Nb 4d Research Articles

The Effect of Silicon Substitution by Boron for the α-Nb5Si3: INSIGHTS into the Constitutive Properties of Nb5Si2B Through Theory and Experimental Approach

We investigated the structural, elastic, vibrational, and electronic properties of the Nb5Si2B compound combining density functional theory (DFT) calculations and experimental methods. We compared our results with the parent compound Nb5Si3 with two non-equivalent Si sites namely Si(4a) and Si(8h). The analysis of elastic constants and phonon spectra indicate that Nb5Si2B is respectively mechanically and dynamically stable. Based on the phonon calculation we evaluate the theoretical constant volume lattice specific heat (CV) for different site occupancies and compare it with experimental specific heat (Cp) measurements. We found an excellent agreement between theoretical and experimental results for Nb5Si2B with the B at the Si(8h) site, which agrees with the calculated formation energy. In addition, we also performed DFT calculations aimed at showing and comparing the total DOS near the Fermi level (EF) for Nb5Si3 and Nb5Si2B. The XPS valence band (VB) of the Nb5Si2B is largely dominated by two characteristic peaks at − 8.7 and − 1.7 eV, respectively. Based on DFT calculations, it follows that the main sharp peak at − 1.7 eV comes as a contribution from Nb 4d states, while the smaller and broader one located at − 8.7 eV results mainly from Si 3s states weakly hybridized with Nb 4d states. In this connection, the majority contribution in the binding energy range from − 12 eV to the EF comes from Nb 4d states, while the contribution from Si and B atoms is very small in this region. The core levels of Nb 3d, Si 2s, 2p, and B 1s were also identified using the XPS technique.

Theoretical study on the mechanism of alcohol photooxidation on Nb2O5 surface.

Theoretical modeling of the solid-state photocatalysis is one of the important issues as various useful photocatalysts have been developed to date. In this work, we investigated the mechanism of the alcohol photooxidation on niobium oxide (Nb2O5) which was experimentally developed, using the density functional theory (DFT)/time-dependent (TD)DFT calculations based on the cluster model. The alcohol adsorption and the first hydrogen transfer from hydroxy group to surface occur in the ground state, while the second hydrogen transfer from CH proceeds in the excited states during the photoirradiation of UV or visible light. The spin crossing was identified and the low-lying triplet states were solved for the reaction pathway. The photoabsorption in the visible light region was characterized as the charge transfer transition from O 2p of alcohol to Nb 4d of the Nb2O5 surface. The spin density and the natural population analysis indicated the generation of spin density in the moiety of carbonyl compound and its dissipation to the interface of the surface, which partly explains the electron paramagnetic resonance measurement. It was confirmed that the rate determining step is the desorption of carbonyl compound and water molecule in agreement with the experimental rate equation analysis. The present findings with the theoretical modeling will provide useful information for the further studies of the solid-state photocatalysis.

Investigation of superconductivity of A15 type cubic Nb3Os material by using density functional theory

In this study, the structural, elastic, electronic, phonon and electron-phonon interaction properties of A15 type cubic Nb3Os compound were investigated by density functional theory. The lattice constant of this compound was obtained in agreement with experiments, and the investigation of elastic properties revealed its ductile nature. From electronic calculations for Nb3Os, the density of states at Fermi level (N(EF)) was found to be 5.31 states/eV with the largest contribution of Nb 4d orbitals. According to the results obtained from phonon calculations, Nb3Os was found to be dynamically stable in the cubic A15 structure and phonon modes were largely formed by the vibrations of Nb atoms. Using the electronic and phonon properties, we determined the Eliashberg spectral function, which illustrates the electron-phonon interaction. The electron-phonon interaction parameter and average phonon frequency values for Nb3Os were found to be 0.41 and 222.97 K, respectively. The calculated superconducting transition temperature of 1.05 K is in excellent agreement with the experimental value of 1.02 K.

