Berry Phase Approach Research Articles

Experimental and theoretical study of Gd2O3 added pseudo-tetragonal Bi3TaTiO9-based ceramics for ferroelectric, electric and high-temperature piezoelectric applications

The present global energy crisis, along with the development of modern technologies, has compelled the scientists to search for smart materials, which possess the multifunctional properties simultaneously. Among such materials, bismuth-layered structure ferroelectrics (BLSFs) containing a perovskite structure have demonstrated the tendency to play a vital role. Herein, we report an engineered Gd2O3 added Bi3TiTa0·5Nb0·5O9 (BTTN) based material with composition Bi3TiTa0·5Nb0·5O9:xwt%Gd2O3 (BTTN:xGd) with x = 0–0.20 ceramics to investigate the ferroelectric, electric, piezoelectric and dielectric properties of the material. Highly dense (relative density of ∼96%) BTTN:0.15Gd has shown the best merits among all other compositions with improved remnant polarization (Pr ∼ 7.86 μC/cm2), energy conversion efficiency (ƞ) of ∼63.48%, resistance of ∼2.2 × 1010 Ω and high piezoelectric co-efficient (d33 ∼ 25 pC/N) at room temperature, which are much improved than pure BTTO or BTTN ceramics. Nonetheless, ceramic has shown the stable resistivity of ∼104 Ω at 500 °C, with a stable d33 value of 22 pC/N (just 12% drop) after annealing at 600 °C. The ferroelectric to paraelectric phase transition of ceramic with the highest dielectric constant is reported at Curie temperature (TC) of 859 °C. Additionally, Density-Functional-Theory (DFT) and Density-Functional-Perturbation-Theory (DFPT) measurements are performed by employing generalized gradient approximation using the Quantum-ESPRESSO code. Structural, ferroelectric and electronic properties of BTTO, BTTN and Gd-added BTTN compounds are investigated. The calculated formation energies confirmed the thermodynamic stability of pseudo-tetragonal Gd-added BTTN compounds. Using the Berry-phase approach for piezoelectric materials, the calculated maximum polarization is 54.00 μC/cm2, which is in agreement with values obtained experimentally. Mentioned outcomes of the material clearly represent the potential of the BTTN:0.15Gd ceramic for the utilization in high-temperature piezoelectric devices.

First-Principles Anharmonic Infrared and Raman Vibrational Spectra of Materials: Fermi Resonance in Dry Ice.

We introduce a computational tool for the quantum-mechanical simulation of anharmonic infrared and Raman vibrational spectra of materials. The approach, implemented in the CRYSTAL software, stems from Taylor's expansion of the potential energy surface (PES) on the basis of normal modes up to cubic and quartic terms. The PES can be sampled with four different numerical schemes at the level of density functional theory (DFT), with local, generalized-gradient, and hybrid density functional approximations. Anharmonic states are obtained by solving Shrödinger's nuclear equation with either the vibrational self-consistent field (VSCF) or vibrational configuration interaction (VCI) methods. Nuclear quantum effects (NQEs) are thus fully accounted for. Infrared intensities are computed numerically through a Berry phase approach or analytically through a coupled-perturbed (CP) approach. Raman intensities are computed analytically via the CP approach. A variety of anharmonic features of vibrational spectra of materials can be simulated, including band shifts, combination bands, overtones, resonances (first-order Fermi, second-order Darling-Dennison), and hot bands. We showcase the effectiveness of the approach on the description of a first-order Fermi resonance (FR) in CO2 dry ice: a challenging test-case given that the FR occurs in the Raman spectrum, requires NQEs, and involves two- and three-mode couplings. Fundamental mechanistic differences with respect to the well-known FR in molecular CO2 are addressed. This application represents the first quantum-mechanical, periodic description of FR in dry ice.

