We theoretically study the stability conditions of the ferroelectric ice of the Cmc21 structure, which has been considered, for decades, one of the most promising candidates of the low temperature proton-ordered phase of pure ice Ih. It turned out that the Cmc21 structure is stable only with a certain amount of dopant and the true proton-ordered phase of pure ice Ih remains to be found at lower temperature. Implication for spin ice is mentioned.
To investigate the possible origin and mechanism of ferroelectricity in polycrystalline spin ices Ho2Ti2O7 and Dy2Ti2O7 a detailed dielectric study has been performed. Experimental finding suggests that both materials have two prominent diffuse ferroelectric phase transitions around 90K and 36K. These transitions are distinctly generated by the lattice distortions at the oxygen sites as confirmed by triggered distortions and order of activation energy. Due to the incompatibility of the gyrotropic order with any phonon mode at the Brillouin zone center, observed diffuse ferroelectric phase tran…
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 disorde…
Water confined between two layers with separation of a few Angstrom forms layered two- dimensional ice structure. Using large scale molecular dynamics simulations with the adoptable ReaxFF interatomic potential we found that flat monolayer ice with a rhombic-square structure nucleates between graphene layers which is non-polar and non-ferroelectric. Two layers of water are found to crystallize into a square lattice close to the experimental found AA-stacking [G. Algara- Siller et al. Nature 519, 443445 (2015)]. Each layer has a net dipole moment which are in opposite direction. Bilayer ice is …
Pyrochlore dielectric materials such as (Bi$_{1.5}$Zn$_{0.5}$)(Nb$_{1.5}$Zn$_{0.5}$)O$_7$ (BZN) have generated interest because they combine high dielectric constants with small dielectric loss tangents and yet are cubic at all temperatures. The recent low-temperature preparation and structural characterization of Bi$_2$Ti$_2$O$_7$, which remains cubic down to 2 K, has provided a good model system for understanding the properties of Bi-based pyrochlores. In this contribution, the electronic structure of cubic Bi$_2$Ti$_2$O$_7$ is visualized and compared with the electronic structure of the Aur…
The influence of the B-site ion substitutions in (1-x)(Bi1/2Na1/2)TiO3-xBaTiO3 system of solid solutions on the relative stability of the ferroelectric and antiferroelectric phases has been studied. The ions of zirconium, tin, along with (In0.5Nb0.5), (Fe0.5Nb0.5), (Al0.5V0.5) ion complexes have been used as substituting elements. An increase in the concentration of the substituting ion results in a near linear variation in the size of the crystal lattice cell. Along with the cell size variation a change in the relative stability of the ferroelectric and antiferroelectric phases takes place ac…
A new phase of ice, named ice XV, has been identified and its structure determined by neutron diffraction. Ice XV is the hydrogen-ordered counterpart of ice VI and is thermodynamically stable at temperatures below ~130 K in the 0.8 to 1.5 GPa pressure range. The regions of stability in the medium pressure range of the phase diagram have thus been finally mapped, with only hydrogen-ordered phases stable at 0 K. The ordered ice XV structure is antiferroelectric, in clear disagreement with recent theoretical calculations predicting ferroelectric ordering.
The removal of residual hydrogen disorder from various phases of ice with acid or base dopants at low temperatures has been a focus of intense research for many decades. As an antipode to these efforts, we now show using neutron diffraction that ammonium fluoride (NH4F) is a hydrogen-disordering agent for the hydrogen-ordered ice VIII. Cooling its hydrogen-disordered counterpart ice VII doped with 2.5 mol% ND4F under pressure leads to a hydrogen-disordered ice VIII with ~31% residual hydrogen disorder illustrating the long-range hydrogen-disordering effect of ND4F. The doped ice VII could be s…
Bounded charges induced by the polarization gradient in finite-size ferroelectrics are known to produce the unfavorable depolarization electric field that suppresses the uniform ferroelectric state. To reduce the depolarization energy the non-uniform vortex (toroidal) state is formed inside ferroelectric nano-particles, nano-disks, and nano-rods. Based on self-consistent solution of Ginzburg-Landau equations coupled with electrostatic equations, we study the multivortex toroidal states appearing in the nanometric easy-plane ferroelectric cylinder. The geometrical textures, critical temperature…
We present the first analytical study of phase transitions in ferroelastic-ferroelectric epitaxial thin films on exactly solvable model. The emerging domain structure with domains of equal width (which may be exponentially large on a "soft" substrate) always remains stable irrespective of the film thickness. The dielectric response of an epitaxial film is {\em smaller} than that of a free film, in striking contrast with assertions in the literature.