First-Principles Investigation on the Adsorption and Diffusion of Oxygen at the B2(110)–O(001) Interface in Ti2AlNb Alloys

The adsorption and diffusion of oxygen at the B2(110)[1¯11]||O(001)[11¯0] interface in Ti2AlNb alloys were investigated via first-principles calculations. Only a 2.6% interfacial mismatch indicates that B2(110)–O(001) is basically a stable coherent interface. The calculated adsorption energies and diffusion energy barriers show that oxygen prefers to occupy the Ti-rich interstitial sites, and once trapped, it hardly diffuses to other interstitial sites, thus promoting the preferential formation of Ti oxides. Under the premise of a Ti-rich environment, a Nb-rich environment is more favorable for oxygen adsorption than an Al-rich environment. The electronic structures suggest that O 2p orbitals mainly occupy the energy region below −5 eV, bonding with its coordinated atoms of Ti, Al, and Nb. However, Al 3p and Nb 4d orbitals near the Fermi level couple with sparsely distributed O 2p orbitals, forming anti-bonding, which is not conducive to oxygen adsorption. Because Nb 4d electrons are more localized than Al 3p electrons are, Nb–O anti-bonding is weaker. O–Ti has almost no contribution to anti-bonding, suggesting good bonding between them. This is consistent with the experimental observations that TiO2 is the main oxidation product.

Bandgap engineering and antiferroelectric stability of tantalum doped silver niobate ceramics from first-principles

Extensive research has been conducted on silver niobite (AgNbO3)-based antiferroelectric ceramics for their promising applications in energy storage applications, with various compositional modifications explored to improve their energy storage capabilities. In this theoretical study, we have systematically investigated the electronic, structural, and chemical bonding properties of AgNb1-xTaxO3 (x = 0.00, 0.125, 0.25, 0.375, 0.50, abbreviated as ANT100x) solid solutions based on first-principles calculation. Our results reveal that the bandgap increases from 1.82 eV to 1.89 eV, due to the higher energy level of Ta 5d orbitals compared to Nb 4d orbitals. The enlarged bandgap, accompanied with oxygen vacancy formation energy (ΔEf,vac), contributes to the enhancement of Eb. The Ta substitution of Nb site suppresses the cation displacement, oxygen octahedral distortion, and bond length and angles, indicating an improved stability of antiferroelectric phase. In addition, the electron localization function (ELF) and Bader charge values show weakened covalent bonding of Ta−O bonds compared to Nb−O bonds. These theoretical findings have the potential to aid in the advancement and creation of novel energy storage applications using lead-free AFE perovskites, as well as facilitate the manipulation of their breakdown electric field through bandgap engineering.

The Effect of Nb Doping on the Properties of Ti-Al Intermetallic Compounds Using First-Principles Calculations.

The crystal structures, stability, mechanical properties and electronic structures of Nb-free and Nb-doped Ti-Al intermetallic compounds were investigated via first-principles calculations. Seven components and eleven crystal configurations were considered based on the phase diagram. The calculated results demonstrate that hP8-Ti3Al, tP4-TiAl, tP32-Ti3Al5, tI24-TiAl2, tI16-Ti5Al11, tI24-Ti2Al5, and tI32-TiAl3 are the most stable phases. Mechanical properties were estimated with the calculated elastic constants, as well as the bulk modulus, shear modulus, Young's modulus, Poisson's ratio and Pugh's ratio following the Voigt-Reuss-Hill scheme. As the Al content increases, the mechanical strength increases but the ductility decreases in the Ti-Al compounds. This results from the enhanced covalent bond formed by the continuously enhanced Al-sp hybrid orbitals and Ti-3d orbitals. Nb doping (~5 at.% in this study) keeps the thermodynamical and mechanical stability for the Ti-Al compounds, which exhibit slightly higher bulk modulus and better ductility. This is attributed to the fact that the Nb 4d orbitals locate near the Fermi level and interact with the Ti-3d and Al-3p orbitals, improving the metallic bonds based on the electronic structures.

Structural, elastic, electronic, bonding, thermo-mechanical and optical properties of predicted NbAlB MAB phase in comparison to MoAlB: DFT based ab-initio insights

In recent times transition metal ternary borides with layered structure have attracted much attention of the materials science community. In this study, we have used density functional theory (DFT) based first-principles investigation of the physical properties of prospective NbAlB compound for the first time. From the analysis of the cohesive energy and enthalpy of formation, it was found that NbAlB is expected to be chemically stable. The physical properties of NbAlB have been compared and contrasted with those obtained for MoAlB. Both these MAB phases are elastically anisotropic, mechanically stable, machinable and brittle materials. Structural and elastic features reflect the layered features. The estimated hardness of NbAlB is 19.0 GPa comparable to that of MoAlB (20.8 GPa) suggesting that predicted NbAlB is a hard compound and is suitable for heavy duty industrial applications. NbAlB is more machinable than MoAlB. Electronic band structure calculations reveal conventional metallic behavior with the electronic density of states at the Fermi level arising mainly due to the Nb 4d orbitals in NbAlB. The electronic density of states at the Fermi level is significantly higher in NbAlB in comparison to MoAlB, indicating that NbAlB is expected to exhibit higher level of electrical conductivity. Electronic dispersion is highly anisotropic for both MoAlB and NbAlB with substantially large electronic effective masses in the out-of-plane directions. The bonding features have been elucidated via the analysis of the band structure and charge density distribution. Both the compounds have mixed covalent, ionic and metallic bonding characteristics. The Fermi surfaces of MoAlB and NbAlB consists of electron and hole like sheets. The Debye temperatures of MoAlB and NbAlB are comparable. The estimated melting temperature of NbAlB is somewhat lower than that of MoAlB. NbAlB shows excellent reflection characteristics suitable to be used as an efficient solar reflector. NbAlB is also expected to absorb ultraviolet radiation very effectively. Unlike elastic, bonding, and electronic anisotropy, the optical spectra are fairly isotropic for NbAlB with respect to the polarization of the incident electric field.