Physical mechanism of Ge doping enhanced Ruddlesden-Popper structure quasi-2D Sr<sub>3</sub>Sn<sub>2</sub>O<sub>7</sub> ceramic hybrid improper ferroelectricity

Hybrid improper ferroelectricity with quasi-two-dimensional (quasi-2D) structure has attracted much attention recently due to its great potential in realizing strong magnetoelectric coupling and room-temperature multiferroicity in a single phase. However, recent studies show that there appears high coercive field and low remnant polarization in ceramics, which severely hinders the applications of this material. In this work, high-quality Sr<sub>3</sub>Sn<sub>2</sub>O<sub>7</sub> and Sr<sub>3</sub>Sn<sub>1.99</sub>Ge<sub>0.01</sub>O<sub>7</sub> ceramics with a Ruddlesden-Popper (R-P) structure are successfully prepared, and their crystal structures and electrical properties are investigated in detail. It is found that the Sr<sub>3</sub>Sn<sub>2</sub>O<sub>7</sub> ceramic exhibits a lower coercive field that is close to that of Sr<sub>3</sub>Sn<sub>2</sub>O<sub>7</sub> single crystal. Moreover, via a small amount of Ge doping, the polarization reaches 0.34 μC/cm<sup>2</sup> for Sr<sub>3</sub>Sn<sub>2</sub>O<sub>7</sub> and 0.61 μC/cm<sup>2</sup> for Sr<sub>3</sub>Sn<sub>1.99</sub>Ge<sub>0.01</sub>O<sub>7</sub>. Combining crystal lattice dynamic studies, we analyze the Raman and infrared responses of the samples, showing the information about the tilting and rotation of the oxygen octahedra in the samples. The improved ferroelectricity after doping may be attributed to the increased amplitude of the tilt mode and the reduced amplitude of rotation mode. Besides, the enhanced ferroelectric properties through Ge doping and its mechanism are further investigated by the Berry phase approach and the Born effective charge method. Furthermore, via the UV-visible spectra, the optical bandgap is determined to be 3.91 eV for Sr<sub>3</sub>Sn<sub>2</sub>O<sub>7</sub> ceramic and 3.95 eV for Sr<sub>3</sub>Sn<sub>1.99</sub>Ge<sub>0.01</sub>O<sub>7</sub> ceramic. Using the Becke-Johnson potential combined with the local density approximation correlation, the bandgap is calculated and is found to be in close agreement with the experimental result. And the electronic excitations can be assigned to the charge transfer excitation from O 2p to Sn 5s (Ge 4s). The effects of Ge doping on the ability of Sr<sub>3</sub>Sn<sub>2</sub>O<sub>7</sub> to gain and lose electrons and the bonding strength of Sn-O bond are analyzed via two-dimensional charge density difference. In conclusion, this study provides insights into the synthesis method and modulation of ferroelectric properties of hybrid improper ferroelectrics Sr<sub>3</sub>Sn<sub>2</sub>O<sub>7</sub>, potentially facilitating their widespread applications in various capacitors and non-volatile memory devices.

Novel Piezoelectricity in Two-Dimensional Metallic/Semimetallic Materials with Out-of-Plane Polarization.

Normally, the good conductivity of metals and semimetals is incompatible with the piezoelectricity since the internal electric current will dismiss any polarization. However, here, we reveal that the out-of-plane piezoelectric effect can exist in two-dimensional (2D) metallic/semimetallic materials due to their giant anisotropy. A method is developed to calculate the out-of-plane polarization in 2D systems, where the modern theory of polarization based on a Berry-phase approach is not applicable. Detailed calculation and analysis on a Dirac material, the FeB2 monolayer, show that it has an out-of-plane polarization of 8.3 pC/m and the piezoelectric coefficient of e31 = -59.3 pC/m and d31 = -0.25 pm/V. This work provides a formalism to discover more piezoelectric materials within the vast 2D metallic/semimetallic materials.