Ferroelectric domain switching in c-axis-oriented epitaxial Pb(Zr0.2Ti0.8)O3 thin films was studied using different field geometries and compared to numerical simulations and theoretical predictions. With carbon nanotubes as electrodes, continuous nanodomains as small as 9 nm in radius in a 270 nm thick film could be switched, remaining stable for over 20 months. Defect pinning of domain walls appears to play a key role in stabilizing such domains, below the predicted thermodynamic size limit.
Phase-field simulation (PFS) have revolutionized the understanding of domain structure and switching behavior in ferroelectric thin films and ceramics. Generally, PFS is based on solution of a (set) of Ginzburg-Landau equations for a defined order parameter field(s) under physical boundary conditions (BCs) of fixed potential or charge. While well-matched to the interfaces in bulk materials and devices, these BCs are generally not applicable to free ferroelectric surfaces. Here, we developed a self-consistent phase-field model with boundary conditions based on electrochemical equilibria. We cho…
Nanoscale ferroelectric topologies such as vortices, anti-vortices, bubble patterns etc. are stabilized in thin films by a delicate balance of both mechanical and electrical boundary conditions. A systematic understanding of the phase stability of bubble domains, particularly when the above factors act simultaneously, remains elusive. Here we present first-principle-based simulations in combination with scanning probe microscopy of ultrathin epitaxial (001) PbZr0.4Ti0.6O3 heterostructures to address this gap. The simulations predict that as-grown labyrinthine domains will transform to bubbles …
We use extensive first-principles quantum mechanical calculations to show that, although the static lattice and harmonic vibrational energies are almost identical, the anharmonic vibrational energy of hexagonal ice is significantly lower than that of cubic ice. This difference in anharmonicity is crucial, stabilising hexagonal ice compared with cubic ice by at least 1.4 meV/H2O, in agreement with experimental estimates. The difference in anharmonicity arises predominantly from molecular O-H bond stretching vibrational modes and is related to the different stacking of atomic layers.
The screening efficiency of a metal-ferroelectric interface plays a critical role in determining the polarization stability and hence the functional properties of ferroelectric thin films. Imperfect screening leads to strong depolarization fields that reduce the spontaneous polarization or drive the formation of ferroelectric domains. We demonstrate that by modifying the screening at the metal-ferroelectric interface through insertion of ultrathin dielectric spacers, the strength of the depolarization field can be tuned and thus used to control the formation of nanoscale domains. Using piezore…
The coexistence of different ferroelectric phases enables the tunability of the macroscopic properties and extensive applications from piezoelectric transducers to nonvolatile memories. Here we develop a thermodynamic model to predict the stability and low-energy orientations of boundaries between different phases in ferroelectrics. Taking lead zirconate titanate and bismuth ferrite as two examples, we demonstrate that the low-energy orientations of interphase boundaries are largely determined by minimizing the electrostatic and elastic energies. Phase-field simulations are employed to analyze…
The stability of the disordered glassy phase in the relaxors PbMg1/3Nb2/3O3 and (PbMg1/3Nb2/3O3)0.88(PbTiO3)0.12, called PMN and PMN-PT, was investigated by preparing partially polarized samples and allowing them to age at zero field in the temperature range for which the phase is history-dependent. The PMN-PT polarization would spontaneously increase until long-range order formed, first appearing as giant polarization noise. Thus the thermodynamically stable phase in PMN-PT appears to be ferroelectric. In contrast, a PMN sample lacking the sharp first-order field-driven transition found in so…
Ferroelectric domain inversion and its effect on the stability of lithium niobate thin films on insulator (LNOI) are experimentally characterized. Two sets of specimens with different thicknesses varying from submicron to microns are selected. For micron thick samples (~28 um), domain structures are achieved by pulsed electric field poling with electrodes patterned via photolithography. No domain structure deterioration has been observed for a month as inspected using polarizing optical microscopy and etching. As for submicron (540 nm) films, large-area domain inversion is realized by scanning…
The discovery of ferroelectric polarization in HfO2-based ultrathin films has spawned a lot of interest due to their potential applications in data storage. Recently, a new R3m rhombohedral phase was proposed to be responsible for the emergence of ferroelectricity in the [111]-oriented Hf0.5Zr0.5O2 thin films, but the fundamental mechanism of ferroelectric polarization in such films remains poorly understood. In this paper, we employ density-functional-theory calculations to investigate structural and polarization properties of the R3m HfO2 phase. We find that the film thickness and in-plane c…
