Comprehensive DFT investigation of transition-metal-based new quaternary Heusler alloys CoNbMnZ (Z = Ge,Sn): compatible for spin-dependent and thermoelectric applications

The hunt for high spin polarization and efficient thermoelectric materials has endured for decades. In this paper, we have explored the structural, mechanical stability, magneto-electronic, and thermoelectric properties of two new quaternary Heusler alloys, CoNbMnZ (Z = Ge, Sn), using first-principles simulation methods. The alloys are stable, showing a Y₁-type phase and ferromagnetic nature. Based on a generalized gradient approximation method, the alloys exhibit metallic nature; upon employing a modified version of the Becke–Johnson potential, both alloys demonstrate half-metallic nature, with gaps of 0.43 and 0.45 eV, which is a precursor for high spin polarization in these alloys. The alloys also follow the necessary Slater–Pauling rule condition M_T = Z_T − 24 for half-metallicity and they have a total magnetic moment of 1 μ_B. Elastic parameters convey the mechanical stabilities of these alloys, with Debye temperatures of 518 K and 445 K. These materials act as anisotropic media with respect to longitudinal and transverse sound velocities. Possible energy efficiency and thermoelectric applications were scrutinized via computing Seebeck coefficients, electrical and electronic lattice thermal conductivities, and, lastly, power factors. The highest S values for Ge- and Sn-based alloys are 60.43 and 68.2 μV K⁻¹, respectively, and the highest power factors are 32 and 35 μW K⁻² cm⁻¹, respectively, suggesting potential efficient applications in thermoelectric power generation.

Possible d–d hybridization for characterizing the electronic profile of the materials.     

Recent advances in transition metal-free catalytic hydroelementation (E = B,Si, Ge,and Sn) of alkynes

Catalytic hydroelementation of alkynes mainly with hydroboranes and hydrosilanes gives a straightforward and atom-economical access to a wide range of vinylmetalloids, which are used as synthetically useful and/or reactive species in both synthetic and materials chemistry. Thus far, although numerous transition metal catalysts with well-defined ligand systems have been developed for alkyne hydroelementation, the employed catalysts are mainly based on expensive and potentially toxic metals such as Rh, Pt, and Ir, and their conventional inner-sphere hydride transfer pathways are susceptible to reaction systems, often making it difficult to control the selectivity. In this regard, transition metal-free catalysts for hydroelementation (E = B, Si, etc.) have intensively been reported as an alternative to the conventional metal catalytic regimes over the last decade. In this review, we describe the recent advances in transition metal-free catalytic procedures for alkyne hydroelementation using hydrides based on Si, B, Sn, and Ge with strong emphasis on the variation in the catalytic working mode depending on the intrinsic nature of the reaction systems.

This review describes the recent advances in the transition metal-free hydroelementation of alkynes with various metalloid hydrides.     

Electronic structure and thermoelectric properties of full Heusler compounds Ca2YZ (Y = Au,Hg; Z = As,Sb, Bi,Sn and Pb)

We investigate the transport properties of bulk Ca₂YZ (Y = Au, Hg; Z = As, Sb, Bi, Sn and Pb) by a combination method of first-principles and Boltzmann transport theory. The focus of this article is the systematic study of the thermoelectric properties under the effect of a spin–orbit coupling. The highest dimensionless figure of merit (ZT) of Ca₂AuAs at optimum carrier concentration are 1.23 at 700 K. Interestingly enough, for n-type Ca₂HgPb, the maximum ZT are close to each other from 500 K to 900 K and these values are close to 1, which suggests that semimetallic material can also be used as an excellent candidate for thermoelectric materials. From another viewpoint, at room temperature, the maximum PF for Ca₂YZ are greater than 3 mW m⁻¹ K⁻², which is very close to that of ∼3 mW m⁻¹ K⁻² for Bi₂Te₃ and ∼4 mW m⁻¹ K⁻² for Fe₂VAl. However, the room temperature theoretical κ_l of Ca₂YZ is only about 0.85–1.6 W m⁻¹ K⁻¹, which is comparing to 1.4 W m⁻¹ K⁻¹ for Bi₂Te₃ and remarkably lower than 28 W m⁻¹ K⁻¹ for Fe₂VAl at same temperature. So Ca₂YZ should be a new type of promising thermoelectric material at room temperature.

