First-Principles Determination of Chain-Structure Instability in KNbO3

Machine learning and atomistic origin of high dielectric permittivity in oxides

Discovering new stable materials with large dielectric permittivity is important for future energy storage and electronics applications. Theoretical and computational approaches help design new materials by elucidating microscopic mechanisms and establishing structure-property relations. Ab initio methods can be used to reliably predict the dielectric response, but for fast materials screening, machine learning (ML) approaches, which can directly infer properties from the structural information, are needed. Here, random forest and graph convolutional neural network models are trained and tested to predict the dielectric constant from the structural information. We create a database of the dielectric properties of oxides and design, train, and test the two ML models. Both approaches show similar performance and can successfully predict response based on the structure. The analysis of the feature importance allows identification of local geometric features leading to the high dielectric permittivity of the crystal. Dimensionality reduction and clustering further confirms the relevance of descriptors and compositional features for obtaining high dielectric permittivity.

Permittivity boosting by induced strain from local doping in titanates from first principles

We examine the effect of isovalent substitutions and co-doping on the ionic dielectric constant of paraelectric titanates (perovskite, Ruddlesden-Popper phases, and rutile) using density functional perturbation theory. Substitutions increase the ionic dielectric constant of the prototype structures, and new dynamically stable structures with ε_ion ~ 10²-10⁴ are reported and analyzed. The boosting of ionic permittivity is attributed to local defect-induced strain, and maximum Ti-O bond length is proposed as a descriptor. The Ti-O phonon mode that is responsible for the large dielectric constant can be tuned by a local strain and symmetry lowering from substitutions. Our findings help explain the recently observed colossal permittivity in co-doped rutile, attributing its intrinsic permittivity boosting solely to the lattice polarization mechanism, without the need to invoke other mechanisms. Finally, we identify new perovskite- and rutile-based systems that can potentially display colossal permittivity.

Dynamic Displacement Disorder of Cubic BaTiO_{3}

The three-dimensional distribution of the x-ray diffuse scattering intensity of BaTiO_{3} has been recorded in a synchrotron experiment and simultaneously computed using molecular dynamics simulations of a shell model. Together, these have allowed the details of the disorder in paraelectric BaTiO_{3} to be clarified. The narrow sheets of diffuse scattering, related to the famous anisotropic longitudinal correlations of Ti ions, are shown to be caused by the overdamped anharmonic soft phonon branch. This finding demonstrates that the occurrence of narrow sheets of diffuse scattering agrees with a displacive picture of the cubic phase of this textbook ferroelectric material. The presented methodology allows one to go beyond the harmonic approximation in the analysis of phonons and phonon-related scattering.

Competing covalent and ionic bonding in Ge-Sb-Te phase change materials

Ge₂Sb₂Te₅ and related phase change materials are highly unusual in that they can be readily transformed between amorphous and crystalline states using very fast melt, quench, anneal cycles, although the resulting states are extremely long lived at ambient temperature. These states have remarkably different physical properties including very different optical constants in the visible in strong contrast to common glass formers such as silicates or phosphates. This behavior has been described in terms of resonant bonding, but puzzles remain, particularly regarding different physical properties of crystalline and amorphous phases. Here we show that there is a strong competition between ionic and covalent bonding in cubic phase providing a link between the chemical basis of phase change memory property and origins of giant responses of piezoelectric materials (PbTiO₃, BiFeO₃). This has important consequences for dynamical behavior in particular leading to a simultaneous hardening of acoustic modes and softening of high frequency optic modes in crystalline phase relative to amorphous. This different bonding in amorphous and crystalline phases provides a direct explanation for different physical properties and understanding of the combination of long time stability and rapid switching and may be useful in finding new phase change compositions with superior properties.

Nanoscale Atomic Displacements Ordering for Enhanced Piezoelectric Properties in Lead-Free ABO3 Ferroelectrics

In situ synchrotron X-ray diffuse scattering and inelastic neutron scattering measurements from a prototype ABO3 ferroelectric single-crystal are used to elucidate how electric fields along a nonpolar direction can enhance its piezoelectric properties. The central mechanism is found to be a nanoscale ordering of B atom displacements, which induces increased lattice instability and therefore a greater susceptibility to electric-field-induced mechanical deformation.

