A density functional theory study of the structural,electronic, and optical properties of XGaO3 (X = V,Nb) perovskites for optoelectronic applications

An ab initio study using density functional theory (DFT) is carried out to explore the structural, electronic, and optical properties of vanadium gallate (VGaO₃) and niobium gallate (NbGaO₃). The structural properties of these compounds are determined by using the full-potential linearized augmented plane wave (FP-LAPW) technique as implemented in WIEN2k with a standard functional, i.e., the Perdew–Burke–Ernzerhof generalized gradient approximation (PBE-GGA). In addition, the local density approximation plus Hubbard parameter (LDA?+?U) is employed to calculate the electronic bandgap and total and partial density of states (TDOS and PDOS), to overcome the limitation of the PBE-GGA functional in terms of underestimation of the electronic bandgap. The values computed for the indirect bandgap of VGaO₃ and NbGaO₃ are 0.45 and 0.51 eV, respectively, indicating that both materials are semiconductors in nature. The PDOS of the studied materials reveal that 3d-states of vanadium atoms, 4d-states of niobium atoms, and 2p-states of oxygen atoms form the valence band. Moreover, the Kramer–Kronig relations are used to compute the optical properties of the title compounds. The dielectric functions, refractive index, optical conductivity, absorption coefficient, extinction coefficient, energy loss function, and reflectivity of these materials are also computed. The results for the studied properties reveal that NbGaO₃ exhibits better properties than VGaO₃ for use in optoelectronic applications.

     

Effect of Ba2+ – Sr2+ co-substitution on the structural and dielectric properties of Lead Titanate

Growing interest in developing new materials for device applications led to study of ferroelectric oxides in a wide range and variety of compositions. In the present work, polycrystalline samples of lead barium strontium titanate (Pb_1-xBa_0.5xSr_0.5xTiO₃) solid solution system have been synthesized. Phase formation studies and crystal structure analysis were carried out by X ray diffractometry at room temperature, which suggested formation of single phase compound with tetragonal structure up to x?=?0.8 and cubic structure for x?=?1.0. The XRD pattern has been analyzed by employing Rietveld method. The phase transition in the system was confirmed by Differential Scanning Calorimetry (DSC). Samples with 0.0????x????0.8 are in ferroelectric state whereas with x?=?1.0 is found to be in paraelectric state at room temperature. Co-substitution of Ba²⁺ and Sr²⁺ into lead titanate shows reduction in anisotropy as well as porosity. The dielectric studies of the system as a function of temperature and frequency were carried out in the range 323?K to 773?K and 100?Hz to 1?MHz respectively. Variation of dielectric constant and loss tangent with temperature shows peaking effect near Curie temperature. Frequency dependant dielectric studies clearly show that the dielectric constant and loss tangent decrease exponentially with increased frequency.     

Structural and electrical properties of SrBi2VxNb2?xO9 ferroelectric ceramics: Effect of temperature and frequency

Aurivillius type bismuth layered materials have received a lot of attention because of their application in ferroelectric non-volatile random access memories. Among bismuth layer structured ferroelectric ceramics SrBi₂Ta₂O₉ (SBT)/SrBi₂Nb₂O₉ (SBN) are of great interest for researchers because of their fatigue resistance and less distorted structure. Recently vanadium substitution in SBN/SBT has shown interesting electric and dielectric properties. In the present work, processing conditions, microstructure and electrical studies of vanadium doped SBN ferroelectric ceramics have been performed. Samples of compositions SrBi₂V _x Nb_2-x O₉, x = 0.0, 0.1, 0.3, 0.5 were prepared by solid-state reaction technique using high purity oxides / carbonates. The samples were calcined at 700 °C and sintered at 800 °C. X-ray diffractograms show that a single phase layered perovskite structure is formed in all the samples. Effect of partial substitution of pentavalent niobium ion (0.68 ?) by smaller pentavalent vanadium ion (0.59 ?) at B site on the microstructure, Curie temperature, Dielectric constant, Dielectric loss and electrical conductivity have been investigated. Dielectric properties of SBVN have been investigated from room temperature to 500 °C and frequency of 100 Hz to 1 MHz. Dielectric constant values at their respective Curie points are observed to increase with increasing vanadium concentration. Curie temperature is observed to be maximum in x = 0.1 vanadium doped sample. Strong relaxor like dielectric relaxation at the transition temperatures have been observed. With increasing vanadium concentration the dielectric loss is observed to increase significantly. It is also observed that dielectric loss increases with increase in temperature. The variation of conductivities in these samples is also reported.     