Experimental data on the structure of ferroelectric oxide Bismuth Titanate suggests two different kinds of structures i.e. orthorhombic and monoclinic. We have performed density functional theory (DFT) based first principle calculations to determine the ground state structure of bismuth titanate, based on experimentally observed monoclinic and orthorhombic phases of Bi4Ti3O12. Both of these phases are optimized to zero pressure and lattice parameters were determined as a = 5.4370 Ang, b = 5.4260 Ang, c = 32.6833 Ang and Z = 4 for structure with space group B2cb and a = 5.4289 Ang, b = 5.4077 A…
We have applied a shell model potential developed by fitting first principle results to describe the behavior of the relaxor-ferroelectric PMN-xPT as function of concentration. The solid solution exhibits three regions with different characteristics according to Ti content in agreement with the experiments. At low Ti the polar and structural disorder dominates, and the symmetry is rhombohedral. The intermediate region is ferroelectric with easy rotation of the polarization direction, and the symmetry appears orthorhombic. Finally, in the high Ti content region, the solid solution adopts a ferr…
The discovery of ferroelectricity in both pure and doped HfO$_2$-based thin films have revitalized the interest in using ferroelectrics for nanoscale device applications. To take advantage of this silicon-compatible ferroelectric, fundamental questions such as the origin of ferroelectricity and better approach to controlled realization of ferroelectricity at the nanoscale need to be addressed. The emergence of robust polarization in HfO$_2$-based thin films is considered as the cumulative effect of various extrinsic factors such as finite-size effects and surface/interface effects of small gra…
Films of all-important compound hafnia (HfO2) can be prepared in an orthorhombic ferroelectric (FE) state that is ideal for applications, e.g. in memories or negative-capacitance field-effect transistors. The origin of this FE state remains a mystery, though, as none of the proposed mechanisms for its stabilization -- from surface and size effects to formation kinetics -- is fully convincing. Interestingly, it is known that doping HfO2 with various cations favors the occurrence of the FE polymorph; however, existing first-principles works suggest that doping by itself is not sufficient to stab…
Recent structural studies of ferroelectric PbZr(1-x)TixO3 (PZT) with x= 0.48, have revealed a new monoclinic phase in the vicinity of the morphotropic phase boundary (MPB), previously regarded as the the boundary separating the rhombohedral and tetragonal regions of the PZT phase diagram. In the present paper, the stability region of all three phases has been established from high resolution synchrotron x-ray powder diffraction measurements on a series of highly homogeneous samples with 0.42 <=x<= 0.52. At 20K the monoclinic phase is stable in the range 0.46 <=x<= 0.51, and this range narrows …
Thermodynamic theory is developed for the ferroelectric phase transition of an ultrathin film in equilibrium with a chemical environment that supplies ionic species to compensate its surface. Equations of state and free energy expressions are developed based on Landau-Ginzburg-Devonshire theory, using electrochemical equilibria to provide ionic compensation boundary conditions. Calculations are presented for a monodomain PbTiO$_3$ (001) film coherently strained to SrTiO$_3$ with its exposed surface and its electronically conducting bottom electrode in equilibrium with a controlled oxygen parti…
We describe length and time rescaling across the paraelectric to ferroelectric phase transition in BaTiO$_3$. Small ferroelectric clusters are unable to grow in the paraelectric phase and exist for a short period of time only. The onset of the phase transition invokes a rescaling of both size and time lenghts as necessary ingredients for ferroelectricity to come into existence. The growing of ferroelectric domains involves correlated chains of Ti-O dipoles, which causes domain wall rigidity and domain shape stability. Ferroelectric nanodomains may host islands of opposite polarization (antifer…
We study the ferroelectric stability and surface structural properties of an oxygen-terminated hexagonal YMnO$_3$ ultra-thin film using density functional theory. Under an open circuit boundary condition, the ferroelectric state with the spontaneous polarization normal to the (0001) surface, is found to be metastable in a single domain state despite the presence of a depolarizing field. We establish a connection between the result and the role of improper ferroelectric transition. Our results imply that improper ferroelectric ultrathin films can have rather unique properties that are distincti…
Using density functional theory (DFT) and density functional perturbation theory (DFPT), we investigate the stability and response functions of CsH$_2$PO$_4$, a ferroelectric material at low temperature. This material cannot be described properly by the usual (semi-)local approximations within DFT. The long-range e$^-$-e$^-$ correlation needs to be properly taken into account, using, for instance, Grimme's DFT-D methods, as investigated in this work. We find that DFT-D3(BJ) performs the best for the members of the dihydrogenated alkali phosphate family (KH$_2$PO$_4$, RbH$_2$PO$_4$, CsH$_2$PO$_…