Ab initio and shell model studies of structural, thermoelastic and vibrational properties of SnO2 under pressure

The pressure dependences of the structural, thermoelastic and vibrational properties of SnO2 in its rutile phase are studied, as well as the pressure-induced transition to a CaCl2-type phase. These studies have been performed by means of ab initio (AI) density functional theory calculations using the localized basis code SIESTA. The results are employed to develop a shell model (SM) for application in future studies of nanostructured SnO2. A good agreement of the SM results for the pressure dependences of the above properties with the ones obtained from present and previous AI calculations as well as from experiments is achieved. The transition is characterized by a rotation of the Sn-centered oxygen octahedra around the tetragonal axis through the Sn. This rotation breaks the tetragonal symmetry of the lattice and an orthorhombic distortion appears above the critical pressure P(c). A zone-center phonon of B1g symmetry in the rutile phase involves such rotation and softens on approaching Pc. It becomes an Ag mode which stabilizes with increasing pressure in the CaCl2 phase. This behavior, together with the softening of the shear modulus (C11-C12)/2 related to the orthorhombic distortion, allows a precise determination of a value for Pc. An additional determination is provided by the splitting of the basal plane lattice parameters. Both the AI and the experimentally observed softening of the B(1g) mode are incomplete, indicating a small discontinuity at the transition. However, all results show continuous changes in volume and lattice parameters, indicating a second-order transition. All these results indicate that there should be sufficient confidence for the future employment of the shell model.

Interface control of emergent ferroic order in Ruddlesden-Popper Sr(n+1)Ti(n)O(3n+1)

We discovered from first principles an unusual polar state in the low n Sr(n+1)Ti(n)O(3n+1) Ruddlesden-Popper (RP) layered perovskites in which ferroelectricity is nearly degenerate with antiferroelectricity, a relatively rare form of ferroic order. We show that epitaxial strain plays a key role in tuning the "perpendicular coherence length" of the ferroelectric mode, and does not induce ferroelectricity in these low-dimensional RP materials as is well known to occur in SrTiO(3). These systems present an opportunity to manipulate the coherence length of a ferroic distortion in a controlled way, without disorder or a free surface.

Correlations and local order parameter in the paraelectric phase of barium titanate

General features of the order parameter distribution in barium titanate in its paraelectric phase and in its ferroelectric phases (tetragonal and orthorhombic) are presented. The density of probability of the polarization [Formula: see text], defined by an average of the local order parameters over regions of various sizes and shapes (L(x) × L(y) × L(z)), is examined by molecular dynamics simulations using a first-principles derived effective Hamiltonian. The free energies [Formula: see text] associated with these probabilities are computed by thermodynamic integration. The evolution of these quantities are explained through the computation of pair correlations, which are found, as stated in several previous works, very anisotropic, 'needle-like', with longitudinal correlations ([Formula: see text]) having much longer range than transverse ones ([Formula: see text]). The correlations explain why the density of probability of the order parameter evolves from a multiple-peaked distribution with maxima along [111] (in the single cell), along [100] for small needle-like regions, towards a single-peaked distribution for larger regions. A useful expression in which the shape-dependence of the free energy is manifest is provided.

New Polaronic-Type Excitons in Ferroelectric Oxides: INDO-Calculations and Experimental Manifestation

The current experimental and theoretical knowledge of new polaronic-type excitons in ferroelectric oxides – charge transfer vibronic excitons (CTVE) – is discussed. It is shown that Hartree-Fock-type INDO calculations as well as photoluminescence studies in ferroelectric oxygen-octahedral perovskites confirm the CTVE-concept. Single CTVE as well as a new phase of strongly correlated CTVEs are analysed.

