DFT Study on Ferroelectricity of BaTiO3

Origins of multi-sublattice magnetism and superexchange interactions in double-double perovskite CaMnCrSbO6

The multi-sublattice magnetism and electronic structure in double-double perovskite compound CaMnCrSbO6is explored using density functional theory. The bulk magnetization and neutron diffraction suggest a ferrimagnetic order (TC∼49 K) between between Mn2+and Cr3+spins. Due to the non-equivalent Mn atoms (labelled as Mn(1) and Mn(2) which have tetrahedral and planar oxygen coordinations, respectively) and the Cr atom in the centre of distorted oxygen octahedron in the unit cell, the exchange interactions are more complex than that expected from a two sublattice magnetic system. The separations between the on-site energies of thed-orbitals of Mn(1), Mn(2) and Cr obtained from Wannier function analysis are in agreement with their expected crystal field splitting. While the DOS obtained from non spin-polarized calculations show a metallic character, starting from HubbardU = 0 eV the spin-polarized electronic structure calculations yield a ferrimagnetic insulating ground state. The band gap increases withUeff(U - J), thereby showing a Mott-Hubbard nature of the system. The inclusion of anti-site disorder in the calculations show decrease in band-gap and also reduction in the total magnetic moment. Due to the ∼90∘superexchange, nearest neighbour exchange constants obtained from DFT are an order of magnitude smaller than those reported for various magnetic perovskite and double-perovskite compounds. The Mn(1)-O-Mn(2) (out of plane and in-plane), Mn(1)-O-Cr and Mn(2)-O-Cr superexchange interactions are found to be anti-ferromagnetic, while the Cr-O-O-Cr super-superexchange is found to be ferromagnetic. The Mn(2)-O-Cr superexchange is weaker than the Mn(1)-O-Cr super-exchange, thus effectively resulting in ferrimagnetism. From a simple 3-site Hubbard model, we derived expressions for the antiferromagnetic superexchange strengthJAFMand also for the weaker ferromagneticJFM. The relative strengths ofJAFMfor the various superexchange interactions are in agreement with those obtained from DFT. The expression for Cr-O-O-Cr super-superexchange strength (J~SS), which has been derived considering a 4-site Hubbard model, predicts a ferromagnetic exchange in agreement with DFT. Finally, our mean field calculations reveal that assuming a set of four magnetic sub-lattices for Mn2+spins and a single magnetic sublattice for Cr3+spins yields a much improvedTC, while a simple two magnetic sublattice model yields a much higherTC.

Structural phase transition driven dielectric and optical properties with reduction in band gap in Sr2+modified BaTiO3ceramics

Ferroelectric materials with crystal symmetry transition from single phase to multiphase coexistence exhibit anomalous photosensitive properties. The optical properties (optical band gap and photosensitive) found on non-centrosymmetric and centrosymmetric systems achieved research interest because of their interesting behavior. In this regard, the lead-free polycrystalline Ba1-xSrxTiO3(BSTO, 0⩽x⩽0.3) has been synthesized to explore its crystal structure, dielectric, light absorption, and photocurrent sensing properties for various applications. Both experimental and theoretical studies on BSTO (0⩽x⩽0.3) ceramics confirm the crystal symmetry transition with the reduction of band gap as compared to pristine BaTiO3. This crystal symmetry transition plays an important role in varying the various physical properties as it involves the transition from the polar phase to the non-polar phase. The optical band gap has been estimated experimentally by the Tauc plot method and found that there is a small variation of energy band gap from 3.615 eV to 3.212 eV with Sr substitution. The highest dielectric constant was found to be 5327 at lower frequency on Ba0.76Sr0.24TiO3after that for further increase in Sr concentration the dielectric constant decreases because of the introduction of the non-polar phase. A strong correlation between crystal structure and physical properties (dielectric, optical, etc.) has been observed. The photocurrent of the samples is significant which reveals that the sample is influenced by the photons. In a nutshell, the present study deepens the understanding of the correlation between crystal structure and various physical properties of BSTO and, hence provides an idea of required design parameters to construct a ferroelectric system for better photosensitive nature suitable for device applications.

