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A new one-dimensional antiferromagnetic and semiconducting quaternary sulfide Ba4FeAgS6 has been synthesized and its physical properties have been investigated in detail.

Single crystals and a polycrystalline sample of BaCeCuS3 were synthesized and characterized. The semiconducting BaCeCuS3 shows ultralow thermal conductivity (0.32 W m−1 K−1 at 773 K) and is promising for thermoelectric and photovoltaic applications.

The relative energetic stability of the structural phases of common antiferromagnetic transition-metal oxides (MnO, FeO, CoO, and NiO) within the semilocal and hybrid density functionals are fraught with difficulties. In particular, MnO is known to be the most difficult case for almost all common semilocal and hybrid density approximations. Here, we show that the meta-generalized gradient approximation (meta-GGA) constructed from the cuspless hydrogen model and Pauli kinetic energy density (MGGAC) can lead to the correct ground state of MnO. The relative energy differences of zinc-blende (zb) and rock-salt (rs) structures as computed using MGGAC are found to be in nice agreement with those obtained from high-level correlation methods like the random phase approximation or quantum Monte Carlo techniques. Besides, we have also applied the onsite hybrid functionals (closely related to DFT+U ) based on GGA and meta-GGA functionals, and it is shown that a relatively high amount of Hartree-Fock exchange is necessary to obtain the correct ground-state structure. Our present investigation suggests that the semilocal MGGAC and onsite hybrids, both being computationally cheap, as methods of choice for the calculation of the relative stability of antiferromagnetic transition-metal oxides having potential applications in solid-state physics and structural chemistry.

Single crystals of a new semiconducting layered quaternary sulfide, Ba3Zr2Cu4S9, are synthesized by the elemental reactions at high temperatures. This phase is the first member of the Ba–Zr–Cu–S system and represents an unprecedented structure type.

A new structure type Ba4Mn2Si2Te9 containing unique MnTe5 units is synthesized. The structure comprises two independent Mn atoms, each with 50% occupancy. It is a narrow bandgap semiconductor (Eg = 0.6(1) eV) consistent with the DFT studies.

Two new metal ternary chalcogenides, NaScSe2 and NaScTe2, have been synthesized via high-temperature reaction.

The single crystals of a quaternary sulfide, Ba

Semiconductors AlSb and GaSb have emerged, in recent years, as important candidates for photovoltaic applications due to their strong absorption coefficients and other photovoltaic properties. In this study, AlSb (GaSb) p-n junction-based solar cell device parameters and properties are studied using the density functional theoretical framework and the non-equilibrium Green function approach. The effect of temperature on various solar cell parameters such as open-circuit voltage, power conversion efficiency, photocurrent density, short-circuit current

Copper/silver-containing chalcogenides have recently attracted significant interest for their promising thermoelectric applications. In this article, we report the syntheses of five new layered quaternary Cu/Ag-containing chalcogenides: SrScCuSe3, SrScCuTe3, BaScCuSe3, BaScCuTe3, and BaScAgTe3. The single-crystal X-ray diffraction studies show that all the Cu-containing compounds are isostructural and crystallize in the orthorhombic crystal system in the Cmcm space group. Interestingly, the crystal structure of isoelectronic BaScAgTe3 does not adopt the same structure type as observed for AkScCuQ3 (Ak = Sr/Ba; Q = Se/Te). Instead, it crystallizes in the primitive orthorhombic Pnma space group. The crystal structures of all these compounds are two-dimensional consisting of 2∞[ScMQ3]2− layers (M = Cu/Ag) separated by Ak2+ cations. The building blocks of these structures are distorted tetrahedral MQ4 and octahedral ScQ6 units. The arrangement and sharing of the MQ4 and ScQ6 units are slightly different for the two structure types, AkScCuQ3 (Ak = Sr and Ba; Q = Se and Te) and BaScAgTe3. Resistivity study confirms the metallic behavior for BaScCuTe3. The positive sign of thermopower values suggests holes as the charge carriers. The value of the thermoelectric figure of merit (zT) for polycrystalline BaScCuTe3 was found to be enhanced on increasing the temperature with a maximum zT value of 0.34 at 779 K. In addition, we have also studied the structural and electronic properties of BaScCuTe3 and BaScAgTe3 within the framework of density functional theory (DFT).

Three new isostructural quaternary tellurides, Ba2Ln1−xMn2Te5 (Ln = Pr, Gd, and Yb), have been synthesized by the molten-flux method at 1273 K. The single-crystal X-ray diffraction studies at 298(2) K showed that Ba2Ln1−xMn2Te5 crystallize in the space group –C2/m of the monoclinic crystal system. There are six unique crystallographic sites in this structure's asymmetric unit: one Ba site, one Ln site, one Mn site, and three Te sites. The Ln site in the Ba2Ln1−xMn2Te5 structure is partially filled, which leaves about one-third of the Ln sites vacant (□) for Pr and Gd compounds. These structures do not contain any homoatomic or metallic bonding and can be charge-balanced as (Ba2+)2(Gd/Pr3+)2/3(Mn2+)2(Te2−)5. The refined composition for the Yb compound is Ba2Yb0.74(1)Mn2Te5 and can be charge-balanced with a mixed valence state of Yb2+/Yb3+. The crystal structures of Ba2Ln1−xMn2Te5 consist of complex layers of 2∞[Ln1−xMn2Te5]4− stacked along the [100] direction, with Ba2+ cations separating these layers. The Ln atoms are bound to six Te atoms that form a distorted octahedral geometry around the central Ln atom. Each Mn atom in this structure is coordinated to four Te atoms in a distorted tetrahedral fashion. These LnTe6 and MnTe4 units are the main building blocks of the Ba2Ln1−xMn2Te5 structure. The optical absorption study performed on a polycrystalline Ba2Gd2/3Mn2Te5 sample reveals a direct bandgap of 1.06(2) eV consistent with the DFT study. A semiconducting behavior was also observed for polycrystalline Ba2Gd2/3Mn2Te5 from the resistivity study. The temperature-dependent magnetic studies on a polycrystalline sample of Ba2Gd2/3Mn2Te5 did not reveal any long-range magnetic order down to 5 K. The effective magnetic moment (μeff) of 10.37μB calculated using the Curie–Weiss law is in good agreement with the theoretical value (μcal) of 10.58μB.

