ZB Phase Research Articles

Superdense Hexagonal BP and AlP with Quartz Topology: Crystal Chemistry and DFT Study

The superdense hexagonal phosphides BP and AlP, whose structures are formed by distorted tetrahedra and characterized by quartz-derived (qtz) topology, were predicted from crystal chemistry and first principles as potential high-pressure phases. From full geometry structure relaxations and ground state energy calculations based on quantum density functional theory (DFT), qtz BP and AlP were found to be less cohesive than the corresponding cubic zinc-blende (zb) phases with diamond-like (dia) topology, but were confirmed to be mechanically (elastic constants) and dynamically (phonons) stable. From the energy–volume equations of state, qtz phases were found to be energetically favorable at small volumes (high pressures), with zb-to-qtz transition pressures of 144 GPa for BP and 28 GPa for AlP. According to the electronic band structures and the site projected density of states, both phosphides exhibit larger band gaps of the zinc-blende phases compared to the qtz phases; the smaller values for the latter result from the smaller volumes per formula unit, leading to increased covalence.

Ab initio study of phase stability and optical properties of TiN and VN nitrides in different phases

Transition metal nitrides are ceramic materials known for their remarkable physical and mechanical characteristics, particularly their hardness. As part of this study, the structural, mechanical, electronic, phonon, and optical properties of nitrides were investigated. We focused on TiN in the NaCl–B1 and ZB-B3 phases and VN in the ZB-B3 using the density functional theory approach. The results show that the TiN equilibrium lattice constant in the NaCl–B1 phase is very close to what has already been reported. This confirms the overall reliability of the different phases. Furthermore, the heat of formation data suggests that TiN in both the NaCl–B1 and ZB-B3 phases is thermodynamically stable. The elastic constants confirm the mechanical stability of TiN in the NaCl–B1, ZB-B3, and VN in ZB phases. The bulk-to-shear ratio highlights that TiN in the NaCl–B1 phase exhibits brittleness, whereas TiN in the ZB-B3 phase displays ductility. Electronic band structure and total density of states calculations for both TiN in NaCl–B1 and ZB-B3 phases demonstrate their conductivity due to a pronounced valence-conduction overlap around the Fermi energy level. The low number of electrons accumulated at the Fermi energy level in the density of states (DOS) indicates electronic stability for TiN in the NaCl–B1 and ZB-B3 phases. The electron charge density plots indicate the presence of electrovalent (ionic) bonds between Ti-3d, V-3d, and N-2p. Additionally, the phonon dispersion curves reveal dynamical stability in both phases. Regarding optical properties in TiN, our predictions establish a distinct correlation between the loss function, reflectivity, and absorbance spectra within the 12.5 nm–25 nm energy range, with TiN in the NaCl phase exhibiting dominance. Notably, the NaCl phase exhibits higher absorption compared to the ZB phase, making it a promising candidate for use as a photocatalyst. Furthermore, the imaginary conductivity spectra align with the electronic properties, collectively indicating that these materials possess conductive characteristics.

The impact of anionic vacancies on the mechanical properties of NbC and NbN: An ab initio study

The development of super-hard materials has recently focused on systems containing heavy transition metal and light non-metallic elements. Niobium carbides and nitrides have previously been identified as potential candidates, however, the volatility of carbon and nitrogen during synthesis make them prone to formation of anionic vacancies, which have the ability of changing the electronic structure, dynamical stability and adversely affecting the mechanical properties. This study presents ab initio Density Functional Theory calculations that probe the occurrence of anionic vacancies as a function of concentration, thereafter, pertinent mechanical properties are investigated. Our results showed that the presence of anionic vacancies in NbC and NbN tend to deteriorate the mechanical properties and ultimately the mechanical hardness due to vacancy softening that can be attributed to defect induced covalent to metallic bond transition. Further, it was observed that anionic vacancies in NbC tend to modify its toughness; in particular, NbC in ZB structure becomes brittle while NbC in WZ phase becomes ductile in presences of C vacancies of up to 6%. The dynamical stability of NbC in RS, ZB, and WZ phases and NbN in WZ phase were found to be insensitive to the presence C and N vacancies, while NbN in RS and ZB becomes dynamically stable with the introduction of N vacancies. In addition, the toughness of NbN was found to be insensitive to defect concentration of even up to 8%. Consequently, stringent control of anionic defects during synthesis of NbC and NbN is critical for realization of the desired mechanical response that can make these materials ideal for super-hard and related applications.