Flexible relaxor materials: Ba(2)Pr(x)Nd(1-x)FeNb(4)O(15) tetragonal tungsten bronze solid solution

Relaxors are very interesting materials but most of the time they are restricted to perovskite materials and thus their flexibility is limited. We have previously shown that tetragonal tungsten bronze (TTB) niobate Ba(2)PrFeNb(4)O(15) was a relaxor below 170 K and that Ba(2)NdFeNb(4)O(15) displays a ferroelectric behavior with a T(C) = 323 K. On scanning the whole solid solution Ba(2)Pr(x)Nd(1-x)FeNb(4)O(15) (x = 0, 0.2, 0.4, 0.5, 0.6, 0.8 and 1), we demonstrate here a continuous crossover between these end member behaviors with a coexistence of ferroelectricity and relaxor in the intermediate range. This tunability is ascribed to the peculiar structure of the TTB networks which is more open than the classical perovskites. This allows for the coexistence of long range and short range orders and thus opens up the range of relaxor materials.

The ferroelectric and cubic phases in BaTiO3 ferroelectrics are also antiferroelectric

Using quantum mechanics (QM, Density Functional Theory) we show that all four phases of barium titanate (BaTiO(3)) have local Ti distortions toward 111 (an octahedral face). The stable rhombohedral phase has all distortions in phase (ferroelectric, FE), whereas higher temperature phases have antiferroelectric coupling (AFE) in one, two, or three dimensions (orthorhombic, tetragonal, cubic). This FE-AFE model from QM explains such puzzling aspects of these systems as the allowed Raman excitation observed for the cubic phase, the distortions toward 111 observed in the cubic phase using x-ray fine structure, the small transition entropies, the heavily damped soft phonon modes, and the strong diffuse x-ray scattering in all but the rhombohedral phase. In addition, we expect to see additional weak Bragg peaks at the face centers of the reciprocal lattice for the cubic phase. Similar FE-AFE descriptions are expected to occur for other FE materials. Accounting for this FE-AFE nature of these phases is expected to be important in accurately simulating the domain wall structures, energetics, and dynamics, which in turn may lead to the design of improved materials.

Frustration of tilts and A-site driven ferroelectricity in KNbO3-LiNbO3 alloys

Density functional calculations for K(0.5)Li(0.5)NbO(3) show strong A-site driven ferroelectricity, even though the average tolerance factor is significantly smaller than unity and there is no stereochemically active A-site ion. This is due to the frustration of tilt instabilities by A-site disorder. There are very large off centerings of the Li ions, which contribute strongly to the anisotropy between the tetragonal and rhombohedral ferroelectric states, yielding a tetragonal ground state even without strain coupling.

Interface effect on ferroelectricity at the nanoscale

Interfaces play a critical role in nanoscale ferroelectricity. We perform a first-principles study of ultrathin KNbO(3) ferroelectric films placed between two metal electrodes, either SrRuO(3) or Pt. We show that bonding at the ferroelectric-metal interfaces imposes severe constraints on the displacement of atoms, destroying the bulk tetragonal soft mode. If the interface bonding is sufficiently strong, the ground-state represents a ferroelectric domain with an interface domain wall, driven by the intrinsic oppositely oriented dipole moments at the two interfaces. The critical thickness for the net polarization of the KNbO(3) film is predicted to be about 1 nm for Pt and 1.8 nm for SrRuO(3) electrodes.

Ferroelectricity at the Nanoscale: Local Polarization in Oxide Thin Films and Heterostructures

Ferroelectric oxide materials have offered a tantalizing potential for applications since the discovery of ferroelectric perovskites more than 50 years ago. Their switchable electric polarization is ideal for use in devices for memory storage and integrated microelectronics, but progress has long been hampered by difficulties in materials processing. Recent breakthroughs in the synthesis of complex oxides have brought the field to an entirely new level, in which complex artificial oxide structures can be realized with an atomic-level precision comparable to that well known for semiconductor heterostructures. Not only can the necessary high-quality ferroelectric films now be grown for new device capabilities, but ferroelectrics can be combined with other functional oxides, such as high-temperature superconductors and magnetic oxides, to create multifunctional materials and devices. Moreover, the shrinking of the relevant lengths to the nanoscale produces new physical phenomena. Real-space characterization and manipulation of the structure and properties at atomic scales involves new kinds of local probes and a key role for first-principles theory.