way to regulate the T_c in ferroelectrics and to design high-T_c molecular ferroelectrics.

An alkali metal phosphate RbPbBi2(PO4)3 with three kinds of disorder: the effect of isolated soft cation units on the crystal structure

We have reported a new phosphate, RbPbBi₂(PO₄)₃ with three kinds of disorder, and a concept of isolated soft units has been firstly put forward to study the origin of structural distortions and disorder.

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.

Deuterium substitutions of the hydrogen-bonded ferroelectrics smoothly raise the Curie point and simultaneously reduce other phase-transition temperatures by a few tens of degrees.

Thermal Expansion of Single-Crystal H_{2}O and D_{2}O Ice Ih

Thermal expansion of H_{2}O and D_{2}O ice Ih with relative resolution of 1 ppb is reported. A large transition in the thermal expansion coefficient at 101 K in H_{2}O moves to 125 K in D_{2}O, revealing one of the largest-known isotope effects. Rotational oscillatory modes that couple poorly to phonons, i.e., lattice solitons, may be responsible.

Spatially resolved proton momentum distributions in KDP from first-principles

The ferroelectric to paraelectric (PE) phase transition of KH₂PO₄ (KDP) is investigated as a stringent test of the first-principles, normal modes framework proposed for calculating anharmonic quantum nuclear motion. Accurate spatially resolved momentum distribution functions (MDFs) are directly calculated from the nuclear wavefunction, overcoming the limitations of path-integral molecular dynamics methods. They indicate coherent, correlated tunneling of protons across hydrogen bonds in the PE phase in agreement with neutron Compton scattering data and reproduces the key features of the experimental MDF. It further highlights the role of Slater's lateral configurations in the PE phase. The analysis in terms of normal modes demonstrates the importance of collective, correlated proton motion and underlines the value of the employed framework in interpreting experimental data. This also makes the framework very attractive for application to deuterated KDP to further elucidate the nature of the PE transition and to systems exhibiting strong quantum nuclear effects in general.

Geometric isotope effect of deuteration in a hydrogen-bonded host-guest crystal

Deuteration of a hydrogen bond by replacing protium (H) with deuterium (D) can cause geometric changes in the hydrogen bond, known as the geometric H/D isotope effect (GIE). Understanding the GIEs on global structures and bulk properties is of great importance to study structure–property relationships of hydrogen-bonded systems. Here, we report a hydrogen-bonded host–guest crystal, imidazolium hydrogen terephthalate, that exemplifies striking GIEs on its hydrogen bonds, phases, and bulk dielectric transition property. Upon deuteration, the donor–acceptor distance in the O–H···O hydrogen bonds in the host structure is found to increase, which results in a change in the global hydrogen-bonded supramolecular structure and the emergence of a new phase (i.e., isotopic polymorphism). Consequently, the dynamics of the confined guest, which depend on the internal pressure exerted by the host framework, are substantially altered, showing a downward shift of the dielectric switching temperature.

Deuterating a hydrogen bond can change the bond’s geometry, a phenomenon known as the geometric isotope effect (GIE). Here, the authors find that a hydrogen-bonded host–guest crystal, imidazolium hydrogen terephthalate, exhibits significant GIE on its hydrogen bonds, changing its crystal phases and bulk dielectric properties.

Magic angle spinning NMR study of the ferroelectric transition of KH2PO4: definitive evidence of a displacive component

Variable temperature magic angle spinning (MAS) NMR measurements are reported on ¹H and ³¹P nuclei in KH₂PO₄ (KDP) in the vicinity of its paraelectric-ferroelectric phase transition temperature, T _c, of 123 K, to examine the transition mechanism, in particular if this is a model order-disorder type or whether it also involves a displacive component. It has been well established that the temperature variation of the isotropic chemical shift, δ _iso, in NMR measurements of the nuclei directly involved in the transition should remain constant or change smoothly through T _c for an order-disorder type transition but it should show an anomalous change for a displacive one. Here we demonstrate that the δ _iso for both ³¹P and ¹H nuclei in KDP show clear anomalies as a function of temperature around KDP's T _c, providing direct evidence of a displacive component for the phase transition of KDP in contrast to the generally accepted notion that it is a model order-disorder type.

Cation-templated cyanometallate-based supramolecular rectangular cage compounds showing dielectric transitions

Relatively small displacements or thermal vibrations of the polar guests in Cd(ii)–Co(iii) cyanometallate frameworks A₂{H(CdCl₂)[Co(CN)₆]} (A = monovalent cation) induce dielectric transitions and relaxations.

Collective proton transfer in ordinary ice: local environments, temperature dependence and deuteration effects

Ordinary ice at low temperature: what about collective nuclear quantum effects in its chiral six rings?

