arXiv subjects

H. Fjellvåg

Publications and source records attributed to H. Fjellvåg.

13 recordsLinked to original sources

arXiv2022

First principle studies on electronic and thermoelectric properties of Fe$_{2}$TiSn based multinary Heusler alloys

The alloys with 8/18/24 valence electron count (VEC) are promising candidates for efficient energy conversion and refrigeration applications at low as well as high temperatures. The full potential linearized augmented plane wave method as implemented in WIEN2k code was used to investigate electronic structure and TE transport properties with the PBE$-$GGA and TB$-$mBJ exchange potentials and Boltzmann transport theory. The calculated single crystal elastic constants, phonon dispersion and phonon density of states confirm that these systems are mechanically and dynamically stable. The TE transport properties is calculated by including the lattice part of thermal conductivity ($κ_{L}$) obtained from two methods one from the calculated elastic properties calculation ($κ^{elastic}_{L}$) and the other from phonon dispersion curve ($κ^{phonon}_{L}$). The strong phonon$-$phonon scattering by large mass difference/strain fluctuation of isovalent/aliovalent substitution at Ti/Sn sites of Fe$_{2}$TiSn reduces the lattice thermal conductivity which results in high \textit{ZT} value of 0.81 at 900\,K for Fe$_{2}$Sc$_{0.25}$Ti$_{0.5}$Ta$_{0.25}$Al$_{0.5}$Bi$_{0.5}$. The comparative analysis of TE transport properties using the band structures calculated with the PBE$-$GGA and TB$-$mBJ functional shows that the \textit{ZT} value obtained from TB$-$mBJ scheme is found to be significantly higher than that based on PBE$-$GGA. The calculated relatively low lattice thermal conductivity and high \textit{ZT} values suggest that isovalent/aliovalent substituted Fe$_{2}$TiSn are promising candidates for medium to high temperature waste heat recovery.

cond-mat.mtrl-sci↗

arXiv2020

The Jahn-Teller active fluoroperovskites $A\mathrm{CrF_3}$ $A=\mathrm{Na^+},\mathrm{K^+}$: thermo- and magneto optical correlations as function of the $A$-site

Chromium (II) fluoroperovskites $A\mathrm{CrF_3}(A\mathrm{=Na^+,K^+})$ are strongly correlated Jahn-Teller active materials at low temperatures. In this paper, we examine the role that the $A$-site ion plays in this family of fluoroperovskites using both experimental methods (XRD, optical absorption spectroscopy and magnetic fields) and DFT simulations. Temperature-dependent optical absorption experiments show that the spin-allowed transitions $E_2$ and $E_3$ only merge completely for $A$= Na at 2 K. Field-dependent optical absorption measurements at 2 K show that the oscillating strength of the spin-allowed transitions in $\mathrm{NaCrF_3}$ increases with increasing applied field. Direct magneto-structural correlations which suppress the spin-flip transitions are observed for ${\rm KCrF_3}$ below its Neél temperature. In ${\rm NaCrF_3}$ the spin-flip transitions vanish abruptly below 9 K revealing magneto-optical correlations not linked to crystal structure changes. This suggests that as the long range ordering is reduced local JT effects in the individual ${\rm CrF_6^{4-}}$ octahedra take control of the observed behavior. Our results show clear deviation from the pattern found for the isoelectronic $A_x{\rm MnF}_{3+x}$ system. The size of the $A$-site cation is shown to be central in dictating the physical properties and phase transitions in $A{\rm CrF}_3$, opening up the possibility of varying the composition to create novel states of matter with tuneable properties.

cond-mat.mtrl-sci↗

arXiv2016

First-principles study of structural stability, dynamical and mechanical properties of Li2FeSiO4 polymorphs

Li2FeSiO4 is an important alternative cathode for next generation Li-ion batteries due to its high theoretical capacity (330 mA h/g). However, its development has faced significant challenges arising from structural complexity and poor ionic conductivity. In the present work, the relative stability, electronic structure, thermodynamics, and mechanical properties of potential cathode material Li2FeSiO4 and its polymorphs have been studied by state-of-the-art density-functional calculations. Among the 11 structural arrangements considered for the structural optimization calculations, the experimentally known monoclinic P21 modification is found to be the ground state structure. The application of pressure originates a sequence of phase transitions according to P21 - Pmn21 - I222, and the estimated values of the critical pressure are found to be 0.38 and 1.93 GPa. The electronic structures reveal that all the considered polymorphs have a non-metallic character, with band gap values varying between 3.0 and 3.2 eV. The energy differences between different polymorphs are small, and most of these structures are dynamically stable. On the other hand, the calculation of single crystal elastic constants reveals that only few Li2FeSiO4 polymorphs are mechanically stable. At room temperature, the diffusion coefficient calculations of Li2FeSiO4 in different polymorphs reveal that Li-ion conductivity of this material is destitute.