Electronic, elastic and piezoelectric properties calculations of perovskites materials type BiXO3(X = Al, Sc): DFT and DFPT investigations

Throughout this work, the structural characteristics are analyzed. Electronic, thermal, and piezo-electric properties of two perovskite-type cubic crystalline structures are presented in BiAlO3 and BiScO3 using full potential-linear augmented plane wave (FP-LAPW) method basing on density function theory (DFT). To accurately describe the exchange and correlation potential, the generalized gradient approximation (GGA) and its corresponding GGA-PBE are employed. The characteristics of the actual states that are calculated are, to a large extent, in agreement with observational data. In this work, for several different types of perovskites, BiMO3 (M = Al, Sc) compounds have indirect-band perovskite. Additionally, the mechanical stability of the mentioned compounds is proven to be substantiated by their elastic characteristics. The various thermal properties by using Debye's quasi-harmonic model to the data collected at various temperatures and pressures are investigated. Finally, the Berry phase approach is used to derive the piezoelectric characteristics of Born and the actual charges.

Progressive Structural Complexity in Ferroelectric 1,2,4-Triazolium Hexabromoantimonate(III): Interplay of "Order-Disorder" and "Displacive" Contributions to the Structural Phase Transitions.

Halobismuthates(III) and haloantimonates(III) with the R3MX6 chemical composition create a new and broadly unexplored class of ferroelectric compounds. In this paper, we report the haloantimonate(III) ferroelectric comprising an aromatic (1,2,4-triazolium) cation, i.e., (C2N3H4)3[SbBr6] (TBA). Temperature-resolved structural and spectroscopic studies indicate that TBA undergoes two solid-solid phase transitions between tetragonal [P42/m (I)] and monoclinic [P21/n (II) and P21 (III)] phases. TBA experiences a paraelectric-ferroelectric phase transition at 271/268 K (II-III) driven by "order-disorder" and "displacive" molecular mechanisms. The ferroelectric properties of phase III have been confirmed by hysteresis loop measurement, and additionally, the acentric order has been further supported by second-harmonic generation measurements. Insight into the molecular origins of the ferroelectric polarization was provided by periodic ab initio calculations using the Berry phase approach at the density functional theory (DFT-D3) method level employed for calculations of spontaneous polarization.

Floquet formulation of the dynamical Berry-phase approach to nonlinear optics in extended systems

We present a Floquet scheme for the ab-initio calculation of nonlinear optical properties in extended systems. This entails a reformulation of the real-time approach based on the dynamical Berry-phase polarisation (Attaccalite and Grüning 2013 Phys. Rev. B 88 1–9) and retains the advantage of being non-perturbative in the electric field. The proposed method applies to periodically-driven Hamiltonians and makes use of this symmetry to turn a time-dependent problem into a self-consistent time-independent eigenvalue problem. We implemented this Floquet scheme at the independent particle level and compared it with the real-time approach. Our reformulation reproduces real-time-calculated 2nd and 3rd order susceptibilities for a number of bulk and two-dimensional materials, while reducing the associated computational cost by one or two orders of magnitude.

Electronic and elastic properties of BN, AlN and GaN

We have carried out first-principles total energy calculations to investigate electronic and elastic properties of both zinc-blende and wurtzite BN, AlN, and GaN. We have calculated lattice parameters, elastic constants, deformation potential constants, phonon frequencies at r point, Born effective charges, and piezoelectric constants. Lattice parameters are fully relaxed by using the first-principles molecular dynamics method with variable cell shape. The internal strain in a strained crystal is also relaxed by the first-principles molecular dynamics method. The internal strain influences the elastic constants, the deformation potential constants, and the piezoelectric constants effectively. We have calculated the wurtzite deformation potential constants D1 – D5 considering the internal strain correction. The piezoelectric constants of wurtzite and also zinc-blende crystals have been calculated using the Berry phase approach and we have found from first principles that those of BN have an inverse sign in contrast to AlN and GaN. Discussions will be given in comparison with results obtained herein with the previous ones. © 1998 American Institute of Physics. [S0021-8979(98)07921-3]