Room-Temperature Transient Hydrogen Uptake of MgH2 Induced by Nb-Doped TiO2 Solid-Solution Catalysts.

The practical applications of MgH2 as a high-density hydrogen carrier depend heavily on efficient and low-cost catalysts to accelerate the dehydriding/hydriding reactions at moderate temperatures. In the present work, this issue is addressed by synthesizing Nb-doped TiO2 solid-solution-type catalysts that dramatically improve the hydrogen sorption performances of MgH2. The catalyzed MgH2 can absorb 5 wt % of H2 even at room temperature for 20 s, release 6 wt % of H2 at 225 °C within 12 min, and the complete dehydrogenation can be achieved at 150 °C under a dynamic vacuum atmosphere. Density functional theory calculations reveal that Nb doping introduces Nb 4d orbitals with stronger interaction with H 1s into the density of states of TiO2. This considerably enhances both the adsorption and dissociation ability of the H2 molecule on the catalysts surface and the hydrogen diffusion across the specific Mg/Ti(Nb)O2 interface. The successful implementation of solid solution-type catalysts in MgH2 offers a demonstration and inspiration for the development of high-performance catalysts and solid-state hydrogen storage materials.

Effect of Mn Incorporated into LiNbO<sub>3</sub> Crystal Structure on the Electronic and Optical Properties Using First-Principles Study

The structural, electronic and optical properties of lithium niobate (LiNbO3) and manganese (Mn)-doped LiNbO3 are investigated using a first-principles study. The first-principles calculation in this work is implemented using CASTEP computer code with GGA-PBE correlation. The band structure and density of states are calculated to analyze the effect of Mn doping on the electronic properties of LiNbO3. Hubbard U correction is applied to Nb 4d state with U= 11 eV and the corrected band gap obtained is 3.771 eV. LiNbO3 doped with Mn shows a reduction in the band gap energy which is 1.9889 eV. The dielectric constant and refractive index of LiNbO3 and Mn-doped LiNbO3 are also calculated. The optical absorption results suggest there is a shift in the absorption edge towards the visible region in comparison with the LiNbO3. The improvement in band gap and optical absorption in Mn-doped LiNbO3 making it a promising material for photovoltaic and photocatalysis applications.

A first principles study on physical properties of Nb-doped LiCoO2 for memristor (CM-3:IL02)

The electronic, magnetic, and vibrational properties of Nb-doped LiCoO2 were studied theoretically via ab initio calculations using density functional theory. We find that the Nb-doping induced sizable mid-gap states due to the strong Nb 4d – O 2p hybridization, which results in the shrinking of the gap between the valence band maximum and the bottom of the conduction band in LixCo1-yNbyO2. Moreover, the oxidation state of Co ions becomes a mixture of Co3+ and Co2+ instead of purely Co3+ in pristine LiCoO2, which indicates the existence of non-zero magnetic moment in the Nb-doped systems. The mixed oxidation states of Co ions are further confirmed by the X-ray absorption spectra calculated for the main three edges, O K-edge, Co K-edge, and Co L-edge, which shows that the Co LIII – edge is shifted to lower energy in Nb-doped LiCoO2. To investigate the structural properties, we calculate phonon spectra for both pristine and doped cases. We find that while the phonon dispersion of LiCoO2 does not have negative-frequency modes, the Nb-doped LiCoO2 has phonon modes with negative frequencies that can be removed by applying Hubbard correction, indicating the importance of the electron correlation upon Nb doping. We argue that the Nb-doping induced modulation of electron correlation could be employed to engineer the electrical transport properties, making LixCo1-yNbyO2 an ideal candidate for memristor.

