Theoretical Investigation of Crystal and Electronic Structure of piezoelectric AgNb0.5Ta0.5O3

We present an ab-initio study of structural and electronic properties of lead-free oxide AgNb0.5Ta0.5O3 (ANTO) in orthorhombic Pbcm space group and monoclinic p2/m space group. In our calculations, both the symmetries (orthorhombic Pbcm and monoclinic p2/m) of AgNb0.5Ta0.5O3 show small relative energy difference ( ~ 0.6062 eV), thus suggesting the coexistence of both the unit cells at the room temperature. Band Structure of ANTO infers that the compound is an insulator or semiconductor with direct band gap. Conduction band of ANTO, consists of Nb-4d and Ta-5d and upper level of valence band is contributed by Ag-4d and O-2p while lower level is contributed by Nb-4d and Ta-5d along with O-2p. Charge density of ANTO is uniformly spherical around Ag deducing an ionic bond between Ag-O and non uniform spherical charge density around Ta/Nb-O suggests an ionic covalent bond due to high electronegativity of O. Large phonon frequency belongs to O atom and the phonon frequency ranging in lower frequency range corresponds to Ag-O and Nb-o/Ta-O vibrations, respectively

Y3Fe0.5SiSe7: A new cation-deficient quaternary mixed transition metal chalcogenide with extremely low thermal conductivity

Single crystals of a new selenide, Y3Fe0.5SiSe7, have been synthesized by a reaction of elements at high temperatures inside a vacuum-sealed fused silica tube. The title phase crystallizes in the hexagonal noncentrosymmetric P63 space group (Pearson symbol hP23) at room temperature (RT), as established by a single-crystal X-ray diffraction study. The structure is non-stoichiometric with refined lattice parameters of a = b = 10.1971(1) Å, c = 5.9799(2) Å, and V = 538.49(2) Å3 (Z = 2). The asymmetric unit of the structure consists of six crystallographically independent atomic positions: one Y, one half-occupied Fe, one Si, and three Se sites. The building blocks of the structure are a distorted FeSe6 octahedron, YSe6 trigonal prism, and SiSe4 tetrahedron that are fused together via sharing of anions to create a three-dimensional periodic network. The charge-balanced formula of the structure can be achieved as (Y3+)3(Fe2+)0.5(Si4+)1(Se2−)7. A polycrystalline sample of Y3Fe0.5SiSe7 was also synthesized by the high-temperature reactions of stoichiometric amounts of elements. A direct bandgap value of 1.1(1) eV for the polycrystalline sample was estimated by a solid-state UV–Vis–NIR absorption spectroscopic study. A resistivity study on the polycrystalline Y3Fe0.5SiSe7 sample corroborates its semiconducting nature. The polycrystalline sample also shows an extremely low thermal conductivity value of ∼0.26 Wm−1K−1 at 773 K. The ab-initio optical and thermoelectric parameters of the title structure have been estimated from the electronic band structure. The relative bond strengths of Y, Si, and Fe atoms with Se atoms are investigated by the COHP study

Study of Structural and Bulk Properties of Tisi2 Compound

In recent years, considerable efforts have been made to study the structural and microscopic properties of transition metal silicides. Silicides are compounds of silicon with more electropositive elements. Silicon is more electropositive than carbon. They are structurally closer to borides (a chemical compound between boron and a less electronegative element) than to carbides (a compound composed of carbon and a less electronegative element). Similar to borides and carbides, the composition of silicides cannot be easily specified as covalent molecules. The chemical bonds in them range from conductive metal-like structures to covalent or ionic

Ferroelectric superlattices at nano scale: A Theoretical Investigation

In ferroelctrics, material size scales play a vital role in deciding its properties. Modeling of nano scale ferroelectric superlattices require different approach than bulk sized ferroelctrics. At smaller scales, thickness becomes a important parameter that can cause variation in critical value of properties of material and to study them choice of appropriate order parameter becomes crucial. This report mainly focuses on approach using spontaneous polarization as order parameter. Variation of Polarization, susceptibility and transition critical temperature with relative thickness of layers of ferroelectric superlattice is shown along comparison with different approach such as total polarization as order parameter, first principle calculation and experimental data using pre-existing curves from references.The most commonly observed anomalous phenomenon of ferroelectric superlattices is that the remanent polarization is much larger than their single-phase thin films.In ferroelctrics, material size scales play a vital role in deciding its properties. Modeling of nano scale ferroelectric superlattices require different approach than bulk sized ferroelctrics. At smaller scales, thickness becomes a important parameter that can cause variation in critical value of properties of material and to study them choice of appropriate order parameter becomes crucial. This report mainly focuses on approach using spontaneous polarization as order parameter. Variation of Polarization, susceptibility and transition critical temperature with relative thickness of layers of ferroelectric superlattice is shown along comparison with different approach such as total polarization as order parameter, first principle calculation and experimental data using pre-existing curves from references.The most commonly observed anomalous phenomenon of ferroelectric superlattices is that the remanent polarization is much larger than their single-phase thin films. Going at very small scale of ferroelectric materials along with superlattice geomatry, one can see very interesting feature of it. In this type of structure, small variation in thickness of layers enhances dielectric constant and polarization of the sample. Going at very small scale of ferroelectric materials along with superlattice geomatry, one can see very interesting feature of it. In this type of structure, small variation in thickness of layers enhances dielectric constant and polarization of the sample