Two new quaternary tellurides, Cs19Ln19Mn10Te48 (Ln = Pr and Gd), were synthesized by the reactive molten-flux method at 1273 K using an excess of CsCl as a reactive flux. The single-crystal X-ray diffraction studies reveal that these compounds are isostructural and crystallize in a new structure type in the space group C32h–C2/m of the monoclinic crystal system with two formula units. There are forty-nine unique crystallographic sites in the unit cell of the structure: ten Cs sites, ten Ln sites, five Mn sites, and twenty-four Te sites. All atoms have a site symmetry of .m. except for Cs(10) and Ln(10), which have a site symmetry of 2/m. The unique crystal structure of Cs19Ln19Mn10Te48 consists of complex layers of 2∞[Ln19Mn10Te48]19− that are separated by filling of Cs+ cations between the layers. The Ln atoms are bonded to six Te atoms that form a distorted octahedral geometry around the central Ln atom, whereas Mn atoms are coordinated to four Te atoms in a distorted tetrahedral fashion. The building blocks of these structures (LnTe6 octahedra and MnTe4 tetrahedra) are fused in a complex fashion to create 2∞[Ln19Mn10Te48]19− layers. These structures do not contain any homoatomic bonding and hence, can be easily charge-balanced as (Cs1+)19(Ln3+)19(Mn2+)10(Te2−)48 as per the Zintl–Klemm concept. The optical absorption study on finely ground single crystals of the Gd-compound reveals a direct bandgap of about 0.5(1) eV. The DFT studies also suggest both systems to be semiconductors with bandgaps of ∼0.5 eV consistent with the experimental value.

Accurate theoretical prediction of the band offsets at interfaces of semiconductor heterostructures can often be quite challenging. Although density functional theory has been reasonably successful to carry out such calculations, efficient, accurate semilocal functionals are desirable to reduce the computational cost. In general, the semilocal functionals based on the generalized gradient approximation (GGA) significantly underestimate the bulk bandgaps. This, in turn, results in inaccurate estimates of the band offsets at the heterointerfaces. In this paper, we investigate the performance of several advanced meta-GGA functionals in the computational prediction of band offsets at semiconductor heterojunctions. In particular, we investigate the performance of r2SCAN (two times revised strongly constrained and appropriately normed functional), rMGGAC (revised semilocal functional based on cuspless hydrogen model and Pauli kinetic energy density functional), mTASK (modified Aschebrock and Kümmel meta-GGA functional), and local modified Becke–Johnson exchange-correlation functionals. Our results strongly suggest that these meta-GGA functionals for supercell calculations perform quite well, especially, when compared to computationally more demanding GW calculations. We also present band offsets calculated using ionization potentials and electron affinities, as well as band alignment via the branch point energies. Overall, our study shows that the aforementioned meta-GGA functionals can be used within the density functional theory framework to estimate the band offsets in semiconductor heterostructures with predictive accuracy.

The interface magnetization and its modulation is explored in oxide heterostructures within the framework of density functional theory. In particular, modulation in interface magnetization is investigated in prototype heterostructure SrRuO3|KNbO3(0 0 1) consisting ferromagnetic SrRuO3 and nonmagnetic KNbO3 as the thickness of KNbO3 is varied. The computed interfacial magnetization is found to alternate between a higher and a lower value with increasing KNbO3 thickness in the heterostructure. The average change in interface magnetization is computed to be ∼−60% as KNbO3 film changes from non-stoichiometric to stoichiometric and ∼+60% as it changes from stoichiometric to non-stoichiometric due to variation in thickness. The magnetic moment per interface cell changes from ∼2.27 μB for non-stoichiometric KNbO3 to ∼1.31 μB for stoichiometric KNbO3 in the heterostructure. The modulation in interface magnetization arises due to spin-dependent screening of effective charge of KNbO3 unit cells by accumulated free carriers at the interface. Large modulation in interface magnetization is also obtained for other heterostructures consisting magnetic substructure and ionized planes at the interface. In addition, the alteration in interface magnetization due to ferroelectric polarization reversal in ferromagnetic|ferroelectric SrRuO3|KNbO3 (0 0 1) superlattice is explored. The resulting interface magnetoelectric effect as well as control of interface magnetization using ionized planes may have interesting implications for technological applications.