First-Principles Study of the Physical Properties of Novel Polytypes of Gallium Phosphide

The polytypism of solids has been established as one of the most effective approaches to alter their physical properties. In this first-principles-based study, we practiced the polytypism of gallium phosphide (GaP) to evolve its physical properties for advanced technological applications. In this regard, new polytypes of GaP in wurtzite (wz)-, sphalerite (sp)-, beryllium oxide (β-BeO)-, and silicon carbide (SiC)-like structures have been generated by applying compressive and tensile pressure on its ground-state zinc blende (zb) structure. Our analysis shows its transformation to a SiC-like structure at a small pressure of 0.60 GPa. On the other hand, transition to wz-, sp-, and β-BeO-like structures has been realized from the zb phase of GaP by applying moderate tensile stress of −0.07, −0.79, and −3.65 GPa, respectively. Calculations of the cohesive energies of the new polytypes are found comparable to those of the zb phase. Similarly, the phonon band structures calculated for these new polytypes have been found free from the imaginary frequencies, indicating their adequate dynamical stability. Like the zb phase, the sp phase exhibited an indirect band gap, whereas wz-GaP, β-BeO-GaP, and SiC-GaP demonstrated direct band gaps. The computed energy gaps for the zb phase, wz phase, sp phase, β-BeO phase, and SiC phase amount to 2.359, 2.528, 1.740, 1.814, and 1.984 eV, respectively. On the other hand, these polytypes exhibited a remarkable absorption of the visible and ultraviolet (UV) light of the order of ∼106 cm–1, which is useful for photovoltaic applications. Investigations of the refractive indices reveal their transparency for light photons having energy less than 7.5 eV. We also estimated plasmon excitation energies as 16.97, 17.075, 16.97, 16.90, and 16.91 eV, respectively, for the zb phase, wz phase, sp phase, β-BeO phase, and SiC phase. Our predictions indicate novel polytypes of GaP as promising candidates for use in electronic and optoelectronic devices.

In situ phase transformation of polytypic zinc-blende/wurtzite copper indium sulfide via a facile polyol method to boost visible-light photocatalytic performance

A zinc-blende/wurtzite (ZB/WZ) copper indium sulfide (CuInS2/CIS) polymorph with high visible-light absorption ability and high charge separation rate was developed by using a facile polyol method. Results showed that when thioacetamide served as a sulfur precursor, the crystalline phase of CIS was zinc-blende. Meanwhile, when thiourea served as a sulfur precursor, the crystalline phase of CIS was wurtzite, which exhibited good photocatalytic acid red 1 (AR1) dye decolorization efficiency. When the precursor/ethylene glycol ratio was 1/50–7/50, the AR1 decolorization efficiency followed the order: T-5-CIS > T-7-CIS > T-3-CIS > T-1-CIS, and the TOC removal efficiency of T-5-CIS was 45.7%. The PL and EIS analyses indicated that T-5-CIS showed the highest charge separation rate. Mott–Schottky analysis demonstrated that the remarkably enhanced photocatalytic decolorization rate was ascribed to the stronger reduction potential of CIS with the mixed ZB/WZ phases and the redox potential difference between the ZB and WZ phases, leading to a good oxidation ability and charge separation. The results indicated that O2− was the main reactive specie in this study, and this study provided a potential photocatalyst in the treatment of dye wastewater.