cond-mat.mtrl-sci↗

arXiv2016

Search for potential precursors for Si-atomic layer deposition- a quantum chemical study

Thin film of silicon is an interesting material for many technological applications in electronic industry and in energy harvesting technologies, but requires a method for controlled growth of thin films. The purpose of this study is to screen a wide variety of Si content precursors for Si ALD reactions using state-of-the-art density-functional calculations. Among the studied 85 Si content precursors we found that C7H12OSi(Methoxy-trivinyl-silane) and C7H9NSi (Benzyliminosilane) show positive indications for ALD reactivity for Si deposition. We believe that this finding will be helpful to develop low-cost, high-energy efficiency thin-film solar cells for future scale up implementation in photovoltaics.

cond-mat.mtrl-sci↗

arXiv2013

Understanding H-defect complexes in ZnO

From state-of-the-art density-functional calculations using hybrid functionals we show that, persistent $n$-type conductivity in ZnO is due to defect complexes formed between H with intrinsic and extrinsic defects. H exhibits cationic, anionic, and electrically-inactive character on interacting with defects in ZnO. The electrically-inactive molecular hydrogen can contribute to $n$-type conductivity in ZnO by activating deep donor levels into shallow levels. By calculating local vibrational mode frequencies, we have identified origins of many H-related Raman and infra-red frequencies and thus confirmed the amphoteric behavior of H.

cond-mat.mtrl-sci↗

arXiv2007

Origin of magnetoelectric behavior in BiFeO$_3$

The magnetoelectric behavior of BiFeO$_3$ has been explored on the basis of accurate density functional calculations. The structural, electronic, magnetic, and ferroelectric properties of BiFeO$_3$ are predicted correctly without including strong correlation effect in the calculation. Moreover, the experimentally-observed elongation of cubic perovskite-like lattice along the [111] direction is correctly reproduced. At high pressure we predicted a pressure-induced structural transition and the total energy calculations at expanded lattice show two lower energy ferroelectric phases, closer in energy to the ground state phase. Band-structure calculations show that BiFeO$_3$ will be an insulator in A- and G-type antiferromagnetic phases and a metal in other magnetic configurations. Chemical bonding in BiFeO$_3$ has been analyzed using various tools and electron localization function analysis shows that stereochemically active lone-pair electrons at the Bi sites are responsible for displacements of the Bi atoms from the centro-symmetric to the noncentrosymmetric structure and hence the ferroelectricity. A large ferroelectric polarization (88.7 $μ$C/cm$^{2}$) is predicted in accordance with recent experimental findings. The net polarization is found to mainly ($>$ 98%) originate from Bi atoms. Moreover the large scatter in experimentally reported polarization values is due to the large anisotropy in the spontaneous polarization.

cond-mat.mtrl-sci↗

arXiv2004

Reply to comment on ``Electronic structure and structural stability study of Li$_3$AlH$_6$''

The nature of the bonding in Li$_3$AlH$_6$ has been re-examined with additional analyses using density-functional calculations. From partial density of states, charge density distribution, charge transfer, electron localization function, crystal orbital Hamilton population and Mulliken population analyses it is concluded that the interaction between Li and AlH$_6$ in Li$_3$AlH$_6$ is ionic as earlier advocated. Based on charge density distribution, electron localization function, and density of states analyses we earlier suggested that the interaction between Al and H is largely of the covalent type. However, the additional analyses indicate that the interaction between Al and H in the AlH$_6$ structural sub-units is of a mixed covalent ionic (iono-covalent) character.