Accurate Determination of the Bandgap Energy of the Rare-Earth Niobate Series.

We report diffuse reflectivity measurements in InNbO4, ScNbO4, YNbO4, and eight rare-earth niobates. A comparison with established values of the bandgap of InNbO4 and ScNbO4 shows that Tauc plot analysis gives erroneous estimates of the bandgap energy. Conversely, accurate results are obtained considering excitonic contributions using the Elliot-Toyozawa model. The bandgaps are 3.25 eV for CeNbO4, 4.35 eV for LaNbO4, 4.5 eV for YNbO4, and 4.73-4.93 eV for SmNbO4, EuNbO4, GdNbO4, DyNbO4, HoNbO4, and YbNbO4. The fact that the bandgap energy is affected little by the rare-earth substitution from SmNbO4 to YbNbO4 and the fact that they have the largest bandgap are a consequence of the fact that the band structure near the Fermi level originates mainly from Nb 4d and O 2p orbitals. YNbO4, CeVO4, and LaNbO4 have smaller bandgaps because of the contribution from rare-earth atom 4d, 5d, or 4f orbitals to the states near the Fermi level.

Band gap anomaly in single-layer Nb<sub>2</sub>SiTe<sub>4</sub>-based compounds

Two-dimensional (2D) niobium silicon telluride (Nb<sub>2</sub>SiTe<sub>4</sub>) with good stability, a narrow band gap of 0.39 eV, high carrier mobility and superior photoresponsivity, is highly desired for applications in mid-infrared (MIR) detections, ambipolar transistors. Intensive investigations on its ferroelasticity, anisotropic carrier transport, anisotropic thermoelectric property, etc., have been reported recently. Motivated by the above prominent properties and promising applications, we systematically study the electronic properties of single-layer (SL) <i>A</i><sub>2</sub><i>BX</i><sub>4</sub> analogues (<i>A</i> = V, Nb, Ta; <i>B</i> = Si, Ge, Sn; <i>X</i> = S, Se, Te) and find a band-gap anomaly with respect to anion change, which differs from conventional 2D metal chalcogenide. In conventional binary chalcogenide, when cations are kept fixed, the bandgap tends to decrease as the atomic number of anions in the same group increases. However, in SL <i>A</i><sub>2</sub><i>BX</i><sub>4</sub>, as atomic number of anions increases, its bandgaps tend to increase, with cations kept fixed. In order to find the underlying mechanism of such an abnormal bandgap, using first-principles calculations, we thoroughly investigate the electronic structures of Nb<sub>2</sub>Si<i>X</i><sub>4</sub> (<i>X</i> = S, Se, Te) surving as an example. It is found that the valance band maximum (VBM) and conduction band minimum (CBM) are mainly derived from the bonding and antibonding coupling between Nb 4d states. The bandwidth of Nb 4d states determines the relative value of the band gap in Nb<sub>2</sub>Si<i>X</i><sub>4</sub>. We demonstrate that the band gap is largely influenced by the competition effect between Nb—Nb and Nb—<i>X</i> interactions in Nb<sub>2</sub>Si<i>X</i><sub>4</sub>. As the anion atomic number increases, the Nb—Nb bond length increases, yielding an increased bandwidth of Nb 4d state and a smaller bandgap of Nb<sub>2</sub>Si<i>X</i><sub>4</sub>. Meanwhile, as Nb—<i>X</i> bond length increases, the bandwidth of Nb 4d however decreases, yielding a larger bandgap. The interaction between Nb and <i>X</i> should be dominant and responsible for the overall bandgap increase of Nb<sub>2</sub>Si<i>X</i><sub>4</sub> compared with the Nb—Nb interaction.

First-principles investigation on structural and photoelectric properties of (Nb, Ta) codoped anatase TiO2

The structural and photoelectric properties of (Nb, Ta) codoped anatase TiO2 were studied by first-principles calculations based on density functional theory and the Hubbard U correction. The results demonstrated that hybridized states composed of Ta 5d and Nb 4d orbitals at the bottom of the conduction band enabled the Fermi level to enter the conduction band and facilitated (Nb, Ta) codoped anatase TiO2 to exhibit n-type metallic characteristics, and the conductivity of Ti0·875Nb0·0625Ta0·0625O2(I) is approximately 3.2 times that of Ti0·9375Nb0·0625O2. Noted that both conductivity and mobility decreased with the increase of the distance between impurity Nb and Ta atoms and the Nb doping concentration. Moreover, the calculated results of optical properties presented the shifts of the absorption edges towards the ultraviolet region, and (Nb, Ta) codoped anatase TiO2 possessed the high transmittance thanks to its low absorption and reflection of visible light.