High temperature electronic properties of BaTiO3 – Bi(Zn1/2Ti1/2)O3 – BiInO3 for capacitor applications

Solid solutions xBaTiO₃ – (1-x)(0.5Bi(Zn_1/2Ti_1/2)O₃ – 0.5BiInO₃), where x?=?0.95–0.60, were prepared by conventional mixed oxide method. The single phase perovskite structure was obtained for the composition with x?≥?0.75. Phase transformation from tetragonal to pseudocubic was observed from x-ray diffraction patterns when x decreased from 0.95 to 0.75. In tetragonal phase region, x?≥?0.90, the increase of Bi(Zn_1/2Ti_1/2)O₃ – BiInO₃ content decreased the tetragonality and the temperature at which the relative permittivity is maximum (T_max). The increase in lattice parameter and T_max were observed in the pseudocubic phase region, x?<?0.90. Additionally, a highly broad and diffuse phase transition was observed from the dielectric data in the pseudocubic phase region. The introduction of Ba vacancies in compositions with x?=?0.80 and 0.75 also improved dielectric loss at high temperatures. The incorporation of BiInO₃ into the BaTiO₃ – Bi(Zn_1/2Ti_1/2)O₃ compound was also found to improve the temperature coefficient of the relative permittivity, with values as low as approximately ?1,000?ppm/K. Overall, ternary perovskite solid solutions based on adding Bi(Zn_1/2Ti_1/2)O₃ – BiInO₃ to BaTiO₃ shows excellent potential for high temperature capacitor applications     

Electronic structure,elastic and magnetic properties of the binary intermetallics RFe2 (R = Eu,Gd and Tb)

Journal of Computational Electronics - Laves-phase (C15) binary intermetallics RFe2 (R?=?Eu, Gd and Tb) are studied using various potentials in the domain of density functional theory...     

Theoretical study of structural,electronic and optical properties of novel ternary alloys $$\hbox {MgO}_{1-{x}}{\hbox {Se}}_{{x}}$$ ($$x =$$x= 0.25, 0.50 and 0.75)

We report on the investigation of the structural, electronic, and optical properties of binary compounds (MgO and MgSe) and their ternary \(\hbox {MgO}_{1-{x}}\hbox {Se}_{{x}}\) (\(x=0.25, 0.5, 0.75\)) alloys within the density functional theory based on the full-potential linearized augmented plane wave method as implemented in the WIEN2k code. We have used the revised Perdew–Burke–Ernzerhof generalized gradient approximation (GGA-PBEsol) to calculate the structural properties and analyze the effect of the Se composition on the lattice constant and the bulk modulus of \(\hbox {MgO}_{1-{x}}\hbox {Se}_{{x}}\). The calculated electronic properties by employing the GGA-PBEsol and TB-mBJ approaches show that \(\hbox {MgO}_{1-{x}}\hbox {Se}_{{x}}\) alloys have a direct band gap \(\Gamma \)–\(\Gamma \) for \(x = 0, 0.25, 0.5\) and 0.75, suggesting the possibility of their use in the long wavelength optoelectronic applications. The optical properties such as the real and imaginary parts of the dielectric function, the refractive index, and the reflectivity of \(\hbox {MgO}_{1-{x}}\hbox {Se}_{{x}}\) are computed by using the accurate TB-mBJ potential. The wide band gaps larger than 3.1 eV mean that \(\hbox {MgO}_{1-{x}}\hbox {Se}_{{x}}\) alloys can be used in the applications of the ultraviolet region of the spectrum. Our data for all studied bowing parameters of \(\hbox {MgO}_{1-{x}}\hbox {Se}_{x}\) may serve as references for future experimental studies.     

Ab-initio study of the structural,optoelectronic, magnetic,hydrogen storage properties and mechanical behavior of novel combinations of hydride perovskites LiXH3 (X = Cr,Fe, Co, & Zn) for hydrogen storage applications

Journal of Computational Electronics - LiXH3 (X?=?Cr, Fe, Co, &amp; Zn) hydride type perovskites have been studied by applying density functional theory (DFT), and their structural,...     

