Showing papers by "S. Shahab Naghavi published in 2019"

Elastic properties and stability of Heusler compounds: Cubic Co2YZ compounds with L21 structure

Abstract: Elastic constants and their derived properties of various cubic Heusler compounds were calculated using the first-principles density functional theory. To begin with, Cu 2MnAl is used as a case study to explain the interpretation of the basic quantities and compare them with experiments. The main part of the work focuses on Co 2-based compounds that are Co 2Mn M with the main group elements M = Al, Ga, In, Si, Ge, Sn, Pb, Sb, Bi, and Co 2 T M with the main group elements Al or Si, and the 3 d transition metals T = Sc, Ti, V, Cr, Mn, and Fe. It is found that many properties of Heusler compounds correlate to the mass or nuclear charge Z of the main group element. Additional representation and compact simplification of the elastic data is useful to investigate and compare their influence on crystal stability and physical properties. Here, Blackman’s and Every’s diagrams are used to compare the elastic properties of the materials, whereas Pugh’s and Poisson’s ratios are used to analyze the relationship between interatomic bonding and physical properties. It is found that Pugh’s criterion on brittleness needs to be revised whereas Christensen’s criterion describes the ductile–brittle transition of Heusler compounds very well. The calculated elastic properties give hint on a metallic bonding with an intermediate brittleness for the studied Heusler compounds. The universal anisotropy of the stable compounds has values in the range of 0.57 < A U < 2.73. The compounds with higher A U values are found close to the middle of the transition metal series. In particular, Co 2ScAl with A U = 0.01 is predicted to be an isotropic material that comes closest to an ideal Cauchy solid as compared to the remaining Co 2-based compounds. Apart from the elastic constants and moduli, the sound velocities, Debye temperatures, and hardness are predicted and discussed for the studied systems. The calculated slowness surfaces for sound waves reflect the degree of anisotropy of the compounds.

Designing chemical analogs to PbTe with intrinsic high band degeneracy and low lattice thermal conductivity.

Abstract: High-efficiency thermoelectric materials require simultaneously high power factors and low thermal conductivities. Aligning band extrema to achieve high band degeneracy, as realized in PbTe, is one of the most efficient approaches to enhance power factor. However, this approach usually relies on band structure engineering, e.g., via chemical doping or strain. By employing first-principles methods with explicit computation of phonon and carrier lifetimes, here we show two full-Heusler compounds Li2TlBi and Li2InBi have exceptionally high power factors and low lattice thermal conductivities at room temperature. The expanded rock-salt sublattice of these compounds shifts the valence band maximum to the middle of the Σ line, increasing the band degeneracy by a factor of three. Meanwhile, resonant bonding in the PbTe-like sublattice and soft Tl-Bi (In-Bi) bonding interaction is responsible for intrinsic low lattice thermal conductivities. Our results present an alternative strategy of designing high performance thermoelectric materials.

Achieving an Ultrahigh Power Factor in Sb2Te2Se Monolayers via Valence Band Convergence.

Abstract: An efficient approach to improve the thermoelectric performance of materials is to converge their electronic bands, which is known as band engineering. In this regard, lots of effort has been made to further improve the thermoelectric efficiency of bulk and exfoliated monolayers of Bi2Te3 and Sb2Te3. However, ultrahigh band degeneracy and thus significant improvement of the power factor have not yet been realized in these materials. Using first-principles methods, we demonstrate that the valley degeneracy of Bi2Te3 and Sb2Te3 can be largely improved upon substitution of the middle-layer Te atoms with the more electronegative S or Se atoms. Our detailed analysis reveals that in this family of materials, two out of four possible valence band valleys merely depend on the electronegativity of the middle-layer chalcogen atoms, which makes the independent modulation of the valleys' position feasible. As such, band alignment of Bi2Te3 and Sb2Te3 largely improves upon substitution of the middle-layer Te atoms with more electronegative, yet chemically similar, S and Se ones. A superior valence band alignment is attained in Sb2Te2Se monolayers where three out of four possible valleys are well aligned, resulting in a giant band degeneracy of 18 that holds the record among all thermoelectric materials. As a result, an outstanding power factor for the hole-doped monolayers is achieved, indicating a highly efficient p-type thermoelectric material.

Phenomenal Observation of Attractive Intermolecular CH⋯HC Interaction in a Mercury (II) Complex: An Experimental and First-Principles Study

Abstract: The nature of the attractive intermolecular C@H…H@C interaction, which could affect the crystal packing and solid-state molecular structure, is yet unknown. Here, a novel mercury (II) complex including N-(2-biphenyl)pyrazine-2-carboxamide ligand, one such system, has been synthesized and characterized by a single crystal X-ray diffraction. The existence of attractive intermolecular C@H⋯H@C interaction (-2.64 to @9.30 kj/mol depending on computational levels) is a notable feature in the crystal packing of this complex, which is the first observation of intermolecular C@H⋯H@C interaction in a metal complex. From crystallographic data, this contact has a distance of 2.172 A which is 9.5% shorter than the sum of the van der Waals radii of two hydrogen atoms, which is the primary condition of having intermolecular interactions. We study the nature C@H…H@C interaction in the synthesized mercury (II) complex using periodic/non-periodic density functional theory in conjunction with quantum theory of atoms in molecules, non-covalent interaction reduced density gradient method, natural bond orbital, and energy decomposition analysis tools. Our results suggest that C@H⋯H@C interaction has closed-shell, donoracceptor, and van der Waals nature.

Achieving ultra-high power factor in Sb2Te2Se via valence band convergence

Abstract: An efficient approach to improve the thermoelectric performance of materials is to converge their electronic bands, which is known as band engineering. In this regard, lots of effort have been made to further improve the thermoelectric efficiency of bulk and exfoliated monolayers of Bi$_{2}$Te$_{3}$ and Sb$_{2}$Te$_{3}$. However, ultra-high band degeneracy and thus significant improvement of power factor have not been yet realized in these materials. Using first-principles methods, we demonstrate that the valley degeneracy of Bi$_{2}$Te$_{3}$ and Sb$_{2}$Te$_{3}$ can be largely improved upon substitution of the middle layer Te atoms with the more electronegative S or Se atoms. Our detailed analysis reveals that in this family of materials two out of four possible valence band valleys merely depend on the electronegativity of the middle layer chalcogen atoms, which makes the independent modulation of the valleys position feasible. As such, band alignment of Bi$_{2}$Te$_{3}$ and Sb$_{2}$Te$_{3}$ largely improves upon substitution of the middle layer Te atoms with more electronegative, yet chemically similar, S and Se ones. A superior valence band alignment is attained in Sb$_{2}$Te$_{2}$Se monolayers where the three out of four possible valleys are well-aligned, resulting in a giant band degeneracy of 18 that holds the record among all thermoelectric materials.