Beyond the ionic radii: A multifaceted approach to understand differences between the structures of LnNbO4 and LnTaO4 fergusonites

Synchrotron X-ray powder diffraction methods have been used to obtain accurate structures of the lanthanoid tantalates, LnTaO4, at room temperature. Three different structures are observed, depending on the size of the Ln cation: P21/c (Ln = La, Pr), I2/a (Ln = Nd-Ho), and P2/c (Ln = Tb-Lu). BVS analysis indicated that TaV is six-coordinate in these structures, with four short bonds and two longer bonds. Synchrotron X-ray powder diffraction methods were also used to observe the impact of Ta doping on the orthoniobates, Ln(Nb1-xTax)O4 (Ln = Pr, Nd, Sm, Gd, Tb, Dy, Ho, Yb, and Lu). Where both the niobate and tantalate oxide were isostructural (fergusonite structure, space group I2/a), complete solid solutions were prepared. In these solid solutions, the unit cell volume decreases as the Ta content increases. The subtle interaction evident between the LnO8 and BO6 sublattices in the fergusonite-type oxides was not observed in the related pyrochlore oxides. A combined synchrotron X-ray and neutron powder diffraction study of the series Ho(Nb1-xTax)O4 was used to determine accurate atomic positions of the anions, and hence, bond lengths. This revealed a change in the (Nb/Ta)-O bond lengths, reflective of the difference in the valence orbitals of Nb(4d) and Ta(5d). Examination of the partial density of states demonstrates differences in the electronics between Nb and Ta, leading to a difference in the bandgap. This study highlights the importance of the long B-O contacts in the fergusonite structures, and its potential impact on the I2/a to I41/a phase transition.

The Local Structure and Metal-Insulator Transition in a Ba3Nb5-xTixO15 System.

The local structure of the filled tetragonal tungsten bronze (TTB) niobate BaNbTiO (x = 0, 0.1, 0.7, 1.0), showing a metal-insulator transition with Ti substitution, has been studied by Nb K-edge extended X-ray absorption fine structure (EXAFS) measurements as a function of temperature. The Ti substitution has been found to have a substantial effect on the local structure, that remains largely temperature independent in the studied temperature range of 80–400 K. The Nb-O bonds distribution shows an increased octahedral distortion induced by Ti substitution, while Nb-Ba distances are marginally affected. The Nb-O bonds are stiffer in the Ti substituted samples, which is revealed by the temperature dependent mean square relative displacements (MSRDs). Furthermore, there is an overall increase in the configurational disorder while the system with Nb 4d electrons turns insulating. The results underline a clear relationship between the local structure and the electronic transport properties suggesting that the metal-insulator transition and possible thermoelectric properties of TTB structured niobates can be tuned by disorder.

Synergistic effects of Eu and Nb dual substitution on improving the thermoelectric performance of the natural perovskite CaTiO3

Driven by the development of sustainable and regenerable energy conversion materials, the mineral perovskite (CaTiO3) is considered to be a potential candidate for large-scale high-temperature applications owing to its abundance, light-weight, non-toxicity, and low cost. A series of compounds with the nominal composition Ca1–xEuxTi0.9Nb0.1O3 (0 ≤ x ≤ 0.4) has been synthesized and studied in this paper. The phase purities and crystal structures were evaluated by powder X-ray diffraction (XRD) and subsequent Rietveld analysis. Through X-ray photoelectron spectroscopy (XPS) characterization, the by far dominating valence states of Ti and Nb are confirmed to be +4 and + 5 in Ca1-xEuxTi0.9Nb0.1O3 compounds, respectively. Dual substitution by Nb and Eu yields a synergistic effect of improving electrical transport properties and simultaneously suppressing thermal conductivity. The former is mainly attributed to the d–f electron exchange induced by the strong hybridization of Eu 4f, Nb 4d, and Ti 3d orbitals. The latter is mostly attributed to the dominant phonon scattering by the mass fluctuation originating from the large mass contrast of Eu and Ca. The results demonstrate the evolution of insulating CaTiO3 to metallic-like conduction performance with increasing Eu content. Due to the largest power factor and lowest thermal conductivity, the sample Ca0.8Eu0.2Ti0.9Nb0.1O3 exhibits the maximum ZT of up to 0.3 at around 1173 K.