Synthesis, electrical and thermal properties of Bi4V2−xZrxO11 (x = 0.0, 0.02, 0.06 and 0.10) ceramics

Polycrystalline ceramic samples of Bi₄V_2?xZr_xO₁₁ (x?=?0.0, 0.02, 0.06 and 0.10) have been synthesized by standard solid state reaction method using high purity oxides. The formation of the compounds have been checked by X-ray diffraction method. The dielectric constant, dielectric loss and ac conductivity as a function of frequency and temperature have been measured. The dielectric studies indicate that the material is highly lossy and hence its ac conductivity increases with the increase of temperature. The dc conductivity of all the materials has been measured as a function of temperature from room temperature to 673 K and its activation energy was calculated using the relation σ?=?σ_oexp (?Ea/kT). The Modulated Differential Scanning Calorimetry (MDSC) has been used to investigate the effect of substitution on the heat capacity and heat flow of the compounds. The results are discussed in detail.     

Structural evolution and microwave dielectric properties of (1 − x)BaZn1/3Nb2/3O3 + xBaMg1/2W1/2O3 ceramics

Structural evolution and microwave dielectric properties of (1 − x)BaZn_1/3Nb_2/3O₃ + xBaMg_1/2W_1/2O₃ (0 ≤ x ≤ 1) system have been investigated in this work. All samples exhibit single perovskite phase except for the samples with x = 0 and x ≥ 0.8 in which barium niobate and BaWO₄ second phase existed, respectively. 1:1 cations ordering existed in the samples with x ≥ 0.1, and the ordering degree increases with the increase of x. Liquid phase sintering was observed in the sample with x ≥ 0.8. Dielectric constant decreases almost linearly from 40.8 to 17.4 with increasing x. Q × f value monotonically increases from 26,162 GHz to 64,705 GHz with increasing x. The τ_f value changes from +30 ppm/°C to −27.8ppm/°C. Near zero τ_f value of −1.4 ppm/°C could be obtained at x = 0.4 composition.     

Synthesis,and measurement of structural and magnetic properties,of La<Subscript>1−x</Subscript>Cd<Subscript>x</Subscript>CoO<Subscript>3</Subscript> perovskite ceramic oxides

Perovskite-type Cd-doped LaCoO₃ materials were synthesized by a simple solution-based combustion process. The synthesized materials were characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), diffuse reflectance (DR) in the UV-VIS, and magnetic property measurements. The parent LaCoO₃ compound showed spin-glass transition at low temperatures, and with progressive Cd doping, showed transition to paramagnetic ordering. The changes in magnetic properties of the materials are correlated to the changes in structural features resulting from the Rietveld structural refinement of the materials.     

A first-principles study of the structural,elastic, electronic,vibrational, and optical properties of BaSe<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Te<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>

The structural, elastic, electronic, vibrational, and optical properties of BaSe_1?xTe_x alloys are investigated by means of the full-potential linearized augmented plane wave method. The exchange–correlation effects are treated with the local density approximation, as well as the GGA-PBE, GGA-PBEsol, and GGA?+?mBJ schemes of the generalized gradient approximation. Ternary BaSe_1?xTe_x compounds have not yet been synthesized. Improved predictions of the structural parameters are obtained using the GGA-PBEsol approach. Calculations of the electronic and optical properties with the GGA?+?mBJ approach yield accurate results. Ternary BaSe_1?xTe_x alloys are wide-band-gap semiconductors with a direct gap Γ–Γ. The upper valence band is mainly due to Se p and Te p states, while the bottom of the conduction band results essentially from Ba d states. The dielectric function, refractive index, reflectivity, absorption coefficient, and energy-loss function are calculated in the range 0–35 eV. The increase in x gives rise to a redshift of the optical spectra. BaSe_1?xTe_x alloys exhibit reflective properties of metals in some energy ranges. The static dielectric constant ?₁(0) and the static refractive index n₀ are calculated. The investigation of the elastic and vibrational properties shows that ternary BaSe_1?xTe_x should be mechanically and dynamically stable, elastically anisotropic, brittle, and relatively soft.     

Study of the electrical properties of Pb(Zn,Ni)1/3Nb2/3O3–PbTiO3 ceramics across the morphotropic phase boundary

Structure and electrical properties of (1???x)Pb(Zn_0.2Ni_0.8)_1/3Nb_2/3O₃–xPbTiO₃ ceramics with x?=?0.24–0.38 were examined in detail. X-ray diffraction measurements reveal that all samples are in pure perovskite phase, and most of them lie within the morphotropic phase boundary (MPB) region. The dielectric behaviors of all compositions are characterized with diffuse phase transition and frequency dispersion. The variable power law and the Vogel–Fulcher relation have been used to describe such dielectric behaviors. The highest dielectric constant and the largest piezoelectric coefficient are simultaneously observed when x?=?0.30. The variation of the ferroelectric property with PT content is also discussed.     