Electronic and elastic properties of BN, AlN and GaN

We have carried out first-principles total energy calculations to investigate electronic and elastic properties of both zinc-blende and wurtzite BN, AlN, and GaN. We have calculated lattice parameters, elastic constants, deformation potential constants, phonon frequencies at r point, Born effective charges, and piezoelectric constants. Lattice parameters are fully relaxed by using the first-principles molecular dynamics method with variable cell shape. The internal strain in a strained crystal is also relaxed by the first-principles molecular dynamics method. The internal strain influences the elastic constants, the deformation potential constants, and the piezoelectric constants effectively. We have calculated the wurtzite deformation potential constants D1 – D5 considering the internal strain correction. The piezoelectric constants of wurtzite and also zinc-blende crystals have been calculated using the Berry phase approach and we have found from first principles that those of BN have an inverse sign in contrast to AlN and GaN. Discussions will be given in comparison with results obtained herein with the previous ones. © 1998 American Institute of Physics. [S0021-8979(98)07921-3]

Controllable second harmonic generation based on topological spin-dependent edge states

Spin-dependent nonlinear harmonic generations from nanostructures may provide a new class of design for future applications such as nonlinear optical switching and modulation and nonlinear information processing. Most reported works are based on the utilization of metasurfaces with specific symmetry (such as C3 and C4) and the application of the Pancharatnam–Berry phase approach, making them sensitive to local defects. In this work, we investigated a controllable second harmonic generation (SHG) based on the topological spin-dependent edge state, which is robust to a broad class of perturbations. Simulation results demonstrated that spin-dependent edge states with strong field localization can be obtained through spin–orbit coupling, resulting in SHG with increased efficiency. In addition, the propagation direction of the enhanced SHG could be dynamically controlled using an excitation source with left/right circular polarizations, exhibiting negligible backscattering and topological protection against local defects. The proposed approach may open a new avenue for manipulating light–light interactions.

Electric-field-induced annihilation of localized gap defect states in amorphous phase-change memory materials

Structural relaxation of amorphous phase-change-memory materials has been attributed to defect-state annihilation from the band gap, leading to a time-dependent drift in the electrical resistance, which hinders the development of multi-level memory devices with increased data-storage density. In this computational study, homogeneous electric fields have been applied, by utilizing a Berry-phase approach with hybrid-density-functional-theory simulations, to ascertain their effect on the atomic and electronic structures associated with the mid-gap states in models of the prototypical glassy phase-change material, Ge2Sb2Te5. Above a threshold value, electric fields remove spatially localized defects from the band gap and transform them into delocalized conduction-band-edge electronic states. A lowering of the nearest-neighbor coordination of Ge atoms in the local environment of the defect-host motif is observed, accompanied by a breaking of 4-fold rings. This engineered structural relaxation, through electric-field tuning of electronic and geometric properties in the amorphous phase, paves the way to the design of optimized glasses.

0D Bismuth(III)-Based Hybrid Ferroelectric: Tris(acetamidinium) Hexabromobismuthate(III)

Ferroelectric properties of haloantimonates(III) and halobismuthates(III) have been detected for as much as 40 structures belonging to 7 different types of anionic networks, with RMX4, R2MX5, R3M2X9, and R5M2X11 stoichiometries being the most frequently reported to host these properties. We report on the first ferroelectric of the halobismuthate(III) family with a R3MX6 stoichiometry, that is, tris(acetamidinium)hexabromobismuthate(III), (CH3C(NH2)2)3[BiBr6] (ABB), characterized by a one-component organic network. While the stoichiometry and crystal packing of ABB might seem uncomplicated, the temperature-resolved structural and spectroscopic studies paint a different picture in which rich polymorphism in the solid state occurs between tetragonal (paraelastic) and triclinic (ferroelastic) crystal phases: I (P42/n) → II (P1̅) at 272/277 K (cooling/heating), II (P1̅) → III (P1̅) at 207 K, and III (P1̅) → IV (P1) at 98/127 K. The ferroelectric properties of phase IV have been confirmed by the pyroelectric current and hysteresis loop measurements; additionally, the acentric symmetry has been further supported by second harmonic generation measurements. Crystallographic analysis of phase III reveals the antiparallel alignment of acetamidinium dipoles, pointing to the antiferrroelectric nature of this phase. In turn, the character of the ferroelectric transition (III → IV) should be considered as “displacive” for both cationic and anionic substructures.) In this report, we also explore the two-photon absorption property of ABB at 800 nm, a property that is unexplored for any halobismuthate(III) thus far. We also present periodic ab initio calculations for ABB crystals. The Berry-phase approach at the Hartree–Fock and density functional theory (DFT-D3) method levels is employed for spontaneous polarization calculations. The origin of ferroelectric polarization is studied using DFT-D3 and RHF electronic structure calculations, emphasizing the relationship between Ps and the relative orientation of organic/inorganic components.