We investigate the transport properties of bulk Ca₂YZ (Y = Au, Hg; Z = As, Sb, Bi, Sn and Pb) by a combination method of first-principles and Boltzmann transport theory.     

A case study of Fe2TaZ (Z = Al,Ga, In) Heusler alloys: hunt for half-metallic behavior and thermoelectricity

We have computed the electronic structure and transport properties of Fe₂TaZ (Z = Al, Ga, In) alloys by the full-potential linearized augmented plane wave (FPLAPW) method. The magnetic conduct in accordance with the Slater–Pauling rule classifies them as non-magnetic alloys with total zero magnetic moment. The semiconducting band profile and the density of states in the post DFT treatment are used to estimate the relations among various transport parameters such as Seebeck coefficient, electrical conductivity, thermal conductivity, and figure of merit. The Seebeck coefficient variation and band profiles describe the p-type behavior of charge carriers. The electrical and thermal conductivity plots follow the semiconducting nature of bands along the Fermi level. The overall measurements show that semi-classical Boltzmann transport theory has well-behaved potential in predicting the transport properties of such functional materials, which may find the possibility of their experimental synthesis for future applications in thermoelectric technologies.

Crystal structure in conventional unit cell for Fe₂TaZ (Z = Al, Ga, In) in Fm$\bar{3}$m configuration.     

Structural exploration of AuxM− (M = Si,Ge, Sn; x = 9–12) clusters with a revised genetic algorithm

We used a revised genetic algorithm (GA) to explore the potential energy surface (PES) of Au_xM⁻ (x = 9–12; M = Si, Ge, Sn) clusters. The most interesting finding in the structural study of Au_xSi⁻ (x = 9–12) is the 3D (Au₉Si⁻ and Au₁₀Si⁻) → quasi-planar 2D (Au₁₁Si⁻ and Au₁₂Si⁻) structural evolution of the Si-doped clusters, which reflects the competition of Au–Au interactions (forming a 2D structure) and Au–Si interactions (forming a 3D structure). The Au_xM⁻ (x = 9–12; M = Ge, Sn) clusters have quasi-planar structures, which suggests a lower tendency of sp³ hybridization and a similarity of electronic structure for the Ge or Sn atom. Au₉Si⁻ and Au₁₀Si⁻ have a 3D structure, which can be viewed as being built from Au₈Si⁻ and Au₉Si⁻ with an extra Au atom bonded to a terminal gold atom, respectively. In contrast, the quasi-planar structures of Au_xM⁻ (x = 9–12; M = Ge, Sn) reflect the domination of the Au–Au interactions. Including the spin–orbit (SO) effects is very important to calculate the simulated spectrum (structural fingerprint information) in order to obtain quantitative agreement between theoretical and future experimental PES spectra.

We used a revised genetic algorithm (GA) to explore the potential energy surface (PES) of Au_xM⁻ (x = 9–12; M = Si, Ge, Sn) clusters.     

Bond-photon-phonon thermal relaxation in the M(X,X2) (M = Mo,Re, Ta,Ge, Sn; X = S,Se, and Te)

We systematically investigated the temperature-dependent bandgap energy and Raman shift on the bond length and bond energy, Debye temperature, and atomic cohesive energy for M(X, X₂) via bond relaxation methods. It is revealed that the thermal decay of both bandgap energy and phonon frequency arose from the thermal integration of the specific heat of Debye approximation. The results indicate that (i) the bandgap energy relaxation is due to the thermal excitation-induced weakening of the bond energy, and the phonon frequency was just a function of bond length and bond energy; (ii) the Debye temperature determines the nonlinear range at low temperatures; (iii) the reciprocal of the atomic cohesive energy governs the linear behavior at high temperatures. Thus, the outcomes of this study include fundamental information about photon, phonon, and the thermal properties of layered semiconductors, which are crucial to develop the new generations of thermal and electronic applications of devices based on layered semiconductors.

We systematically investigated the temperature-dependent bandgap energy and Raman shift on the bond length and bond energy, Debye temperature, and atomic cohesive energy for M(X, X₂) via bond relaxation methods.     