Semiconducting silicides are highly promising materials for applications in thermoelectric, photovoltaic and optoelectronic devices. Here, using ab-initio density functional theory, the lattice dynamics, dielectric properties, Infrared (IR) refletivity and Raman intensity spectra of Nowotny chimney-ladder semiconducting silicide Ru2Si3 are investigated. The zone-center phonon mode frequencies are found to be in the range 80–510 cm−1. The Born effective charge tensor, oscillator strength and Infrared activity is found to largest for the mode with frequency 422 cm−1 which is primarily contributed by displacements of Si atoms. The largest Raman activity is obtained for mode with frequency 479 cm−1 which is also dominated by vibrations of Si atoms. Modified Becke-Johnson (MBJ) exchange potential is used to compute the band gap of Ru2Si3 which improves the band gap by ∼8% as compared to that obtained using LDA. The computed Infrared reflectivity and Raman intensity spectra are expected to provide benchmark first-principles theoertical results for comparison with the experiments.

Two-dimensional (2D) layered III-VI semiconductors such as GaSe have attracted a lot of attention in recent years. Bulk GaSe consists of stacks of layers held together by weak interlayer interaction generally assumed to be of van der Waals type. However, proper justification of this assumption has been lacking in the reported studies. In this paper, we explore in detail the interlayer coupling in GaSe by studying lattice dynamics using first-principles density functional theory. Our study strongly suggests that contrary to common assumption, the contribution of Coulomb interaction in interlayer coupling can be significantly higher than that of van der Waals interaction in GaSe and other similar 2D layered semiconductors. The suggested predominance of electrostatic over van der Waals interaction in interlayer coupling may have important implications for various physical properties of GaSe and related layered semiconductors. Further, we study polarized Raman spectra, infrared (IR) activities, mode symmetry assignments, and Born-effective charge tensors for bulk GaSe polytypes (β, ϵ, γ). The Raman mode intensities are calculated for different light polarization setups and signature Raman and IR active modes are identified for each GaSe polytype (structure). In addition, the influence of film thickness and strain on Raman and IR mode frequencies and intensities of GaSe are explored and compared with available experiments.

Gallium sulfide (GaS) is a highly promising two-dimensional layered semiconductor owing to its remarkable thickness dependent electronic and physical properties. In this article, we perform a comprehensiveab initiostudy of lattice dynamics, mode symmetry assignments, polarized Raman and infrared (IR) reflectivity spectra of GaS system. Polarized Raman spectra are obtained for different light polarization set-ups of incoming and scattered light. The frequencies of all allowed vibrational modes at the zone-centre are calculated and symmetry labels are assigned. Furthermore, the variation of frequencies & intensities of Raman/IR active modes of ultrathin GaS films (few layers) as function of film thickness is studied. In addition, we also explore the nature of weak interlayer coupling in GaS. The weak forces between the GaS layers are usually assumed to be due to interlayer van der Waals (vdW) interaction. However, this assumption has not been reasonably explained in reported experimental studies. Our study strongly suggests that weak interlayer interactions in GaS may be primarily electrostatic (Coulomb) in nature and therefore the contribution of vdW interactions to layer-layer coupling and lattice dynamics may be significantly lower than that of electrostatic interaction. The suggested nature of interlayer coupling in GaS and related III-VI semiconductors may have important implications in determination of their various physical properties.

Three new isostructural quaternary tellurides, Ba

The correct treatment of d electrons is of prime importance in order to predict the electronic properties of the prototype chalcopyrite semiconductors. The effect of d states is linked with the anion displacement parameter u, which in turn influences the bandgap of these systems. Semilocal exchange-correlation functionals which yield good structural properties of semiconductors and insulators often fail to predict reasonable u because of the underestimation of the bandgaps arising from the strong interplay between d electrons. In the present study, we show that the meta-generalized gradient approximation (meta-GGA) obtained from the cuspless hydrogen density (MGGAC) [Phys. Rev. B 100, 155140 (2019)] performs in an improved manner in apprehending the key features of the electronic properties of chalcopyrites, and its bandgaps are comparative to that obtained using state-of-art hybrid methods. Moreover, the present assessment also shows the importance of the Pauli kinetic energy enhancement factor, $\alpha=(\tau-\tau^W)/\tau^{unif}$ in describing the d electrons in chalcopyrites. The present study strongly suggests that the MGGAC functional within semilocal approximations can be a better and preferred choice to study the chalcopyrites and other solid-state systems due to its superior performance and significantly low computational cost., Comment: 12 pages, 8 figures

The solar cell properties of crystalline BaSi2 and BaGe2 p–n homojunctions are explored using density functional theory combined with a nonequilibrium Green function method. In particular, the quantitative estimates of solar cell parameters such as photocurrent, open-circuit voltage [Formula: see text], short-circuit current [Formula: see text], and efficiency [Formula: see text] are obtained for LDA and GGA-1/2 functionals. The effect of temperature on solar cell parameters is included through electron–phonon coupling (EPC) using the special thermal displacements method. The magnitudes of [Formula: see text], [Formula: see text], and [Formula: see text] for BaSi2 (BaGe2) at 300 K are found to be 27.35 mA/cm2 (26.1 mA/cm2), 0.84 V (0.78 V), and 18.0% (16.6%), respectively. Our study strongly suggests that the phonon-assisted photon absorption and thereby EPC significantly affect the photocurrent, and its inclusion is necessary for a proper description of various solar cell parameters. The computed solar cell parameters for BaSi2 (BaGe2) p–n homojunctions can be used as benchmark ab-initio quantum mechanical results and can be used in simulations based on continuum models.