Influence of Ca doping in structural, electronic, optical and mechanical properties of Ba1-xCaxTiO3 perovskite from first-principles investigation

Nowadays, perovskite materials are well known for electronics and optoelectronics applications. We have investigated a potential candidate for those applications to compare the applicability in optoelectronics, photorefractive and photovoltaic (PV) devices. The systematic comparative study of the structural, electronic, optical, mechanical, and thermodynamic properties of pure BaTiO₃ and Ca doped BaTiO₃ (Ba_1-xCa_xTiO₃ where x = 0.125, 0.25, 0.375, 0.500, 0.625) perovskite have been carried out using first-principles and density-functional-theory calculations as recently this material was mostly experimented. The measured structural parameters from the geometrically optimized structure of cubic BT ceramic compared with the other theoretical values. A crystal phase transition occurs when doping content x = 0.25. The electronic band structure shows that the nature of the bandgap is changed from indirect bandgap to direct bandgap energy at G-point after doping the Ca atom into BaTiO₃ (BT) crystal. Doping of Ca into BT has led to bandstructure modification including conduction band (CB) shifting toward the higher energy level. Electronic properties have been reported to examine the contribution of different orbitals to the CB and to the valance band (VB). This study investigated the modification of optical properties such as absorption, reflectivity, refractive index, extinction coefficient, conductivity, dielectric function and loss function at the energy range from 0 to 30 eV. The prominent absorption peak and optical energy were observed at the UV light energy region. Based on the optical behavior of the material this theoretical research suggests that the doped BT solution is a suitable candidate for photorefractive and optoelectronic devices. Different elastic constants reveal mechanical stability and the existence of the covalent bond of those compounds. Debye temperature increases with doping content. Hence modification of BaTiO₃ crystal by Ca atom significantly develop various properties that led it to multifunctional applications.

A first-principles study on the multiferroicity of semi-modified X2M (X = C, Si; M = F, Cl) monolayers

The research of two-dimensional multiferroic materials has attracted extensive attention in recent years. In this work, we systematically investigated the multiferroic properties of semi-fluorinated and semi-chlorinated graphene and silylene X₂M (X = C, Si; M = F, Cl) monolayers under strain using first principles calculations based on density functional theory. We find that the X₂M monolayer has a frustrated antiferromagnetic order, and a large polarization with a high reversal potential barrier. When increasing the applied biaxial tensile strain, the magnetic order remains unchanged, but the polarization flipping potential barrier of X₂M gradually decreases. When the strain increases to 35%, although the energy required to flip the fluorine and chlorine atoms is still very high in the C₂F and C₂Cl monolayers, it goes down to 312.5 meV and 260 meV in unit cells of the Si₂F and Si₂Cl monolayers, respectively. At the same time, both semi-modified silylenes exhibit metallic ferroelectricity with a band gap of at least 0.275 eV in the direction perpendicular to the plane. The results of these studies show that Si₂F and Si₂Cl monolayers may become a new generation of information storage materials with magnetoelectric multifunctional properties.

Insights into the relationship between ferroelectric and photovoltaic properties in CsGeI3 for solar energy conversion