The mechanical,electronic, optical and thermoelectric properties of two-dimensional honeycomb-like of XSb (X = Si,Ge, Sn) monolayers: a first-principles calculations

Herein, by using first-principles calculations, we demonstrate a two-dimensional (2D) of XSb (X = Si, Ge, and Sn) monolayers that have a honey-like crystal structure. The structural, mechanical, electronic, thermoelectric efficiency, and optical properties of XSb monolayers are studied. Ab initio molecular dynamic simulations and phonon dispersion calculations suggests their good thermal and dynamical stabilities. The mechanical properties of XSb monolayers shows that the monolayers are considerably softer than graphene, and their in-plane stiffness decreases from SiSb to SnSb. Our results shows that the single layers of SiSb, GeSb and SnSb are semiconductor with band gap of 1.48, 0.77 and 0.73 eV, respectively. The optical analysis illustrate that the first absorption peaks of the SiSb, GeSb and SnSb monolayers along the in-plane polarization are located in visible range of light which may serve as a promising candidate to design advanced optoelectronic devices. Thermoelectric properties of the XSb monolayers, including Seebeck coefficient, electrical conductivity, electronic thermal conductivity, power factor and figure of merit are calculated as a function of doping level at temperatures of 300 K and 800 K. Between the studied two-dimensional materials (2DM), SiSb single layer may be the most promising candidate for application in the thermoelectric generators.

Herein, by using first-principles calculations, we demonstrate a two-dimensional (2D) of XSb (X = Si, Ge, and Sn) monolayers that have a honey-like crystal structure.     

Correction: The mechanical,electronic, optical and thermoelectric properties of two-dimensional honeycomb-like of XSb (X = Si,Ge, Sn) monolayers: a first-principles calculations

Correction for ‘The mechanical, electronic, optical and thermoelectric properties of two-dimensional honeycomb-like of XSb (X = Si, Ge, Sn) monolayers: a first-principles calculations’ by Asadollah Bafekry et al., RSC Adv., 2020, 10, 30398–30405, DOI: 10.1039/D0RA05587E.

The authors regret that the name of one of the authors (Fazel Shojaei) was shown incorrectly in the original article. The corrected author list is as shown above.The Royal Society of Chemistry apologises for these errors and any consequent inconvenience to authors and readers.     

Bonding and stability of dinitrogen-bonded donor base-stabilized Si(0)/Ge(0) species [(cAACMe–Si/Ge)2(N2)]: EDA-NOCV analysis

Recently, dinitrogen (N₂) binding and its activation have been achieved by non-metal compounds like intermediate cAAC-borylene as (cAAC)₂(B-Dur)₂(N₂) [cAAC = cyclic alkyl(amino) carbene; Dur = aryl group, 2,3,5,6-tetramethylphenyl; B-Dur = borylene]. It has attracted a lot of scientific attention from different research areas because of its future prospects as a potent species towards the metal free reduction of N₂ into ammonia (NH₃) under mild conditions. Two (cAAC)(B-Dur) units, each of which possesses six valence electrons around the B-centre, are shown to accept σ-donations from the N₂ ligand (B ← N₂). Two B-Dur further provide π-backdonations (B → N₂) to a central N₂ ligand to strengthen the B–N₂–B bond, providing maximum stability to the compound (cAAC)₂(B-Dur)₂(N₂) since the summation of each pair wise interaction accounted for the total stabilization energy of the molecule. (cAAC)(B-Dur) unit is isolobal to cAAC–E (E = Si, Ge) fragment. Herein, we report on the stability and bonding of cAAC–E bonded N₂-complex (cAAC–E)₂(N₂) (1–2; Si, Ge) by NBO, QTAIM and EDA-NOCV analyses (EDA-NOCV = energy decomposition analysis coupled with natural orbital for chemical valence; QTAIM = quantum theory of atoms in molecule). Our calculation suggested that syntheses of elusive (cAAC–E)₂(N₂) (1–2; Si, Ge) species may be possible with cAAC ligands having bulky substitutions adjacent to the C_cAAC atom by preventing the homo-dimerization of two (cAAC)(E) units which can lead to the formation of (cAAC–E)₂. The formation of E E bond is thermodynamically more favourable (E = Si, Ge) over binding energy of N₂ inbetween two cAAC–E units.

Dinitrogen (N₂) binding and its activation have been achieved by non-metal compounds like intermediate cAACborylene with the general formula of (cAAC)₂(B-Dur)₂(N₂) [cAAC = cyclic alkyl(amino)carbene; Dur = aryl group, 2,3,5,6-tetramethylphenyl; B-Dur = aryl-borylene].     