A monophasic polycrystalline sample of Ba2Ge2Te5 has been synthesized for the first time using the sealed tube solid-state method. The Rietveld refinement of a polycrystalline Ba2Ge2Te5 and a single crystal X-ray diffraction study confirm that Ba2Ge2Te5 crystallizes in the orthorhombic polar C 2 v 9 -Pna21 space group. Each of the Ge atoms in the Ba2Ge2Te5 structure is covalently connected to one Ge and three Te atoms making one-dimensional (1D) chains of 1 ∞ [Ge2Te5]4− that are separated by Ba2+ cations. The (Ba2+)2(Ge3+)2(Te2−)5 can be charge-balanced as per the Zintl-Klemm concept. A resistivity study of Ba2Ge2Te5 shows a metallic behavior till 18 K below which metal to insulator transition was observed. Thermal conductivity of Ba2Ge2Te5 was found to decrease gradually on heating the sample with a minimum of about 0.41 Wm–1K–1 at 773 K. The DFT studies predict semiconducting nature for Ba2Ge2Te5 with a narrow indirect bandgap of about 0.6 eV.

The Pt/MgO (100) interface electronic properties have been analyzed with first principles density functional theory plus nonequilibrium greenes function (NEGF) framework. We analysed n-type schottky barrier hight (SBH) of metal-insulator (Pt/MgO) interface for various biase voltage using two probe model. The applied bias voltage +0.5 V for left and right electrode of supercell, we found large n-SBH is 3.12 eV due to strong interface dipoles. Similarly, in the case of bias voltage −0.5 V for left and +0.5 V for right electrode the n-SBH is lower by 1.91 V. These large SBH values are useful in electronic applications.

In this study, we investigated the phonon modes, dielectric properties, infrared (IR) reflectivity, and Raman intensity spectrum of semiconducting silicide BaSi2 within the framework of the first principles density functional theory. The zone-center phonon mode frequencies formed two major bands in the ranges of 50–150 cm−1 and 270–500 cm−1. The modes in the frequency band 100–150 cm−1 had high Born effective dynamical charges and oscillator strengths, and they contributed significantly to the mode effective charge, static dielectric permittivity, IR activities, and reflectivity spectrum. These modes were attributable to displacements of isolated Si clusters relative to one another without internal distortions. The computed Raman activities and intensities were significant for modes in the frequency band of 270–500 cm−1. These modes were primarily associated with silicon atom vibrations in the Si clusters in BaSi2 and the internal modes of the isolated Si clusters with Td symmetry. The band gap was also computed for BaSi2 using the modified Becke–Johnson exchange potential and it was in good agreement with the previously reported experimental values. The computed Raman intensity and IR reflectivity spectrum can be used as benchmark first principles theoretical results to compare with experimental results.

Using density functional theory, the interface local magnetic moment and its variation are explored in SrRuO3|LaAlO3|vacuum (001) heterostructures with increasing number of LaAlO3 unit cells. The computed magnetic moment projected on interface planes is found to modulate nearly periodically as the number of LaAlO3 unit cells is increased in SrRuO3|LaAlO3|vacuum structure. The magnetic moment localized at the interface RuO2 plane shifts from a lower value of $\sim 0.7~\mu _{B}$ to a higher value of $\sim 1.35~\mu _{B}$ and changes by ~42% as LaAlO3 changes from non-stoichiometric to stoichiometric with increase in its thickness. However, the magnitude shifts from higher to lower value as LaAlO3 changes from stoichiometric to non-stoichiometric with further increase in thickness. The large variation and nearly periodic modulation of magnetic moment localized at the interface with increasing LaAlO3 thickness arise due to spin-dependent screening of effective total charge of LaAlO3 film by accumulated free carriers at the interface in heterostructure. The modulation of interface local magnetic moment using ultrathin LaAlO3 film at the oxide heterointerfaces can be exploited as an additional functionality, in conjunction with those reported recently such as control of band alignment and enhancement of the electric conductivity at the interface.

We investigate electronic structure, vibrational, ferroic and thermoelectric properties of AgTaO 3 within the frame of density functional theory. The atomic structure is fully relaxed and the structural parameters are found to differ by less than 1–5% from experimental data. The associated electronic band structure and density of states presented. We obtain the phonon frequencies at the center of Brillouin zone, Born effective charge tensors using density functional perturbation theory and the computed frequencies at the Γ- point agree with the experimental reported values. The born effective charge tensor suggests that the weak ferroelectricity arises because of displacement of Ta and O atoms. The thermoelectric properties such as thermopower, electrical and thermal conductivity and power factor has been discussed with variation of temperature and concentration doping. The highest power factor obtained ∼91 × 10 14 W cm −1 K −1 at the optimum carrier concentration 8 × 10 21 cm −3 for n-type doping at the 800 K. This results suggests that n-type doping can enhance the thermoelectric properties of AgTaO 3 material in the high temperature region.

Polycrystalline AgNb1/2Ta1/2O3 powder was prepared by solid state reaction method. Preliminary x-ray diffractogram analysis of some aspects of crystal structure showed that a single phase compound formed exhibiting a monoclinic system. Impedance spectroscopy showed the presence of both bulk and grain boundary effects in the material. The relaxation behavior was studied by fitting electric modulus with Bergman function confirms us the existence of non-Debye type of relaxation the material. The ac conductivity spectrum obeyed Funke's double power law and fitting in results, the hopping parameters n1,n2 were indicating the existence of small and large range polaron hopping in the material. The band gap of the material 3.02 eV measured by using UV visible spectroscopy.