WloopPHI: A tool for ab initio characterization of Weyl semimetals

WloopPHI is a Python code that expands the features of WIEN2k, a full-potential all-electron density functional theory package, by the characterization of Weyl semimetals. It enables the calculation of the chirality (or “monopole charge”) associated with Weyl nodes and nodal lines. The theoretical methodology for the calculation of the chirality is based on an extended Wilson loop method and a Berry phase approach. We validate the code using TaAs, which is a well-characterized Weyl semimetal, both theoretically and experimentally. Afterwards, we applied the method to the characterization of YRh6Ge4 and found two sets of Weyl points (ca. 0.2 eV below the Fermi energy) together with a topological nodal line (protected by mirror symmetry) crossing the Fermi energy and mapped their chiralities.

Theoretical and Experimental Overview of Structural, Dielectric, Crystallographic, Electronic, Optical, and Physical Tensors of α-DIPAB and Iodine-Doped α-DIPAB Molecular Ferroelectric Crystals

Self-consistent ab initio calculations of the structural, crystallographic, electronic, optical, and physical properties as well as spontaneous polarization of P21 (α)-ferroelectric phase of diisopropylammonium bromide (α-DIPAB) molecular crystal are carried out. Examination of the total density of states as obtained using the generalized gradient approximation and hybrid exchange–correlation functional (HSE06) methods yields an optical bandgap of ∼ 5 eV and 6 eV, respectively. Furthermore, by using the Berry phase approach, we found that polar DIPAB exhibits a spontaneous polarization of 22.64 μC/cm2, indicating that this remarkable ferroelectric molecule could be used as an alternative to replace several known ferroelectrics for piezoelectric and optoelectronic applications such as capacitors, sensors, and data storage as well as microelectromechanical system (MEMS) devices. Using the vdW + DF2 approximation, we identify the majority of the vibrational modes in the Raman spectra and analyze the modes generated by the bromine (Br) deficiency. We find that the Br deficiency strongly affects the electric and elastic properties of α-DIPAB. Furthermore, the elastic, dielectric, and piezoelectric tensors of α-DIPAB are calculated and interpreted. α-DIPAB is found to exhibit a static dielectric tensor of ∼ 2.5, i.e., slightly smaller than that of typical perovskite-based ferroelectrics. In addition, the clamped-ion piezoelectric tensor is calculated. We found that the piezoelectric coefficient e21 corresponding to applying uniaxial strain in the direction perpendicular to the N–C bonds is smaller than the e22 principle coefficient. Moreover, the components e15 = 0.220 C/m2 and e35 = −0.2032 C/m2 correspond to applying shear strain η31, indicating a reasonable piezoelectric response of this polar crystal and making it a cheap attractive candidate for piezoelectric applications. The components of the elastic moduli tensor are calculated and interpreted. The α-DIPAB exhibits Young’s modulus of up to 50 GPa along specific crystalline directions despite its relatively low hardness. We attribute such a large value to the presence of a dense H–Br bond network between the Br and DIPAB molecule. The Poisson ratio is also strongly anisotropic, with values ranging from 0.5 to 0.2. The DIPAB systems are brittle based on the ratio between the bulk and shear elastic constants, suggesting that this material has potential for use in flexible electronic applications. A real-part dielectric anomaly at T = 140 K induced by iodine doping of α-DIPAB exhibits switchable behavior. This abnormality can be attributed to suppression of ferroelectricity induced by trapping of vortices at dislocation defects generated by iodine. Another dielectric anomaly found in the vicinity of the Curie temperature of α-DIPAB can be explained in terms of a structural phase-induced ferroelectric-to-paraelectric transition.