Atomic and electronic structure of solids of Ge2Br2PN,Ge2I2PN,Sn2Cl2PN,Sn2Br2PN and Sn2I2PN inorganic double helices: a first principles study

We report the results of density functional theory calculations on the atomic and electronic structure of solids formed by assembling A₂B₂PN (A = Ge and Sn, B = Cl, Br, and I) inorganic double helices. The calculations have been performed using a generalized gradient approximation for the exchange–correlation functional and including van der Waals interactions. Our results show that the double helices crystallize in a monoclinic lattice with van der Waals type weak interactions between the double helices. In all cases except Ge₂Cl₂PN, the solids are stable with a binding energy between the double helices ranging from 0.06 eV per atom to 0.09 eV per atom and inter-double helices separation of more than 3.33 Å. All the solids are semiconducting. Further calculations have been done by using meta-GGA with a modified Becke–Johnson functional to obtain better band gaps, which are found to lie in the range of 0.91 eV to 1.49 eV. In the case of Ge₂Br₂PN the solid is a direct band gap semiconductor although the isolated double helix has an indirect band gap and it is suggested to be interesting for photovoltaic, and other optoelectronic applications. The charge transfer between the atoms has been studied using Bader charge analysis and the DDEC6 method in the CHARGEMOL program, which suggests charge transfer from the outer helix to the inner helix.

We report the results of density functional theory calculations on the atomic and electronic structure of solids formed by assembling A₂B₂PN (A = Ge and Sn, B = Cl, Br, and I) inorganic double helices.     

Structural,elastic and optoelectronic properties of inorganic cubic FrBX3 (B = Ge,Sn; X = Cl,Br, I) perovskite: the density functional theory approach

Inorganic metal-halide cubic perovskite semiconductors have become more popular in industrial applications of photovoltaic and optoelectronic devices. Among various perovskites, lead-free materials are currently most explored due to their non-toxic effect on the environment. In this study, the structural, electronic, optical, and mechanical properties of lead-free cubic perovskite materials FrBX₃ (B = Ge, Sn; X = Cl, Br, I) are investigated through first-principles density-functional theory (DFT) calculations. These materials are found to exhibit semiconducting behavior with direct bandgap energy and mechanical phase stability. The observed variation in the bandgap is explained based on the substitutions of cations and anions sitting over B and X-sites of the FrBX₃ compounds. The high absorption coefficient, low reflectivity, and high optical conductivity make these materials suitable for photovoltaic and other optoelectronic device applications. It is observed that the material containing Ge (germanium) in the B-site has higher optical absorption and conductivity than Sn containing materials. A systematic analysis of the electronic, optical, and mechanical properties suggests that among all the perovskite materials, FrGeI₃ would be a potential candidate for optoelectronic applications. The radioactive element Fr-containing perovskite FrGeI₃ may have applications in nuclear medicine and diagnosis such as X-ray imaging technology.

Inorganic metal-halide cubic perovskite semiconductors have become more popular in industrial applications of photovoltaic and optoelectronic devices.     

High efficiency Mg2(Si,Sn)-based thermoelectric materials: scale-up synthesis,functional homogeneity,and thermal stability

Considering the need for large quantities of high efficiency thermoelectric materials for industrial applications, a scalable synthesis method for high performance magnesium silicide based materials is proposed. The synthesis procedure consists of a melting step followed by high energy ball milling. All the materials synthesized via this method demonstrated not only high functional homogeneity but also high electrical conductivity and Seebeck coefficients of around 1000 Ω⁻¹ cm⁻¹ and −200 μV K⁻¹ at 773 K, respectively. The measured values were similar for all the samples extracted from the ∅50 mm and ∅70 mm compacted pellets and were stable upon thermal cycling. Thermal stability experiments from 168 hours to 720 hours at 723 K revealed no significant change in the material properties. The low thermal conductivity of ∼2.5 W m⁻¹ K⁻¹ at 773 K led to a maximum figure of merit, zT_max, of 1.3 at the same temperature and an average value, zT_avg, of 0.9 between 300 K and 773 K, which enables high efficiency in future silicide-based thermoelectric generators.

Considering the need for large quantities of high efficiency thermoelectric materials for industrial applications, a scalable synthesis method for high performance magnesium silicide based materials is proposed.     