Nickel monosilicide (NiSi) and Platinum monosilicide (PtSi) are highly promising for applications as contact materials in micro- and nanoelectronic devices. In this article, the interface electronic structure and Schottky barriet height in Si/NiSi(010) and Si/PtSi(010) heterostructures are explored using first-principles density functional theory. Variations in Schottky barrier in Si/NiSi(010) and Si/PtSi(010) systems and workfunctions of NiSi(010), PtSi(010) surfaces due to interface defects are also estimated. In Si/NiSi(010) system, resonance interface states decaying sharply in Si away from the interface are found at ∼8.5 eV below the Fermi energy. On the other hand, localized interface states decaying sharply in Si as well as PtSi away from the interface are found at ∼8.5 eV below the Fermi energy in Si/PtSi(010) system. The Schottky barrier heights in Si/NiSi and Si/PtSi systems are also obtained using MIGS empirical model and bond polarization model and compared with available experimental data and those obtained from first-principles method.

Single crystals of two new chalcogenides CsSc5Te8 and Cs0.6(1)Ti6Se8 were synthesized by the high-temperature solid-state sealed tube method using a reactive flux (CsCl) at 1223 K. The crystal structures of CsSc5Te8 and Cs0.6(1)Ti6Se8 were determined by single-crystal X-ray diffraction method. The compound CsSc5Te8 crystallizes in the monoclinic space group C 2 h 3 −C2/m with the lattice parameters a = 21.3376(15) A, b = 4.1434(3) A, c = 10.2853(7) A, and β = 103.925(2)° having the two formula units (Z = 2). The asymmetric unit of CsSc5Te8 contains eight crystallographically independent atomic sites: Cs1 (site symmetry: 2/m), Sc1 (m), Sc2 (m), Sc3 (2/m), Te1 (m), Te2 (m), Te3 (m), and Te4 (m). The structure of CsSc5Te8 is built up of the three-dimensional anionic framework of 3 ∞ [ Sc 5 Te 8 ] 1 − where the Sc atoms are octahedrally coordinated with six Te atoms forming the one-dimensional tunnels approximately along the b-axis where the Cs+ cations are present. The compound Cs0.6(1)Ti6Se8 crystallizes in the Nb3Te4 structure type with Z = 1, in the hexagonal space group C 6 h 2 −P63/m having cell dimensions of a = b = 9.9520(1) A and c = 3.5710(1) A. The asymmetric unit of Cs0.6(1)Ti6Se8 structure is composed of four crystallographically independent sites with atoms Cs1 (site symmetry: 6 ‾ ..), Ti1 (m..), Se1 (m..), and Se2 ( 6 ‾ ..). The Ti atoms are making distorted octahedral units by coordinating with six Se atoms. These TiSe6 distorted octahedra share edges and corners with the adjacent TiSe6 units to form three-dimensional anionic networks that generate one-dimensional tunnels approximately along the c-direction. The optical absorption measurements show that CsSc5Te8 is a semiconductor having a direct bandgap of 1.2(1) eV at room temperature consistent with the DFT studies.

The interface magnetoelectric (ME) effect is promising for novel technological applications as it allows the control of interface magnetization at heterointerfaces by the external electric field. Here in this article, we explore the interface ME effect and its sensitivity on interface atomic structure and bonding in ferromagnetic-ferroelectric oxide hetrostructures within the framework of density functional theory. In particular, we consider Fe/AgNbO3 and SrRuO3/AgNbO3 superlattices with different possible defect-free interfaces. Our results suggest that interface magnetization and thereby interface ME effect can critically depend on interface structure. The interface magnetization is found to depend on atomic bondings which are sensitive to atomic displacements at the interface. This leads to ME coupling due to induced change in the interface magnetization as the electric polarization is reversed in the ferroelectric film. In addition, the contribution to interface ME coupling also comes from the change in exchange-splitting between spin-polarized electrons and consequently change in the interface magnetic moments due to ferroelectric polarization reversal. The change in exchange splitting, in turn, is caused by spin-dependent screening of the bound polarization charges at the interface. In Fe/AgNbO3 system, the interface ME coefficient is found to exhibit opposing trend for Fe|AgO and Fe|NbO2 interfaces. In addition to interface ME effect, the formation of ferroelectric interface domain wall (IDW) in these systems is also explored. The IDW is found to form only in SrRuO3/AgNbO3 system with SrO|NbO2 interface due to oppositely oriented strong dipole moments at the two interfaces. The strong dependence of interface ME effect on interface atomic structure may have implication for its experimental observation in oxide heterostructures. In particular, the interface defects may render the conclusive observation of this phenomenon challenging due to resulting fluctuations in the observed data.

Platinum silicide (PtSi) is highly promising material for applications in microelectronic devices. In this article, the surface electronic structure, surface energetics and work functions of stoichiometric and non-stoichiometric PtSi(010) surfaces are explored within the framework of first-principle density functional theory. The surface rumpling is found to be significant only for the top surface layer. The computed values of the rumpling parameter for the top three layers are ~ 11.0%, ~ 0.9% and ~ 1.9%. Further, the interlayer relaxation is found to be largest for the top layer and decreases rapidly for inner layers. Localized surface states are obtained in the valence band at ~ 9.0 eV below the Fermi level. Under rich Pt and Si growth conditions, nonstoichiometric (010) terminations are found to have the lowest surface energies, whereas stoichiometric termination has the lowest surface energy (~ 1.74 J/m2) under mixed conditions. The work function of stoichiometric (010) termination is computed to be 5.15 eV and differ as much as by ± 0.5 eV for nonstoichiometric terminations.