Effects of Nb concentration on Nb-doped anatase TiO2: DFT + U calculations

The crystal structure, formation energy, electronic structure, electrical properties and optical properties of anatase TiO2 with various Nb concentrations were studied by first-principles calculations based on density functional theory (DFT) and the Hubbard U correction. Firstly, the crystal structures of TiO2 with various Nb concentrations were optimized successfully. The higher concentrations of Nb facilitate the synthesis of Nb-doped TiO2 systems in an O-rich environment. Furthermore, the band structures and density of states prove that the electronic structure of Nb-doped TiO2 is mainly transformed by the hybridization of Ti 3d, Nb 4d and O 2p states. Next, broadening of the valence band and occupied states at the bottom of the conduction band enhance the conductivity of Nb-doped TiO2, and the conductivity reaches the maximum value at the high Nb-doping concentration of 6.25 at.%. Last but not least, the calculated optical properties show that the absorption edge of Nb-doped TiO2 produces an blue shift. TiO2 with Nb-doping concentration below 14.58 at.% exhibits the high transmittance in the visible region.

Superimposed Effect of La Doping and Structural Engineering to Achieve Oxygen-Deficient TiNb2O7 for Ultrafast Li-Ion Storage.

TiNb2O7 (TNO) is a competitive candidate of a fast-charging anode due to its high specific capacity. However, the insulator nature seriously hinders its rate performance. Herein, the La3+-doped mesoporous TiNb2O7 materials (La-M-TNO) were first synthesized via a facile one-step solvothermal method with the assistance of polyvinyl pyrrolidone (PVP). The synergic effect of La3+ doping and the mesoporous structure enables a dual improvement on the electronic conductivity and ionic diffusion coefficient, which delivers an impressive specific capacity of 213 mAh g-1 at 30 C. The capacity retention (@30C/@1C) increases from 33 to 53 and 74% for TNO, M-TNO, and La-M-TNO (0.03), respectively, demonstrating a step-by-step improvement of rate performance by making porous structures and intrinsic conductivity enhancement. DFT calculations verify that the enhancement in electronic conductivity due to La3+ doping and oxygen vacancy, which induce localized energy levels via slight hybridization of O 2p, Ti 3d, and Nb 4d orbits. Meanwhile, the GITT result indicates that PVP-induced self-assembly of TNO accelerates the lithium ion diffusion rate by shortening the Li+ diffusion path. This work verifies the effectiveness of the porous structure and highlights the significance of electronic conductivity to rate performance, especially at >30C. It provides a general approach to low-conductivity electrode materials for fast Li-ion storage.

Rational Crystal Structure Design and Nonlinear-Optical Properties of Noncentrosymmetric RbCu2NbS4.

Crystal structure design based on known materials is an efficient strategy for exploring new compounds with evident second-harmonic-generation (SHG) effects. By the introduction of Rb+ ions into the 3D framework of Cu3NbS4, the new compound RbCu2NbS4 (space group Ama2) composed of layers is obtained. The band gap of RbCu2NbS4 is 2.1 eV, indicating that this compound is a semiconductor. Band-structure calculations indicate that the electronic transition mainly occurs from the S 3p/Cu 3d to Nb 4d states. The intercalation of Rb+ ions induces a high degree of local distortion and symmetry reduction, which results in a dipole moment of 11.7 Debye for RbCu2NbS4. RbCu2NbS4 shows a moderate SHG response of 0.33 × AgGaS2 (particle size of 20-41 μm).

First-principles study of the structural and optoelectronic properties of ANbO3 (A = Na, K and Rb) in four crystal phases

In the present work structural and optoelectronic properties of alkali niobates ANbO3 (A = Na, K and Rb) are investigated in different crystal structures e.g. cubic, tetragonal, orthorhombic and rhombohedral using generalized gradient approximation (GGA), GGA with Hubbard potential (GGA + U) and modified Becke-Johnson (mBJ) approach. Structural properties show that ferroelectric rhombohedral crystal structure is the most stable structure of these compounds. The anti-ferroelectric and paraelectric are their higher energy crystal structures. The obtained results of the electronic band gaps show that mBJ is the appropriate exchange-correlation approximation for the calculations of electronic and optical properties of these niobates. They are indirect band gap semiconductors, except the ferroelectric rhombohedral and antiferroelectric orthorhombic NaNbO3. The valence band is predominantly contributed by O 2p orbitals and conduction band is made of Nb 4d orbitals. Real and imaginary parts of dielectric function and other optical parameters such as energy loss function, sum rule and refractive index are also investigated.