Study of half metallicity,structural and mechanical properties in inverse Heusler alloy Mn2ZnSi(1−x)Gex and a superlattice

The electronic and magnetic properties of Mn₂ZnSi_(1−x)Ge_x (x = 0.0, 0.125, 0.25, 0.375, 0.5, 0.625, 0.75, 0.875, and 1.0) inverse Heusler alloys and Mn₂ZnSi/Mn₂ZnGe superlattice have been investigated using first-principles calculations. All these alloys are stable in the fcc magnetic phase and satisfies the mechanical and thermal stability conditions as determined from the elastic constants and negative formation energy. The spin-polarized electronic band structures and the density of states indicate half-metallicity with 100% spin polarization at the Fermi energy level for x = 0.0, 0.125, 0.25, 0.50, and 1.0, with the integral values of the total magnetic moments per formula unit at their equilibrium lattice constants, following the Slater–Pauling rule. The electronic properties and the magnetic moments are mostly contributed by two Mn atoms and are coupled anti-parallel to each other, making them ferrimagnetic in nature. The presence of the half-metallic bandgap with an antiparallel alignment of Mn atoms makes these Heusler alloys a potential candidate for spintronic applications.

The electronic and magnetic properties of Mn₂ZnSi_(1−x)Ge_x (x = 0.0, 0.125, 0.25, 0.375, 0.5, 0.625, 0.75, 0.875, and 1.0) inverse Heusler alloys and Mn₂ZnSi/Mn₂ZnGe superlattice have been investigated using first-principles calculations.     

First-principles study on structural,electronic and optical properties of perovskite solid solutions KB1−xMgxI3 (B = Ge,Sn) toward water splitting photocatalysis

Perovskite materials have been recently attracting a great amount of attention as new potential photocatalysts for water splitting hydrogen evolution. Here, we propose lead-free potassium iodide perovskite solid solutions KBI₃ with B-site mixing between Ge/Sn and Mg as potential candidates for photocatalysts based on systematic first-principles calculations. Our calculations demonstrate that these solid solutions, with proper Goldschmidt and octahedral factors for the perovskite structure, become stable by configurational entropy at finite temperature and follow Vegard’s law in terms of lattice constant, bond length and elastic constants. We calculate their band gaps with different levels of theory with and without spin–orbit coupling, revealing that the hybrid HSE06 method yields band gaps increasing along the quadratic function of Mg content x. Moreover, we show that the solid solutions with 0.25 ≤ x ≤ 0.5 have appropriate band gaps between 1.5 and 2.2 eV, reasonable effective masses of charge carriers, and suitable photoabsorption coefficients for absorbing sunlight. Among the solid solutions, KB_0.5Mg_0.5I₃ (B = Ge, Sn) is found to have the most promising band edge alignment with respect to the water redox potentials with different pH values, motivating experimentalists to synthesize them.

We propose lead-free potassium iodide perovskite solid solutions KBI₃ with B-site mixing between Ge/Sn and Mg as potential candidates for photocatalysts based on systematic first-principles calculations.     

DFT study on the adsorption of 5-fluorouracil on B40, B39M,and M@B40 (M = Mg,Al, Si,Mn, Cu,Zn)

Based on density functional theory, the adsorption behavior of 5-fluorouracil (5-Fu) on B₄₀ and its derivatives has been explored. It was observed that 5-Fu prefers to combine with the corner boron atom of the B₄₀ cage via one of its oxygen atoms, forming a strong polar covalent B–O bond. The adsorption energy of 5-Fu on B₄₀ was calculated to be −11.15 kcal mol⁻¹, and thus, it can be duly released from B₄₀ by protonation in the slightly acidic environment of tumor tissue, which makes for reducing the toxic and side effects of this drug. Additionally, the substituent and embedding effect of Mg, Al, Si, Mn, Cu, and Zn atoms on the drug delivery performance of B₄₀ have been also considered. We hope this work could offer some implications for the potential application of boron-based nanomaterials, such as B₄₀ in drug delivery.

The adsorption of 5-fluorouracil on B₄₀ and its derivatives has been theoretically studied to provide some implications for the potential application of B₄₀ in drug delivery.     