The modulation of Schottky barrier height (SBH) due to defect migration has been suggested to be an important driving mechanism for resistive switching in metal–oxide–metal structures. Here, we explore the SBH and its modulation due to different interface structures and defects in the Pt|MgO|Pt(001) system using hybrid Heyd–Scuseria–Ernzerhof density functional theory. The computed magnitudes of SBH at Pt|MgO interfaces obtained using the generalized gradient approximation (local density approximation) functional are found to be significantly underestimated as compared to those obtained using hybrid functional. Furthermore, the magnitudes of SBH are found to depend critically on interface structures. In the case of defect-free Pt|MgO interfaces, the p-type SBH is found to be 4.13 eV and 3.04 eV for interfaces having adjacent Pt–O and Pt–Mg bonds, respectively. In addition, the SBH magnitudes are found to exhibit significant variation primarily due to nominal effective charges on interface layers which, in turn, are induced by interface defects such as O and Mg vacancies. The magnitudes of p-type SBH are found to increase (decrease) by ∼1.0–1.5 eV as the ionic layers with charge +1e (−1e) are introduced at the interface. The modulation in SBH due to interface ionic/polar layer is explained using a micro-capacitor model. Furthermore, the SBH is found to shift by ∼0.2 eV with the varying distance of O and/or Mg vacancies from the interface. Our results suggest that fluctuations in experimental resistive switching data in Pt|MgO structures may originate due to fluctuations in SBH induced by changes in interface atomic structure. The study also shows that SBH in Pt|MgO and related structures may be modulated in a controlled way by the insertion of interface polar layers. The lower and upper bounds of the SBH magnitudes are also estimated using a semi-empirical model expressed in terms of parameters such as charge neutrality level, ionization potential, pinning parameter, and metal work function. The quantitative results on the SBH modulation presented in the study may be expected to have important implications for resistive switching phenomenon in Pt|MgO and similar other structures.

Surface electronic structure and energetics of (001) surfaces of relaxor ferroelectric Na1/2Bi1/2TiO3 (NBT) are explored theoretically within the framework of ab-initio density functional theory. In particular, polar and nonpolar surfaces of tetragonal NBT with bulk P4mm and P4bm symmetries are explored. For P4mm symmetry, (NaO)-, (BiO)+ and (TiO2)0 terminated surfaces and for P4bm symmetry, (Na1/2Bi1/2O)0 and (TiO2)0 -terminated surfaces are considered. The surface electronic structures for different terminations are found to differ with respect to bulk and with each other due to different polarity compensation mechanisms. In case of (BiO)+ and (NaO)- terminated surfaces, the Fermi level shifts slightly above the conduction band minimum (CBM) and below the valence band minimum (VBM) respectively. The resulting localized surface gap states near the VBM and CBM provides sufficient positive and negative charge required for compensation in agreement with the polarity compensation criteria. Furthermore, localized surface states are also formed at energies (~12 eV) deep in the gap below the VBM. The relative thermodynamic stability, reconstructions and relaxations of NBT surfaces may be expected to be correlated with the formation of these deep energy localized surface states. The magnitudes of surface relaxations are found to depend strongly on the nominal charges of the surfaces. The surface energies of polar NaO, BiO and nonpolar TiO2 terminations are found to be significantly larger than that of nonpolar Na1/2Bi1/2O termination. Thus the arrangement of nonpolar-nonpolar planes forming the sequence –Na1/2Bi1/2O–TiO2–Na1/2Bi1/2O–TiO2- may be expected to be more favourable than the sequence of polar-nonpolar planes -NaO-TiO2-BiO-TiO2-NaO-. This favourable arrangement of nonpolar planes may likely influence the degree of A-site cation ordering in Na1/2Bi1/2TiO3 which in turn may influence the degree of coexistence of R3C and CC phases of NBT at the room temperature.

The single crystals of Cs3FeCl5 were synthesized at 973 K using the sealed tube solid-state molten flux method using CsCl as a reactive flux. The polycrystalline sample of Cs3FeCl5 was obtained by the stoichiometric reaction of CsCl and FeCl2 powders at 823 K by the sealed tube solid-state method. The crystal structure of Cs3FeCl5 was determined by single-crystal X-ray diffraction study at 298 (2) K. This ternary halide crystallizes in the body-centered tetragonal crystal system in I4/mcm space group with cell constants of a = b = 9.279 (1) A and c = 14.824 (3) A with four formula units per cell. The asymmetric unit of Cs3FeCl5 contains five crystallographically independent atomic sites: Cs1 (site symmetry: m.2 m), Cs2 (422), Fe1 ( 4 ¯ 2 m), Cl1 (..m), and Cl2 (4/m..). Each Fe atom in Cs3FeCl5 structure is bonded to four Cl1 atoms in a slightly distorted tetrahedral fashion to form isolated FeCl42− units. These FeCl42− units are separated by the Cs+ cations and infinite [CsCl] linear chains. Charge balance in this closed-shell compound can be achieved by 3 × Cs+, 1 × Fe2+, and 5 × Cl−. Bond valence sum (BVS) calculation also supports this assignment of formal oxidation states of elements in Cs3FeCl5 structure. The electronic structure calculation for Cs3FeCl5 performed within a density functional theoretical framework predicts a band gap of 3.5 eV, which is in good agreement with the experimental band gap of 3.71 (2) eV, that was estimated from the UV–vis absorption edge study of polycrystalline Cs3FeCl5.