Materials such as oxide and halide perovskites that simultaneously exhibit spontaneous polarization and absorption of visible light are called photoferroelectrics. They hold great promise for the development of applications in optoelectronics, information storage, and energy conversion. Devices based on ferroelectric photovoltaic materials yield an open-circuit voltage that is much higher than the band gap of the corresponding active material owing to a strong internal electric field. Their efficiency has been proposed to exceed the Shockley-Queisser limit for ideal solar cells. In this paper, we present theoretical calculations of the photovoltaic properties of the ferroelectric phase of the inorganic germanium halide perovskite (CsGeI3). Firstly, the electronic, optical and ferroelectric properties were calculated using the FP-LAPW method based on density functional theory, and the modern theory of polarization based on the Berry phase approach, respectively. The photovoltaic performance was evaluated using the Spectroscopic Limited Maximum Efficiency (SLME) model based on the results of first-principles calculations, in which the power conversion efficiency and the photocurrent density-voltage (J-V) characteristics were estimated. The calculated results show that the valence band maximum (VBM) of CsGeI3 is mainly contributed by the I-5p and Ge-4s orbitals, whereas the conduction band is predominantly derived from Ge-4p orbitals. It can be seen that CsGeI3 exhibits a direct bandgap semiconductor at the symmetric point of Z with a value of 1.53 eV, which is in good agreement with previous experimental results. The ferroelectric properties were therefore investigated. With a switching energy barrier of 19.83 meV per atom, CsGeI3 has a higher theoretical ferroelectric polarization strength of 15.82 μC cm-2. The SLME calculation also shows that CsGeI3 has a high photoelectric conversion efficiency of over 28%. In addition to confirming their established favorable band gap and strong absorption, we demonstrate that CsGeI3 exhibits a large shift current bulk photovoltaic effect of up to 40 μA V-2 in the visible region. Thus, this material is a potential ferroelectric photovoltaic absorbed layer with high efficiency.

Highly heterogeneous epitaxy of flexoelectric BaTiO3-δ membrane on Ge

The integration of complex oxides with a wide spectrum of functionalities on Si, Ge and flexible substrates is highly demanded for functional devices in information technology. We demonstrate the remote epitaxy of BaTiO₃ (BTO) on Ge using a graphene intermediate layer, which forms a prototype of highly heterogeneous epitaxial systems. The Ge surface orientation dictates the outcome of remote epitaxy. Single crystalline epitaxial BTO_3-δ films were grown on graphene/Ge (011), whereas graphene/Ge (001) led to textured films. The graphene plays an important role in surface passivation. The remote epitaxial deposition of BTO_3-δ follows the Volmer-Weber growth mode, with the strain being partially relaxed at the very beginning of the growth. Such BTO_3-δ films can be easily exfoliated and transferred to arbitrary substrates like Si and flexible polyimide. The transferred BTO_3-δ films possess enhanced flexoelectric properties with a gauge factor of as high as 1127. These results not only expand the understanding of heteroepitaxy, but also open a pathway for the applications of devices based on complex oxides.

The integration of epitaxial complex oxides on semiconductor and flexible substrates is required but challenging. Here, the authors report the highly heterogeneous epitaxy of transferrable BaTiO_3-δ membrane with enhanced flexoelectricity on Ge (011).

Metal–Organic Frameworks (MOFs) as Potential Hybrid Ferroelectric Materials

This chapter presents new findings of intrinsic and induced ferroelectricity in Metal–Organic Frameworks (MOFs) with a polar system, capable of forming an electronic structure in an asymmetric lattice. Multiple experimental techniques and simulation methods are reviewed in detail. The characteristics of ferroelectrics such as discontinuity in temperature-dependent dielectric constant, polarization hysteresis loops, etc. have been observed from several MOF large crystals and crystalline powders. A relationship between polarization and bond polarity for MOFs has been established. In addition, we emphasize the significance of mechanical strength of MOFs in real applications. This chapter reviews MOF materials for energy storage and utilization, aiming to provide an insight into the design of novel MOF-based ferroelectrics.