Nano ferrites (AFe2O4, A = Zn,Co, Mn,Cu) as efficient catalysts for catalytic ozonation of toluene

Nano ferrites (AFe₂O₄, A = Zn, Co, Mn, Cu) were supported on the surface of γ-Al₂O₃ support by hydrothermal synthesis to prepare a series of novel composite catalysts (AFe₂O₄/γ-Al₂O₃) for catalytic ozonation for elimination of high concentration toluene at ambient temperature. The characterization results showed that the high-purity nano-AFe₂O₄ particles were uniformly loaded on mesoporous γ-Al₂O₃. Further, it was confirmed that among the several catalysts prepared, the amount of oxygen vacancies (O_vs), Lewis acid sites (LAS), and Brønsted acid sites (BAS) of the ZnFe₂O₄/γ-Al₂O₃ catalyst were the highest. This meant that the ZnFe₂O₄/γ-Al₂O₃ catalyst had a strong adsorption capacity for toluene and ozone (O₃), and had a strong catalytic activity. When the temperature was 293 K and the space velocity was 1500 h⁻¹, the mol ratio of O₃ to toluene was 6, the degradation rate of toluene (600 mg m⁻³) can reach an optimum of 99.8%. The results of electron paramagnetic resonance (EPR) and Fourier infrared (FT-IR) proved superoxide radicals and hydroxyl radicals by catalytic ozonation. Moreover, the GC-MS analysis results indicated that the toluene degradation began with the oxidation of methyl groups on the benzene ring, eventually producing CO₂ and H₂O. After repeated experiments, the toluene degradation rate remained stable, and the residual content of O₃ in each litre of produced gas was less than 1 mg L⁻¹, thereby indicating that the ZnFe₂O₄/γ-Al₂O₃ catalyst had excellent reusability and showed great potential for the treatment of toluene waste gas.

Nano ferrites (AFe₂O₄, A = Zn, Co, Mn, Cu) were supported on the surface of γ-Al₂O₃ by hydrothermal synthesis to prepare a series of novel catalysts (AFe₂O₄/γ-Al₂O₃) for catalytic ozonation of high concentration toluene at ambient temperature.     

Electronic,magnetic, optical and thermoelectric properties of co-doped Sn1−2xMnxAxO2 (A = Mo,Tc): a first principles insight

The electronic, magnetic, optical and thermoelectric (TE) properties of Sn_1−2xMn_xA_xO₂ (A = Mo/Tc) have been examined using density functional theory (DFT) based on the FP-LAPW approach. The results suggested that all the doped compounds show a half-metallic ferromagnet property with a 100% spin polarization at the Fermi level within GGA and mBJ. Moreover, doping SnO₂ with double impurities reduces the bandgap. The reduced bandgaps are the result of impurity states which arise due to the Mn and Mo/Tc doping, leading to the shifts of the minima of the conduction band towards the Fermi energy caused by substantial hybridization between transition metals 3d–4d and O-2p states. Also, the (Mn, Mo) co-doped SnO₂ system exhibits a ferromagnetic ground state which may be explained by the Zener double exchange mechanism. While the mechanism that controls the ferromagnetism in the (Mn, Tc) co-doped SnO₂ system is p–d hybridization. Therefore, the role of this study is to illustrate the fact that half-metallic ferromagnet material is a good absorber of sunlight (visible range) and couples to give a combined effect of spintronics with optronics. Our analysis shows that Sn_1−2xMn_xMo_xO₂ and Sn_1−2xMn_xTc_xO₂ are more capable of absorbing sunlight in the visible range compared to pristine SnO₂. In addition, we report a significant result for the thermoelectric efficiency ZT of ∼0.114 and ∼0.11 for Sn_1−2xMn_xMo_xO₂ and Sn_1−2xMn_xTc_xO₂, respectively. Thus, the coupling of these magnetic, optical, and thermoelectric properties in (Mn, A = Mo or Tc) co-doped SnO₂ can predict that these materials are suitable for optoelectronic and thermoelectric systems.

The electronic, magnetic, optical and thermoelectric properties of Sn_1−2xMn_xA_xO₂ (A = Mo/Tc) have been examined using density functional theory (DFT) based on the FP-LAPW approach.     