We perform a comprehensive theoretical study of electronic band gaps of semiconducting single-walled carbon nanotubes (SWNTs) with different sets of chiral indices using semi-empirical tight binding and density functional (DFT) based ab-initio methods. In particular, self-consistent extended Huckel (EH-SCF) and self-consistent Slater Koster (SK-SCF) tight binding models are used as semi-empirical methods, whereas the DFT based LDA-1/2 and Tran-Blaha (TB09) meta-GGA schemes are used as ab-initio methods. The calculations are performed for 1) (n, m) chiral SWNTs for which experimental optical gaps have been reported 2) (9, 0), (12, 0) and (15, 0) ‘metallic’ zigzag SWNTs for which small bad gaps have been reported 3) Pairs of SWNTs having same diameters but different chiral angles 4) (n, 0) zigzag SWNTs with 10 ≤ n ≤ 30 . From the comparison of bands gaps of tubes with same diameter, the electronic band gaps are found to vary with chiral angles with opposing trend as compared to that reported for experimental optical band gaps. This result may be expected to have important implications for self-energy corrections and/or exciton binding energies and their dependence on chiral angles. The hopping parameter γ 0 obtained from fitting EH-SCF and SK-SCF bandgap data, is found to be in good agreement with that obtained from fitting experimental data. In general, the band gap values of SWNTs computed using semi-empirical EH-SCF and SK-SCF methods are quite close (within ∼ 5%) to those computed using DFT-based LDA-1/2 and TB09 meta-GGA methods. The results suggest that self-consistent semi-empirical methods can be expected to provide similar accuracy in results as that expected from more computationally challenging ab-intio DFT based LDA-1/2 and TB09 meta-GGA methods.

Mg2Si is an important semiconducting silicide with several promising applications in photovoltaics, thermoelectrics, and optoelectronics. In this article, we perform a comprehensive density functional study of surface electronic structure, formation of localized surface states and their influence on relaxation and thermodynamic energies of (100), (110) and (111) surfaces of Mg2Si. The Tran–Blaha (TB09) meta-GGA exchange-correlation (xc) functional is used in order to correctly describe the surface electronic structures and the band gaps. The band gap of bulk Mg2Si computed using TB09 xc-functional is found to be 0.71 eV in excellent agreement with reported experimental values of 0.65–0.74 eV. Mg2Si(100) surfaces are found to be semiconducting in contrast to previous studies wherein these surfaces were reported as metallic with zero band gaps computed using local density approximation (LDA). The surface band gap is found to be 0.32 eV for Mg-terminated (100)-(1 × 1) surface whereas it vanishes for Si-termination. However, reconstructed Si-terminated (100)-(2 × 1) surface is found to be semiconducting with band gap ∼0.42 eV. The band gap for (110) surface is computed to be 0.73 eV. For (111) orientation, three different terminations are considered and are found to be semiconducting. Localized surface states are formed near valence band maximum (VBM) extending in the band gap for both (100) and (110) surfaces. In addition, localized surface gap states are also formed in the gap at ∼7 eV below the VBM for Si-terminated (100) surfaces. These localized gap states are expected to have important implications for relaxations, reconstructions and thermodynamic energies of Mg2Si surfaces. In case of (100) surfaces, interlayer relaxation is found to be significantly large for Si termination as compared to that for Mg termination. The surface energy is found to be largest for Si-terminated (100)-(1 × 1) surface with magnitude ∼2.0 J/m2. The reconstructed Si-terminated (100)-(2 × 1) surface is found to be lower in energy by ∼0.2 J/m2 than that of (100)-(1 × 1) surface. The surface energy is found to be lowest at ∼0.7 J/m2 for (111) orientations.

Theoretical investigations are carried to explore surface electronic structure and surface energetics of (1 × 1) polar and nonpolar (001) surfaces of room temperature tetragonal phase of lead-free relaxor ferroelectric K1/2Bi1/2TiO3 (KBT) within the framework of density functional theory. In particular, polar (KO)-, (BiO)+ and non-polar (TiO2)0 terminations with bulk P4mm symmetry and non-polar (K1/2Bi1/2O)0 and (TiO2)0 terminations with bulk P4bm symmetry are explored. The electronic structures of different terminations differ significantly with respect to bulk and with each other due to surface charge compensation. In case of BiO termination, the Fermi level shifts just above the conduction band minimum (CVM), whereas in case of KO termination, it shifts just below the valence band maximum (VBM) resulting in formation of localized surface states in the band gap with conduction and valence band character. In addition, localized surface states are also formed in the gap at ~11 eV below the VBM. These deep energy localized surface states may be expected to have important implications for surface relaxation and thermodynamic stability of surfaces and their reconstructions. For polar surfaces, surface relaxations are found to be strongly dependent on surface charge and are quite disparate as compared to that for non-polar surfaces. Surface energy of K1/2Bi1/2O termination is found to be much smaller than KO, BiO and TiO2 terminations, suggesting that stacking sequence -K1/2Bi1/2O–TiO2–K1/2Bi1/2O–TiO2- of non-polar planes is significantly favorable energetically than the sequence -KO-TiO2-BiO-TiO2- KO- of polar and non-polar planes in KBT. The preferred stacking sequence of non-polar planes may be expected to have significant influence on the degree of A-site cation ordering in K1/2Bi1/2TiO3.