Microstructural Design and Properties of LaCoO3/La2(Zr,Y)2O7−δ Composites

LaCoO_3–/La₂(Zr,Y)₂O₇-based composites were designed in view of three main properties: electrical conductivity, thermal expansion and resistance to thermal cycling. Composition and processing conditions were investigated on the basis of an experimental plan according to the Taguchi method. The phase distribution was estimated from image analysis and related to electrical and thermo-mechanical behavior. Results indicate that an homogeneous and fine grained phase distribution is essential in order to obtain materials with the desired thermal expansion, electrical properties and thermomechanical properties.     

Investigation of the BaTiO3–BaMg1/3Nb2/3O3 system: Structural, dielectric, ferroelectric and electromechanical studies

Lead-free (1?x) BaTiO₃ –x BaMg_1/3Nb_2/3O₃ ceramics with x?=?0.03, 0.04, 0.05 and 0.06 were prepared by solid-state synthesis. The effects of the Ba(Mg_1/3Nb_2/3)O₃ addition on the phase composition, dielectric and ferroelectric properties, as well as the electromechanical response of the classic ferroelectric BaTiO₃ were investigated. The room-temperature X-ray diffraction analyses of all the ceramics revealed a perovskite phase after sintering at 1300 °C with a composition-dependent symmetry. The samples with a lower concentration of Ba(Mg_1/3Nb_2/3)O₃, i.e., x?=?0.03 and 0.04, were tetragonal, while the samples with x?=?0.05 and 0.06 were found to be cubic. The result is in agreement with the dielectric, ferroelectric and electromechanical properties. With x increasing from 0.03 to 0.06 the temperature of the diffused maximum of the dielectric permittivity decreased from 348 K to 265 K. All the ceramics showed a large electromechanical response: the calculated room-temperature electrostrictive coefficient M ₃₃ of the sample with x?=?0.06 was 1.4 · 10^?16?m²/V², which is comparable to the value measured for Pb(Mg_1/3Nb_2/3)O₃ ceramics.     

Sintering behavior, structures and microwave dielectric properties of a rutile solid solution system: (AxNb2x)Ti1–3xO2 (A=Cu, Ni)

The sintering behavior, structures and microwave dielectric properties in a rutile solid solution system—(A_xNb_2x)Ti_1–3xO₂ (A=Cu, Ni)—were investigated and the samples were prepared by conventional solid state reaction method. Single phase of tetragonal rutile structure has been obtained through the entire range of compositions (0.02 ≤ x ≤ 0.20). The sintering temperature was lowered to 900°C by (Cu _{x /3}Nb_2x/3)⁴⁺ substituting for Ti⁴⁺ in the solid solution. Comparing with that of rutile TiO₂ (465 ppm/°C), the temperature coefficient of resonant frequency (TCF) of the rutile solid solution is much lower (about 250 ppm/°C), and the dielectric constant and the quality factor (Qf value) of the solid solution are about 70~80 and 7,000G Hz. The substitution of (Cu _{x /3}Nb_2x/3)⁴⁺or (Ni _{x /3}Nb_2x/3)⁴⁺ for Ti⁴⁺ in the solid solution improved the microwave dielectric properties of the rutile TiO₂ ceramics.     

Microstructure and microwave dielectric properties of (1 − x)Ca(Mg1/3Nb2/3)O3/xCa0.61Nd0.26TiO3 complex perovskite ceramics

Ceramics in the (1?x)Ca(Mg_1/3Nb_2/3)O₃/xCa_0.61Nd_0.26TiO₃ system were prepared and characterized. Single phase solid solutions were obtained up to x?=?0.4, and the crystal structure belonged to monoclinic space group P21(4) for x?=?0.1 and orthorhombic space group Pbnm(62) for x above 0.1. The secondary phase of Ca₂(Ti,Nb)₂O₆ was observed for x?=?0.6 and 0.8. Good combination of microwave dielectric properties was achieved in the present ceramics with x?=?0.4: ? _r?=?46.5, Qf?=?14,136 GHz, τ _f?=??12.5 ppm/°C.     

Structural,morphological and electrical properties of new type Dy doped Ca6-xNa2Y2(SiO4)6(OH)2 hydroxyapatite compound synthesized by co – precipitation method

Journal of Electroceramics - In this study Dy3+ doped Ca6-xNa2Y2(SiO4)6(OH)2 (x?=?0 – 0.05&nbsp;mol%) hydroxyapatite compound was synthesized by co – precipitation...