DFT insights into Nb-based 211 MAX phase carbides: Nb2AC (A = Ga,Ge, Tl,Zn, P,In, and Cd)

In this study, we performed the first-principles calculations to study the 211 MAX phase carbides: Nb₂AC (A = Ga, Ge, Tl, Zn, P, In, Cd, and Al). The structural characteristics are in good agreement with those of the prior studies. The mechanical behavior has been explored by calculating the stiffness constants, elastic moduli, and Vickers hardness. The stiffness constants and phonon dispersion curves were used to check the structural stability of the selected compounds. 2D and 3D plotting of elastic moduli and calculated anisotropy indices disclosed the anisotropy of the elastic properties. We utilized the Mulliken atomic and bond overlap population to explain the mixture of ionic and covalent bonding among these carbides. The metallic behavior has been confirmed by calculating the band structure and density of states (DOS). Partial DOS was also used to discuss the bonding nature and strength among the different states. The optical properties of these phases have also been computed and analyzed to reveal possible relevance in diverse fields. The Debye temperature (Θ_D), Grüneisen parameter (γ), melting temperature (T_m), and minimum thermal conductivity (K_min) were studied to bring out their possible relevance in high-temperature technology. The outcomes of this research indicate that the titled carbides are suitable for use as solar radiation-protecting coating and thermal barrier coating (TBC) materials.

Comparison of (a) stiffness constants and (b) elastic moduli of Nb₂AC (A = Ga, Ge, Tl, Zn, P, In, Cd, and Al) MAX phases.     

Electronic and optical properties of orthorhombic (CH3NH3)BX3 (B = Sn,Pb; X = F,Cl, Br,I) perovskites: a first-principles investigation

Lead halide perovskites have generated considerable interest in solar cell, sensor, and electronics applications. While great focus has been placed on (CH₃NH₃)PbI₃, an organic–inorganic hybrid perovskite, comparatively little work has been done to understand some of its existing crystal phases and analogous materials after substituting with Sn and/or other halogens in the framework. Here, first-principles density functional theory calculations are performed to comprehensively evaluate the electronic and optical properties of (CH₃NH₃)BX₃ (B = Sn, Pb; X = F, Cl, Br, I) in a low-temperature orthorhombic phase. Bulk modulus, electronic structures, and several optical properties of these perovskite systems are further calculated. The obtained results are first confirmed by comparing with existing perovskite systems in literature. The shifting trends on those physical properties when extending to other barely studied systems of (CH₃NH₃)BX₃ is further revealed. The band gap of these perovskites is found to decrease when varying halogen anion in “X” sites from F to I, and/or substituting Pb cations with Sn in “B” sites. Notably, the less toxic Sn-containing perovskites, (CH₃NH₃)SnI₃ in particular, display higher absorption coefficients in the visible light range than their Pb-containing counterparts. An orthorhombic (CH₃NH₃)PbF₃ is predicted to exist at low temperature, and adsorb strongly UV energy. Our systematical examination efforts on the two groups of perovskites provide valuable physical insights in these materials, and the accompanied new findings warrant further investigation on such subjects.

The electronic and optical properties of orthorhombic (CH₃NH₃)BX₃ (B = Sn, Pb; X = F, Cl, Br, I) were investigated by first-principles density functional theory.     

Strain-induced structural phase transition,electric polarization and unusual electric properties in photovoltaic materials CsMI3 (M = Pb,Sn)

The structural phase transition, ferroelectric polarization, and electric properties have been investigated for photovoltaic films CsMI₃ (M = Pb, Sn) epitaxially grown along (001) direction based on the density functional theory. The calculated results indicate that the phase diagrams of two epitaxial CsPbI₃ and CsSnI₃ films are almost identical, except critical transition strains varying slightly. The epitaxial tensile strains induce two ferroelectric phases Pmc2₁, and Pmn2₁, while the compressive strains drive two paraelectric phases P2₁2₁2₁, P2₁2₁2. The larger compressive strain enhances the ferroelectric instability in these two films, eventually rendering them another ferroelectric state Pc. Whether CsPbI₃ or CsSnI₃, the total polarization of Pmn2₁ phase comes from the main contribution of B-position cations (Pb or Sn), whereas, for Pmc2₁ phase, the main contributor is the I ion. Moreover, the epitaxial strain effects on antiferrodistortive vector, polarization and band gap of CsMI₃ (M = Pb, Sn) are further discussed. Unusual electronic properties under epitaxial strains are also revealed and interpreted.

The structural phase transition, ferroelectric polarization, and electric properties have been investigated for photovoltaic films CsMI₃ (M = Pb, Sn) epitaxially grown along (001) direction based on the density functional theory.