cond-mat.mtrl-sci↗

arXiv2002

Pressure-induced structural transitions in MgH${_2}$

The stability of MgH$_2$ has been studied up to 20~GPa using density-functional total-energy calculations. At ambient pressure $α$-MgH${_2}$ takes a TiO$_2$-rutile-type structure. $α$-MgH$_2$ is predicted to transform into $γ$-MgH${_2}$ at 0.39~GPa. The calculated structural data for $α$- and $γ$-MgH${_2}$ are in very good agreement with experimental values. At equilibrium the energy difference between these modifications is very small, and as a result both phases coexist in a certain volume and pressure field. Above 3.84~GPa $γ$-MgH${_2}$ transforms into $β$-MgH${_2}$; consistent with experimental findings. Two further transformations have been identified at still higher pressure: i) $β$- to $δ$-MgH${_2}$ at 6.73 GPa and (ii) $δ$- to $ε$-MgH${_2}$ at 10.26~GPa.

cond-mat.mtrl-sci↗

arXiv2001

Electronic structure, phase stability and chemical bonding in Th$_2$Al and Th$_2$AlH$_4$

We present the results of theoretical investigation on the electronic structure, bonding nature and ground state properties of Th$_2$Al and Th$_2$AlH$_4$ using generalized-gradient-corrected first-principles full-potential density-functional calculations. Th$_2$AlH$_4$ has been reported to violate the "2 Årule" of H-H separation in hydrides. From our total energy as well as force-minimization calculations, we found a shortest H-H separation of 1.95 Å in accordance with recent high resolution powder neutron diffraction experiments. When the Th$_2$Al matrix is hydrogenated, the volume expansion is highly anisotropic, which is quite opposite to other hydrides having the same crystal structure. The bonding nature of these materials are analyzed from the density of states, crystal-orbital Hamiltonian population and valence-charge-density analyses. Our calculation predicts different nature of bonding for the H atoms along $a$ and $c$. The strongest bonding in Th$_2$AlH$_4$ is between Th and H along $c$ which form dumb-bell shaped H-Th-H subunits. Due to this strong covalent interaction there is very small amount of electrons present between H atoms along $c$ which makes repulsive interaction between the H atoms smaller and this is the precise reason why the 2 Å rule is violated. The large difference in the interatomic distances between the interstitial region where one can accommodate H in the $ac$ and $ab$ planes along with the strong covalent interaction between Th and H are the main reasons for highly anisotropic volume expansion on hydrogenation of Th$_2$Al.

cond-mat.mtrl-sci↗

arXiv2001

Spin, charge and orbital ordering in ferrimagnetic insulator YBaMn$_2$O$_5$

The oxygen-deficient (double) perovskite YBaMn$_2$O$_5$, containing corner-linked MnO$_5$ square pyramids, is found to exhibit ferrimagnetic ordering in its ground state. In the present work we report generalized-gradient-corrected, relativistic first-principles full-potential density-functional calculations performed on YBaMn$_2$O$_5$ in the nonmagnetic, ferromagnetic and ferrimagnetic states. The charge, orbital and spin orderings are explained with site-, angular momentum- and orbital-projected density of states, charge-density plots, electronic structure and total energy studies. YBaMn$_2$O$_5$ is found to stabilize in a G-type ferrimagnetic state in accordance with experimental results. The experimentally observed insulating behavior appears only when we include ferrimagnetic ordering in our calculation. We observed significant optical anisotropy in this material originating from the combined effect of ferrimagnetic ordering and crystal field splitting. In order to gain knowledge about the presence of different valence states for Mn in YBaMn$_2$O$_5$ we have calculated $K$-edge x-ray absorption near-edge spectra for the Mn and O atoms. The presence of the different valence states for Mn is clearly established from the x-ray absorption near-edge spectra, hyperfine field parameters and the magnetic properties study. Among the experimentally proposed structures, the recently reported description based on $P$4/$nmm$ is found to represent the stable structure.

cond-mat.str-el↗

arXiv2001

Itinerant metamagnetism and possible spin transition in LaCoO$_3$ by temperature/hole doping