We study the free energy landscape of a minimal model for relaxor ferroelectrics. Using a variational method which includes leading correlations beyond the mean-field approximation as well as disorder averaging at the level of a simple replica theory, we find metastable paraelectric states with a stability region that extends to zero temperature. The free energy of such states exhibits an essential singularity for weak compositional disorder pointing to their necessary occurrence. Ferroelectric states appear as local minima in the free energy at high temperatures and become stable below a coex…
With numerical simulations of the mW model of water, we investigate the energetic stability of crystalline clusters for both Ice I (cubic and hexagonal ice) and for the metastable Ice 0 phase as a function of the cluster size. Under a large variety of forming conditions, we find that the most stable cluster changes as a function of size: at small sizes the Ice 0 phase produces the most stable clusters, while at large sizes there is a crossover to Ice I clusters. We further investigate the growth of crystalline clusters with the seeding technique and study the growth patterns of different cryst…
Results of detailed investigations of stability of phases in substances with the small difference in the energies of the ferroelectric and the antiferroelectric types of dipole ordering are presented. It is shown that interaction of locally separated domains of the ferroelectric and antiferroelectric phases stabilizes a nonuniform state of the samples. The analysis of physical phenomena caused by the presence of domains of coexisting ferroelectric and antiferroelectric phases in different dipole ordered substances is presented. The consideration of peculiarities of behavior of systems with non…
For over 70 years, ferroelectric materials have been remaining one of the central research topics for condensed matter physics and material science, the interest driven both by fundamental science and applications. However, ferroelectric surfaces, the key component of ferroelectric films and nanostructures, still present a significant theoretical and even conceptual challenge. Indeed, stability of ferroelectric phase per se necessitates screening of polarization charge. At surfaces, this can lead to coupling between ferroelectric and semiconducting properties of material, or with surface (elec…
We report observation of 90-degree ferroelectric domain structures in transmission electron microscopy (TEM) of epitaxially-grown films of PbTiO3. Using molecular dynamics (MD) simulations based on first-principles effective Hamiltonian of bulk PbTiO3, we corroborate the occurance of such domains showing that it arises as metastable states only in cooling simulations (as the temperature is lowered) and establish characteristic stability of 90-degree domain structures in PbTiO3. In contrast, such domains do not manifest in similar simulations of BaTiO3. Through a detailed analysis based on ener…
SrTiO$_{3}$ is known to be an incipient ferroelectric. It is thought that ferroelectric stability in SrTiO$_{3}$ is suppressed by a delicate competition with quantum fluctuation and antiferrodistortion. The ferroelectric phase can, however, be stabilized by doping, isotope manipulation and strain engineering etc. Till date ferroelectricity in SrTiO$_{3}$ thin films was observed to exist up to room temperature -- that was when the films were grown on specially engineered substrates using complex growth techniques. It was possible to write and erase ferroelectric domains on the specially enginee…
In the last decade five new ice phases were experimentally prepared. Two of them are empty clathrate hydrates and three of them represent hydrogen ordered counterparts of previously known disordered ice phases. Here, we report on hydrogen ordering in ice VI samples upon cooling at pressures up to 1.8 GPa. Using calorimetry, dielectric relaxation spectroscopy, Raman spectroscopy, and powder X-ray diffraction we provide evidence for the existence of a second hydrogen ordered phase related to ice VI, that we call ice β-XV. This phase is more ordered than ice XV by 14% and directly transforms to i…
The time dependent cluster approximation called the path probability method (PPM) is applied to a pseudo-spin Ising Hamiltonian of the Slater-Takagi model for KH2PO4-type hydrogen-bonded ferroelectrics in order to calculate the homogeneous dynamical susceptibility above and below the ferroelectric transition temperature. Above the transition temperature all the calculations are carried out analytically in the cactus approximation of the PPM. Below the transition temperature the dynamical susceptibility is also calculated accurately since the analytical solution of spontaneous polarization in t…
The structural evolution of a polydomain ferroelectric Pb(Zr0.2Ti0.8)O3 film was studied by temperature dependent X-ray diffraction. Two critical temperatures were evidenced: T*=740K, corresponding to a change in the domain structure (a/c/a/c to a1/a2/a1/a2), and Tc_film = 825K where the film undergoes a ferroelectricparaelectric phase transition. The films remains tetragonal on the whole range of temperature investigated. The evolutions of the domain structure and lattice parameters were found to be in very good agreement with the calculated domain stability map and theoretical temperature-mi…