Quantum simulation of collective proton tunneling in hexagonal ice crystals

The effect of proton tunneling on many-body correlated proton transfer in hexagonal ice is investigated by quantum simulation. Classical single-particle hopping along individual hydrogen bonds leads to charge defects at high temperature, whereas six protons in ringlike topologies can move concertedly as a delocalized quasiparticle via collective tunneling at low temperature, thus preventing the creation of high-energy topological defects. Our findings rationalize many-body quantum tunneling in hydrogen-bonded networks and suggest that this phenomenon might be more widespread than previously thought.

Communication: ab initio simulations of hydrogen-bonded ferroelectrics: collective tunneling and the origin of geometrical isotope effects

Ab initio simulations that account for nuclear quantum effects have been used to examine the order-disorder transition in squaric acid, a prototypical H-bonded antiferroelectric crystal. Our simulations reproduce the >100 K difference in transition temperature observed upon deuteration as well as the strong geometrical isotope effect observed on intermolecular separations within the crystal. We find that collective transfer of protons along the H-bonding chains - facilitated by quantum mechanical tunneling - is critical to the order-disorder transition and the geometrical isotope effect. This sheds light on the origin of isotope effects and the importance of tunneling in squaric acid which likely extends to other H-bonded ferroelectrics.

Quantum Effects in the Diffusion of Hydrogen on Ru(0001)

An understanding of hydrogen diffusion on metal surfaces is important not only for its role in heterogeneous catalysis and hydrogen fuel cell technology but also because it provides model systems where tunneling can be studied under well-defined conditions. Here we report helium spin-echo measurements of the atomic-scale motion of hydrogen on the Ru(0001) surface between 75 and 250 K. Quantum effects are evident at temperatures as high as 200 K, while below 120 K we observe a tunneling-dominated temperature-independent jump rate of 1.9 × 109 s-1, many orders of magnitude faster than previously seen. Quantum transition-state theory calculations based on ab initio path-integral simulations reproduce the temperature dependence of the rate at higher temperatures and predict a crossover to tunneling-dominated diffusion at low temperatures. However, the tunneling rate is underestimated, highlighting the need for future experimental and theoretical studies of hydrogen diffusion on this and other well-defined surfaces.

Ultrafast large-amplitude relocation of electronic charge in ionic crystals

The interplay of vibrational motion and electronic charge relocation in an ionic hydrogen-bonded crystal is mapped by X-ray powder diffraction with a 100 fs time resolution. Photoexcitation of the prototype material KH(2)PO(4) induces coherent low-frequency motions of the PO(4) tetrahedra in the electronically excited state of the crystal while the average atomic positions remain unchanged. Time-dependent maps of electron density derived from the diffraction data demonstrate an oscillatory relocation of electronic charge with a spatial amplitude two orders of magnitude larger than the underlying vibrational lattice motions. Coherent longitudinal optical and tranverse optical phonon motions that dephase on a time scale of several picoseconds, drive the charge relocation, similar to a soft (transverse optical) mode driven phase transition between the ferro- and paraelectric phase of KH(2)PO(4).

Unexpected isotopic effect of a deuteriumed countercation on the spin-Peierls-type transitions in quasi-one-dimensional bis(maleonitriledithiolato)nickelate monoanion spin systems

Two deuteriumed quasi-one-dimensional (quasi-1D) spin-Peierls-type compounds, 4-X-benzylpyridinium-d(5) bis(maleo-nitriledithiolato)nickelate (the substituent X = Br or Cl), were structurally characterized. Compared with the corresponding non-deuteration compounds, the transition temperature T(C) shifts to higher temperature. The isotopic effect of countercations on T(C) is probably related to the change of phonon frequency ω(0) and 'chemical pressure' resulted from the substitution of pyridine by pyridine-d(5).

Thermally activated proton hopping in lawsonite, the ferroelectric transition at 125 K, and the co-elastic phase transition at 270 K

Lawsonite, CaAl(2)Si(2)O(7)(OH)(2)H(2)O, is a novel ferroelectric material which is dominated by ordering and disordering of protons. We report related elastic anomalies in lawsonite. A new approach to extracting acoustic data from resonant ultrasonic spectroscopy has been applied to investigate the ordering and disordering of protons. This approach is based on analysis of entire spectra, rather than the more conventional fitting of individual resonance peaks and captures three features of the transitions. Firstly, structural disorder of protons persists down to low temperatures. Secondly, the structural transition between two paraelectric phases near 270 K is mainly of the order/disorder type but is not directly coupled to the proton mobility. Thirdly, the ferroelectric transition at 125 K shows a direct link with the proton mobility with an activation energy of 0.03 eV. The latter transition occurs in the large class of ferroelectric crystals where the molecular groups are linked by hydrogen and its thermodynamic potential is almost identical to that of KDP (KH(2)PO(4)), albeit with a smaller isotope effect. While some damping is seen in small temperature intervals below the transition points, these anomalies are much smaller than those that occur at ferroelastic phase transitions.