First principle study of band gap nature, spontaneous polarization, hyperfine field and electric field gradient of desirable multiferroic bismuth ferrite (BiFeO3)

Electronic band structure, spontaneous polarization, hyperfine fields and electric field gradients (EFG) of the desirable multiferroic BiFeO3 (BFO) are explored in R3c space group, using density functional theory. Improved TB-mBJ approach resolves the long-standing inconsistency between the theoretical and experimental band gap nature and value of BFO. In this study the calculated direct band gap is 2.84 eV, which was reported indirect band structures in other theoretical studies. The ferroelectric spontaneous polarization 88 μC/cm2 by PBE-GGA and 81 μC/cm2 by TB-mBJ were obtained using the Berry phase approach. Furthermore, the hyperfine interaction and EFG of the compound are also investigated.

First-Principles Calculation of Physical Tensors of α-Diisopropylammonium Bromide (α-DIPAB) Molecular Ferroelectric Crystal

We report accurate calculations of tonsorial elements of α-Diisopropylammonium bromide (α-DIPAB) molecular ferroelectric crystal. In particular, elastic, piezoelectric and dielectric tensors were calculated using Vienna ab initio simulation package (VASP) within the framework of density functional (DFT) theory. The determination of above parameters allows an accurate description of the energy landscape for modeling of realistic devices at finite temperatures. We determine the major physical tensors in energy expansion of total energy per volume of un-deformed crystal to provide experimentalists with valuable information for designing and fabrication of pyroelectric detectors, capacitors, piezoelectric ferroelectric memories, display devices, electro-optic devices, and ultrasound transducers based on α-DIPAB. The spontaneous polarization P_s was calculated using Berry phase approach and found to be 22.64 μC⁄〖cm〗^2 in agreement with reported theoretical value. The dynamical Born effective charge tensor is reported for all atoms in the unit cell of α-DIPAB to get a deeper insight into the bonding network. The neighboring layers of DIPA molecules were found to be strongly crenelated due to the strong short-ranged electrostatic repulsion between Br sites in the DIPAB crystal structure. The organization of species in DIPA molecular layer as well as in the bromine “stitching” layer is essential for accurate calculation of DIPAB elastic properties. Having understood the actual network bonding in α-DIPAB, we calculated the components of the elastic moduli tensor. Our results indicate that a Young’s modulus of 10-180 GPa and a shear modulus of 4-26 GPa were found. Thus, α-DIPAB phase has a great potential to be a terrific candidate for flexible electronic device applications. The value of the principle component of electronic contribution to the static dielectric tensor of α-DIPAB is found to be ≈ 2.5, i.e. 50% smaller than that in typical perovskite-based ferroelectrics. Therefore, α-DIPAB is anticipated to exhibit creative materials' innovations. It offers highly customized features for specific applications, including insulation on polymer thick film conductive inks in the manufacture of membrane switches and flex circuits. Furthermore, clamped-ion piezoelectric tensor is calculated. Our results indicate a reasonable piezoelectric response of this polar crystal making it a low cost attractive candidate for piezoelectric applications.

First-principles study of the coexisting ferroelectric and ferromagnetic properties of the La0.75Bi0.25CrO3 compound