Enhanced room-temperature electromechanical coupling in the lead-free ferroelectric system $(1-x)$BaZr$_{0.2}$Ti$_{0.8}$O$_{3}$ - $x$Ba$_{0.7}$Ca$_{0.3}$TiO$_{3}$ (abbreviated as BZCT) at $x=0.5$ is attributed to the existence of a morphotropic phase region (MPR) containing an intermediate orthorhombic ($O$) phase between terminal rhombohedral ($R$) BZT and tetragonal ($T$) BCT phases. However, there is ambiguity regarding the morphotropic phase transition in BZCT at room temperature - while some experiments suggest a single $O$ phase within the MPR, others indicate coexistence of three polar …
We used first-principles calculations to investigate the existence and origin of the ferroelectric in- stability in the ABF3 fluoro-perovskites. We find that many fluoro-perovskites have a ferroelectric instability in their high symmetry cubic structure, which is of similar amplitude to that commonly found in oxide perovskites. In contrast to the oxides, however, the fluorides have nominal Born effective charges, indicating a different mechanism for the instability. We show that the instability originates from ionic size effects, and is therefore in most cases largely insensitive to pressure a…
The stability of high-symmetry $P4mm$ polar phase in eleven ferroelectric perovskite superlattices with the polar discontinuity is studied from first principles. In most superlattices, this phase exhibits either the ferroelectric or the antiferrodistortive instability, or both of them. The structure of the ground state and, for a number of systems, also of metastable phases in these superlattices is found. The spontaneous polarization and piezoelectric properties of superlattices are calculated. The appearance of high piezoelectric coefficients (up to 150-270 pC/N) in some superlattices is ass…
Piezoelectric and ferroelectric ceramics with a high Curie temperature (Tc) have attracted a growing attention owning to their applications under severe environments. In this work, phase structure, dielectric, ferroelectric and piezoelectric properties of (0.975-x)BiScO3-xPbTiO3-0.025Pb(Cd1/3Nb2/3)O3 ceramics (x = 0.58-0.64) were studied. A composition-induced structural transformation occurs from rhombohedral phase to tetragonal phase through an intermediate monoclinic phase with the increasing PT concentration. The relationship between structure and electrical properties of the system were d…
Ferroelectric oxides interfaced with transition metal dichalcogenides (TMDCs) offer a promising route toward ferroelectric-based devices due to lack of dangling bonds on the TMDC surface leading to a high-quality and abrupt ferroelectric/TMDC interface. In this work, the thermal stability of this interface is explored by first depositing hafnium zirconium oxide (HZO) directly on geological MoS2 and as-grown WSe2, followed by sequential annealing in ultra-high vacuum (UHV) over a range of temperatures (400-800 °C), and examining the interface through X-ray photoelectron spectroscopy (XPS). We s…
We have constructed the surface phase diagram of the BaTiO$_{3}$(001) surface by employing an evolutionary algorithm for surface structure prediction, where the ferroelectric polarization is included as a degree of freedom. Among over 1000 candidate structures explored, a surface reconstruction of (2$\times$1)-TiO is discovered to be thermodynamically stable and have the \emph{p2mm} plane group symmetry as observed experimentally. We find that the influence of ferroelectric polarization on the surface free energy can be either negligibly small or sizably large (over 1 eV per ($2 \times 1$) sup…
Comprehensive review of investigations of substances with a small difference in the energies of the ferroelectric and antiferroelectric types of dipole ordering is presented. Detailed analysis of the stability of homogeneous phases and conditions for existence of the inhomogeneous state of the substance containing domains of the coexisting ferroelectric and antiferroelectric phases is presented. It is shown that the interaction of locally separated domains of the ferroelectric and antiferroelectric phases stabilizes an inhomogeneous state of these materials. The analysis of physical phenomena …
We have performed molecular dynamics simulations using a shell model potential developed by fitting first principles results to describe the behavior of the relaxor-ferroelectric (1-x)PbMg1/3Nb2/3O3-xPbTiO3 (PMN-xPT) as function of concentration and temperature, using site occupancies within the random site model. In our simulations, PMN is cubic at all temperatures and behaves as a polar glass. As a small amount of Ti is added, a weak polar state develops, but structural disorder dominates, and the symmetry is rhombohedral. As more Ti is added the ground state is clearly polar and the system …
The structural, dielectric, dynamical, elastic, piezoelectric and nonlinear optical (second-order susceptibility and Pockels tensors) properties of Bi2$WO6 in its P2_1ab ferroelectric ground state are determined using density functional theory. The calculation of infrared and Raman spectra on single crystal allowed us to clarify the assignment of experimental phonon modes, considering the good agreement between the calculated and the experimental Raman spectra obtained on polycrystal. The calculation of the elastic constants confirms the elastic stability of the crystal and allow us to estimat…