Randomly arranged cation-ordered nanoregions in lead-free relaxor ferroelectric K1/2Bi1/2TiO3: Prediction from first-principles study

First-principles density functional calculations are performed to investigate the lattice dynamics, Infrared reflectivity, and Raman intensity spectra of a lead-free ferroelectric K1/2Bi1/2TiO3 system. In particular, the A-site cation ordering in K1/2Bi1/2TiO3 and its effects on lattice dynamics and the Raman spectrum are explored. The results suggest that the cation ordering at the A-site in K1/2Bi1/2TiO3 significantly influences its Raman spectra. From the analysis of theoretical and experimental Raman spectra, it is suggested that randomly arranged cation ordered nanoregions with different A-site orderings are formed in K1/2Bi1/2TiO3 samples. The random arrangement is favored by entropy contributions to free energy and may explain the lack of observed long-range A-site cation ordering in K1/2Bi1/2TiO3. Further, it is suggested that partial A-site cation ordering may also occur in K1/2Bi1/2TiO3 favored by kinetic factors during sample preparation. The Born effective charges of K and Bi ions at the A-site are computed and found to be significantly disparate, thereby suggesting hetero-polar activity at the A-site in K1/2Bi1/2TiO3. The formation of A-site hetero-polar cation ordered nanoregions and their random or/and partially ordered arrangement in K1/2Bi1/2TiO3 may play an important role in the determination of its relaxor properties apart from the dominant role played by polar nanoregions. The computed Infrared reflectivity and Raman intensity spectra are expected to provide benchmark first-principles results for further analysis of experimental spectra and results

Enhancement of magnetic and electrical properties in Sc substituted BiFeO3 multiferroic

Polycrystalline BiFe1-xScxO3 (x=0, 0.05, 0.1 and 0.15) compounds are prepared using solid state reaction. The XRD patterns show that all compounds are crystallized in rhombohedral structure with R3c space group. An induced weak ferromagnetism in Sc substituted BiFeO 3 due to suppression of spiral modulated spin structure is revealed. In addition, a spin glass like behaviour is observed from the zero field cooled (ZFC) and field cooled (FC) magnetization curves in the low temperature region. Further, the coupling between the ferroelectric and (anti) ferromagnetic orders is evident from the appearance of anomaly in the dielectric data near the magnetic Néel temperature (373 °C). The reduction of oxygen vacancies due to Sc substitution is evident from the ac conductivity data and the suppressed anomaly in dielectric data at 220 °C. The temperature dependence of ac conductivity is consistent with correlated barrier hopping (CBH) model. The temperature dependent ac conductivity and activation energies indicate that electronic conduction, oxygen vacancies movement and creation of defects are the prime contributors to the ac conductivity in measured temperature regions. The improved magnetic and electrical properties due to the structural modification are prominent for novel device applications

Prediction of a Switchable Two-Dimensional Electron Gas at Ferroelectric Oxide Interfaces

The demonstration of a quasi-two-dimensional electron gas (2DEG) in LaAlO3=SrTiO3 heterostructures has stimulated intense research activity in recent years. The 2DEG has unique properties that are promising for applications in all-oxide electronic devices. For such applications it is desirable to have the ability to control 2DEG properties by external stimulus. Here, based on first-principles calculations we predict that all-oxide heterostructures incorporating ferroelectric constituents, such as KNbO3=ATiO3 (A = Sr, Ba, Pb), allow creating a 2DEG switchable between two conduction states by ferroelectric polarization reversal. The effect occurs due to the screening charge at the interface that counteracts the depolarizing electric field and depends on polarization orientation. The proposed concept of ferroelectrically controlled interface conductivity offers the possibility to design novel electronic devices

Dielectric Studies of Agta0.5Nb0.5O3

The complex perovskite oxide AgNb0.5Ta0.5O3 (ATN) synthesized by solid state reaction techinique has single phase with monoclinic structure.The scanning electron micro graph of the sample shows that grains and grain boundary are there but grain size is not definable from there. The field dependence dielectric response and loss tangent were measured in the temperature range from 30o C to 430 o C and in the frequency range from 100Hz to 1MHz. An analysis of the dielectric constant (ϵ’) and loss tangent (ta

Aliovalent cation ordering, coexisting ferroelectric structures, and electric field induced phase transformation in lead-free ferroelectric Na0.5Bi0.5TiO3

Using first-principles calculations, we show that a specific chemical ordering of Na and Bi in Na0.5Bi0.5TiO3 is responsible for the co-existence of its ferroelectric phases with rhombohedral R3c and monoclinic Cc structures, which are relevant to its morphotropic phase boundary and large piezoelectric response. We identify the signatures of chemical ordering in the calculated phonon spectra and establish the prevalence of A-type ordering through comparison with experiment. We uncover a mechanism of the observed electric field induced Cc to lower energy R3c structural transformation promoted by a hybrid soft mode involving a combination of TiO6 rotations and a polar component