Electro-optic Response in Germanium Halide Perovskites

Electro-optic materials that can be solution-processed and provide high-crystalline quality are sought for the development of compact, efficient optical modulators. Here we present density functional theory investigations of the linear electro-optic coefficients of candidate materials cesium and methylammonium germanium halide perovskites. As with their lead halide counterparts, these compounds can be solution-processed, but in contrast, they possess the noncentrosymmetric crystal structures needed to provide a linear electro-optic effect. We find substantial electro-optic responses from these compounds; in particular, for the r₅₁ tensor element of CsGeI₃, we predict an electro-optic coefficient of 47 pm·V^-1 at the communications wavelength of 1550 nm, surpassing the strongest coefficient of LiNbO₃ at 31 pm·V^-1. The strong electro-optic responses of the germanium compounds are driven by high nonlinear susceptibilities and dynamics of the germanium atoms that ultimately arise from the distorted crystal structures. Alongside the electro-optic coefficient calculations, we provide the frequency responses for the linear and nonlinear electronic susceptibilities.

Single Atomic Layer Ferroelectric on Silicon

A single atomic layer of ZrO₂ exhibits ferroelectric switching behavior when grown with an atomically abrupt interface on silicon. Hysteresis in capacitance-voltage measurements of a ZrO₂ gate stack demonstrate that a reversible polarization of the ZrO₂ interface structure couples to the carriers in the silicon. First-principles computations confirm the existence of multiple stable polarization states and the energy shift in the semiconductor electron states that result from switching between these states. This monolayer ferroelectric represents a new class of materials for achieving devices that transcend conventional complementary metal oxide semiconductor (CMOS) technology. Significantly, a single atomic layer ferroelectric allows for more aggressively scaled devices than bulk ferroelectrics, which currently need to be thicker than 5-10 nm to exhibit significant hysteretic behavior (Park, et al. Adv. Mater. 2015, 27, 1811).

Monolayer AgBiP2Se6: an atomically thin ferroelectric semiconductor with out-plane polarization

Using first-principles calculations, we designed a two-dimensional material, monolayer AgBiP₂Se₆, with the thickness of only 6 Å, which exhibited out-plane ferroelectricity. The ground state of the monolayer AgBiP₂Se₆ was not purely ferroelectric since the out-plane ferroelectricity originated from the compensated ferrielectric state: the off-centering antiparallel displacements of Ag⁺ and Bi³⁺ ions. The compensated ferrielectric ordering has superiority on reducing the depolarization field to stabilize the ferroelectricity. Furthermore, together with strong visible-light adsorption and suitable band edge alignments, we proposed the monolayer AgBiP₂Se₆ as a visible-light photocatalyst for water-splitting as the out-plane polarization could enhance the electron-hole separation. Our results offer a new way to overcome the critical thickness limitation of nanoscale ferroelectrics. The out-plane ferroelectricity in monolayer AgBiP₂Se₆ has great potential for developing various devices with desirable applications.

Surface effects on converse piezoelectricity of crystals

The contribution of surface units to bulk properties are often neglected in theoretical and computational studies of crystalline systems. We demonstrate that this assumption has to be made with caution in the case of (electric field) polarization.

Photoelectron transport tuning of self-assembled subbands

Conventionally, electrical transport of quantum subbands occurs at very high electric fields, indicating that the medium is easy to break down. In the experiments and practical applications, the extreme condition is difficult to satisfy. For quantum information transmission, low power consumption and convenient implementation are what we expect. In this paper, we engineered a special quantum dot array (QDA) embedded in a single crystal matrix. By external optical field excitation, we found a series of subbands made of the self-assembled QDA discretely located in the matrix. Changing the spacing between the quantum dots leads to the variation of subband spacing. Artificially manipulating the microcosmic QDA system can bring interesting macroscopic effects, such as an enhanced absorption intensity in the ultraviolet range, a blue-shift of the surface plasmon resonance peak and nonlinear absorption changed from two-photon absorption to saturated absorption. The intrinsic mechanism of the subband optical response was revealed due to the strong quantum confinement effect and dominant intraband transitions. The weak surface plasmon resonance absorption of Ni QDA gave an excellent figure of merit of the order of 10(-10). The composite films are expectation enough to become a prime candidate for nonlinear applications near 532 nm. Therefore with interplay of the weak optical field and subbands, we achieved a tunable photoelectron transport process.