The electronic structure of the perovskite La$_{1-x}$Sr$_x$CoO$_3$ has been obtained as a function of Sr substitution and volume from a series of generalized-gradient-corrected, full-potential, spin-density-functional band structure calculations. The energetics of different spin configurations are estimated using the fixed-spin-moment (FSM) method. From the total energy vs spin magnetic moment curve for LaCoO$_3$ the ground state is found to be nonmagnetic with the Co ions in a low-spin (LS) state, a result that is consistent with the experimental observations. Somewhat higher in energy, we find an intermediate-spin (IS) state with spin moment $\sim$1.2 $μ_{B}$/f.u. From the anomalous temperature dependent susceptibility along with the observation of an IS state we predict metamagnetism in LaCoO$_3$ originating from an LS-to-IS transition. The IS state is found to be metallic and the high-spin (HS) state of LaCoO$_3$ is predicted to be a half-metallic ferromagnet. With increasing temperature, which is simulated by a corresponding change of the lattice parameters we have observed the disappearence of the metamagnetic solution that is associated with the IS state. The FSM calculations on La$_{1-x}$Sr$_x$CoO$_3$ suggest that the hole doping stabilizes the IS state and the calculated magnetic moments are in good agreement with the corresponding experimental values. Our calculations show that the HS state cannot be stabilized by temperature or hole doping since the HS state is significantly higher in energy than the LS or IS state. Hence the spin-state transition in LaCoO$_3$ by temperature/hole doping is from an LS to an IS spin state and the present work rules out the other possibilities reported in the literature.

cond-mat.str-el↗

arXiv2001

Detailed electronic structure studies on superconducting MgB$_2$ and related compounds

In order to understand the unexpected superconducting behavior of MgB$_2$ compound we have made electronic structure calculations for MgB$_2$ and closely related systems. Our calculated Debye temperature from the elastic properties indicate that the average phonon frequency is very large in MgB$_2$ compared with other superconducting intermetallics and the exceptionally high $T_c$ in this material can be explained through BCS mechanism only if phonon softening occurs or the phonon modes are highly anisotropic. We identified a doubly-degenerate quasi-two dimensional key-energy band in the vicinity of $E_{F}$ along $Γ$-A direction of BZ which play an important role in deciding the superconducting behavior of this material. Based on this result, we have searched for similar kinds of electronic feature in a series of isoelectronic compounds such as BeB$_2$, CaB$_2$, SrB$_2$, LiBC and MgB$_2$C$_2$ and found that MgB$_2$C$_2$ is one potential material from the superconductivity point of view. There are contradictory experimental results regarding the anisotropy in the elastic properties of MgB$_2$ ranging from isotropic, moderately anisotropic to highly anisotropic. In order to settle this issue we have calculated the single crystal elastic constants for MgB$_2$ by the accurate full-potential method and derived the directional dependent linear compressibility, Young's modulus, shear modulus and relevant elastic properties. We have observed large anisotropy in the elastic properties. Our calculated polarized optical dielectric tensor shows highly anisotropic behavior even though it possesses isotropic transport property. MgB$_2$ possesses a mixed bonding character and this has been verified from density of states, charge density and crystal orbital Hamiltonian population analyses.

cond-mat.supr-con↗

arXiv2001

Ground state and excited state properties of LaMnO$_3$ from full-potential calculation

The ground state and excited state properties of the perovskite LaMnO$_3$ are calculated based on the generalized-gradient-corrected relativistic full-potential method. The electronic structure, magnetism and energetics of various spin configurations for LaMnO$_3$ in the ideal cubic perovskite structure and the experimentally observed distorted orthorhombic structure are obtained. The excited state properties such as the optical, magneto-optical, x-ray photoemission (XPS), bremsstrahlung isochromat (BIS), x-ray absorption near edge structure (XANES) spectra are calculated and found to be in excellent agreement with available experimental results. The present results suggest that the correlation effect is not significant in LaMnO$_3$ and the presence of ferromagnetic coupling within the $ab$ plane as well as the antiferromagnetic coupling perpendicular to this plane can be explained through the itinerant band picture. As against earlier expectations, our calculations show that the Mn 3$d$ $t_{2g}$ as well as the $e_{g}$ electrons are present in the whole valence band region. In particular significantly large amounts of $t_{2g}$ electrons are present in combination with the $e_{g}$ electrons at the top of the valence band against the common expectation of presence of only pure $e_{g}$ electrons. We have calculated the hyperfine field parameters for the A-type antiferromagnetic and the ferromagnetic phases of LaMnO$_3$ and compared the findings with the available experimental results. The role of the orthorhombic distortion on electronic structure, magnetism and optical anisotropy are analyzed.

cond-mat.str-el↗