Mg2Si is an important semiconducting silicide material with promising applications in optoelectronics, thermoelectrics, and photovoltaics. Mg2Si(001)-(1 × 1) surfaces have been suggested to be metallic from density functional (DFT) studies performed within local density (LDA) and generalized gradient approximations (GGA). Here in this article, we revisit and further explore surface electronic structure, surface reconstruction and stability of Mg2Si(100) surfaces within DFT framework in combination with Heyd-Scuseria-Ernzerhof (HSE06) hybrid functional as well as Tran-Blaha (TB09) meta-GGA functional for exchange-correlation. Our results suggest that reconstructed Mg2Si(100) surfaces are semiconducting which are incorrectly found as metallic in absence of reconstructions with higher surface periodicities and when computed within LDA (GGA) approximations. In particular, the Si-terminated Mg2Si(100) surface exhibits (2 × 1) reconstruction qualitatively similar to buckled asymmetric dimer type reconstruction of Si(100)-(2 × 1) surface. The band gap of Mg-terminated relaxed Mg2Si(100)-(1 × 1) and reconstructed Si-terminated Mg2Si(100)-(2 × 1) surfaces are found to be in the range ~ 0.3–0.5 eV. The Si-terminated reconstructed (2 × 1) surface is found to be lower in energy by ~0.21 J/m2 than the (1 × 1) relaxed surface. Further, localized surface states are formed in the band gap near valence band maximum (VBM) as well as at ~7 eV below the VBM depending on the surface termination. The surface localized gap states may be expected to have important implications for relaxations and thermodynamic energies of Mg2Si(100) surfaces.

Using ab initio density-functional theory, the Born-effective charge tensors and zone-center phonon mode frequencies are computed for AgNb1/2Ta1/2O3 in monoclinic P2/m and orthorombic Pbcm symmetries. The experimental mode frequencies are obtained from deconvolution of Raman spectrum of prepared AgNb1/2Ta1/2O3 samples and are compared with computed mode frequencies. The Raman modes with high (>350 cm−1) and low frequencies (

Polycrystalline BiFe 1− x Sc x O 3 ( x = 0, 0.05, 0.10 and 0.15) compounds were prepared using solid state reaction route. The X-ray diffraction patterns showed that Sc substitution stabilized the crystal structure in rhombohedral structure with R3c space group along with nominal percentage of monoclinic structure with Cm space group. Raman and FTIR measurements revealed that the substituent replaced Fe-ions and caused structural distortions. Co-existence of ferroelectric and antiferroelectric phases were observed in Sc substituted BiFeO 3 and antiferroelectric phase is found to be evolved at the expense of ferroelectric phase with the Sc content. The electric field driven effects indicated that antiferroelectric phase was suppressed and ferroelectric phase was enriched. Frequency dependence of dielectric constant ( ɛ r ) and dielectric loss tangent (tan δ ) at different temperatures were investigated. Further, grain resistances and grain boundary resistances were increased with the Sc content. Activation energies estimated from dielectric, impedance and modulus data indicated that these relaxations originated presumably due to the oxygen vacancy movements.

High alloyed β-phase stabilized titanium alloys are known to provide comparable Young's modulus as that to the human bones (~ 30 GPa) but is marred by its high density. In the present study the low titanium alloyed compositions of binary Ti–Nb and ternary Ti–Nb–Zr alloy systems, having stable β-phase with low Young's modulus are identified using first principles density functional framework. The theoretical results suggest that the addition of Nb in Ti and Zr in Ti–Nb increases the stability of the β-phase. The β-phase in binary Ti–Nb alloys is found to be fully stabilized from 22 at.% of Nb onwards. The calculated Young's moduli of binary β-Ti–Nb alloy system are found to be lower than that of pure titanium (116 GPa). For Ti–25(at.%)Nb composition the calculated Young's modulus comes out to be ~ 80 GPa. In ternary Ti–Nb–Zr alloy system, the Young's modulus of Ti–25(at.%)Nb–6.25(at.%)Zr composition is calculated to be ~ 50 GPa. Furthermore, the directional Young's moduli of these two selected binary (Ti–25(at.%)Nb) and ternary alloy (Ti–25(at.%)Nb–6.25(at.%)Zr) compositions are found to be nearly isotropic in all crystallographic directions.

Cobalt disilicide (CoSi 2 ) is an interesting and promising metallic material with numerous applications in silicon microelectronics. In this article, various thermodynamic properties of CoSi 2 are studied using first-principles density-functional theory. In particular, Gruneisen parameter, thermal expansion coefficients, Debye temperature and temperature dependent heat capacities are determined using quasiharmonic approximation. The Gruneisen parameter dispersion curves are obtained from the variation of phonon frequencies with volume. The frequencies of zone center phonon modes are calculated to be 348.2 cm −1 (IR active) and 347.1 cm −1 (Raman active). The calculated linear thermal expansion coefficient (∼9.1 × 10 −6 /K) and Debye temperature (∼547 K) are found to be in good agreement with reported experimental values. Further, the thermodynamic Gruneisen parameter is calculated to be 1.66. The presented results are expected to generate further interest in experimental investigations of thermal properties of CoSi 2 .

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 BiFeO3 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 Neel 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.

Ti alloys are one of the most commonly used structural biomaterials due to their unique properties, such as high specific strength, superior biocompatibility, excellent corrosion, and adequate mechanical properties. The present work involves designing and fabrication of suitable ternary Ti-Nb-Sn alloy compositions, with low elastic modulus, through a combination of Density Functional Theory (DFT) calculations and powder metallurgy route. Designed alloys consist of stable Ti-Nb-Sn system with low elastic modulus that can reduce 'stress shielding effect'. These ternary Ti-Nb-Sn alloys were characterized using XRD for phase analysis, SEM-EBSD for microstructural characterization and tensile test for mechanical properties. It is envisaged that these designed Ti-Nb-Sn alloy compositions using first principles calculations and fabricated through P/M route would be a potentially good candidate for orthopedic implants.