Despite the interesting properties of La1-xBixCrO3, the origin of its multiferroic properties has not been yet established by first-principles calculations. Toward this effect, in this work we present a comparative study between the structural, electronic, magnetic and ferroelectric properties of pristine LaCrO3 and La0.75Bi0.25CrO3 using density functional theory (DFT) plus the Hubbard potential (U) formalism and the Berry phase approach for the polarization. Additionally, we present a comparative analysis between reported experimental results and the theoretical calculations of this work, from which we have determined that LaCrO3 is stable under a G-type antiferromagnetic configuration, while the La0.75Bi0.25CrO3 is stable in a ferromagnetic one. This is due to the decrease of the Cr-O-Cr angle which is strongly related to the high degree of covalence of the spin-up Cr-t2g and O-2p orbitals in the Cr-O bonds. On the other hand, La0.75Bi0.25CrO3 turns out to be ferroelectric due to the structural distortion induced by the lone pair of Bi in site A, which predicts a spontaneous polarization of 30.41 μC/cm2 in the [011] direction. That is, first-principles calculations satisfactory explain the origin of ferromagnetism and ferroelectricity of the La1−xBixCrO3 compound.

Hybrid improper ferroelectricity and possible ferroelectric switching paths in Sr3Hf2O7

Hybrid improper ferroelectricity, which is induced by the coupling of oxygen octahedral rotation and tilt in oxide materials, has great potential to create single-phase multiferroic materials with intrinsic electric field-controlled magnetism. Based on previous experiments with bi-layered Ruddlesden-Popper Sr-based materials, hybrid improper ferroelectricity in Sr3Hf2O7 was investigated by first-principle calculations in the present work. The results show that the ferroelectric A21am phase has the lowest energy among all possible phases, indicating that it is the ground state. The spontaneous polarization calculated by the Berry phase approach and Born effective charge model is approximately 4.6 μC/cm2, in agreement with those reported for other Sr-based compounds. Compared to Ca3Ti2O7, the smaller polarization of Sr3Hf2O7 is attributed to the negative contribution of the HfO2 layer to global polarization. The possible ferroelectric switching path was determined using the nudged elastic band method. The energy barriers of the two four-step paths across the non-polar Pbnm and Pnab phases are almost the same, and they are the lowest energies among all possible paths. Competition between these paths can help us understand complex phase transitions in analogous compounds. In summary, hybrid improper ferroelectricity was predicted in Sr3Hf2O7 for the first time, and the possible ferroelectric switching paths were determined.

First Principle Study of the Structural and Piezoelectric Properties of Perovskite ZrTi(PbO<sub>3</sub>)<sub>2</sub> Materials for Naval SONAR

First principle calculation within the Density Functional Theory (DFT) and Density Functional Perturbation Theory (DFPT) using Local Density Approximation as implemented in Quantum ESPRESSO has been significantly used to investigate the structural and Piezoelectric, properties of Perovskite ZrTi(PbO3)2. From structural properties calculation, the ground state total energy of -2417.12 eV has been obtained which led to an equilibrium lattice constant of a= 5.620Å for ZrTi(PbO3)2. Our obtained optimized atomic positions and atomic effective charge shows that the optimized ZrTi(PbO3)2 is stable and the Piezoelectric stress tensor is calculated using Berry-phase approach within density functional perturbation theory (DFPT). From our calculation, we have obtained the stress tensor elements with values of d1,5 = 6.81, d3,1 = 1.69, and d3,3 = 6.18, which is in agreement with the values obtained for tetragonal PbTiO3.

Magnetic and transport properties of equiatomic quaternary Heusler CoFeVSi epitaxial films

We study magnetic and transport properties of an equiatomic quaternary CoFeVSi Heusler alloy. By using a molecular beam epitaxy technique at $400{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$, CoFeVSi epitaxial films with stoichiometry on MgO substrate have been grown. The saturation magnetic moment at 10 K and Curie temperature are $\ensuremath{\sim}2.1\phantom{\rule{4pt}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}$/f.u. and $\ensuremath{\sim}260$ K, respectively. From electrical and magnetotransport measurements, we find the small temperature dependence of the electrical resistivity, temperature-dependent ambipolar carrier transport, and positive linear magnetoresistance at low temperatures. An anomalous Hall conductivity of $\ensuremath{\sim}67$ S/cm at 10 K is slightly deviated from the calculated value from a Berry phase approach. On the basis of the experimental data and first-principles calculations, the origin of the peculiar magnetotransport properties of the CoFeVSi film is discussed.