Electronic, structural and optical properties of zincblend and wurtizite cadmium selenide (CdSe) using density functional theory and hubbard correction

Zinc blend (zb) and wurtizite (wz) structure of cadmium selenide (CdSe) is determined using density-functional theory within local density approximation (LDA), generalized gradient approximation (GGA), Hubbard-correction (GGA+U) and Hybrid functional approximation (PBE0 or HSE06). The first principle pseudopotential plane wave is used and the relaxed atomic position for the CdSe in zb and wz structure was obtained by using total energy and force minimization method following the Hellmann Feynman approach. The convergence test of total energy with respect to cutoff energy and k-point sampling is performed . The equilibrium lattice constant and unit cell volume of CdSe in both phases are calculated and the obtained value is compared` with experimental values. In addition the band gap of CdSe is analyzed using DFT within LDA, GGA, DFT+U and PBE0 to approximate the unknown exchange correlation functional. The band gap values obtained using LDA and GGA are severally under estimated due to their poor approximation of exchange-correlation potential. This problem was improved by using projector augmented-wave pseudopotential within Hubbard-correction (GGA+U) and the hybrid functional approximation. Optical properties: complex and real parts of dielectric function, energy loss spectrum and absorption coefficient of CdSe in both ZB and WZ phase were studied.

Vectorial carriers interfacial migration of Cd1-xZnxS heterophase junctions for visible-light water splitting

Twin structure in Cd1-xZnxS solid solution based on sphalerite/wurtzite (ZB/WZ) heterophase junction is considered as a quite promising approach to enhance the charge separation efficiency and inhibit the charge recombination during the photocatalytic process. Herein, Cd1-xZnxS nanocrystals based on ZB/WZ heterophase junction with different molar ratios of Cd/Zn were synthesized by one-step solvothermal process. XRD analysis and HRTEM images verified the formation of nano-twin structures in the Cd0·6Zn0·4S solid solution. Photoluminescence (PL) spectra, time-resolved photoluminescence (TRPL) spectra, Electrochemical impedance spectroscopy (EIS) test and Photocurrent response revealed that the recombination of photogenerated charge carriers has been reduced due to the presence of ZB/WZ heterophase junction. The experimental results showed that the photocatalytic activity of Cd0·6Zn0·4S without noble metal load was the best under visible light irradiation, and the hydrogen production efficiency was up to 472 μmol h−1 and an apparent quantum efficiency of 9.8% at 420 nm. The periodic type-Ⅱ staggered energy band arrangement formed between ZB and WZ phases can orientate the photoinduced electrons and holes to ZB and WZ regions, respectively, and redox reactions can occur on the surface. Meanwhile, Cd0·6Zn0·4S exhibited good stability for hydrogen production. The result provides an available progress for the development of photocatalytic mechanism in heterophase junction between ZB and WZ phases, leading to the vectorial interfacial transfer of photogenerated e−/h+ pairs and prolonged carrier lifetime which can effectively enhance the photocatalytic reactivity.

Nonlayered CdSe Flakes Homojunctions

Abstract2D homojunctions have stimulated extensive attention because of their perfect thermal and lattice matches, as well as their tunable band structures in 2D morphology, which provide fascinating opportunities for novel electronics and optoelectronics. Recently, 2D nonlayered materials have attracted the attention of researchers owing to their superior functional applications and diverse portfolio of the 2D family. Therefore, 2D nonlayered homojunctions would open the door to a rich spectrum of exotic 2D materials. However, they are not investigated due to their extremely difficult synthesis methods. Herein, nonlayered CdSe flakes homojunctions are obtained via self‐limited growth with InCl3 as a passivation agent. Interestingly, two pieces of vertical wurtzite‐zinc blende (WZ‐ZB) homojunctions epitaxially integrate into WZ/ZB lateral junctions. These homojunctions show a divergent second‐harmonic generation intensity, strongly correlated to the multiple twinned ZB phase, as identified by aberration‐corrected scanning transmission electron microscopy and theoretical calculations. Impressively, the photodetector based on this WZ/ZB CdSe homojunction shows excellent performances, integrating a high photoswitching ratio (3.4 × 105) and photoresponsivity (3.7 × 103 A W−1), suggesting promising potential for applications in electronics and optoelectronics.

Density functional theory studies of the SrC and SrN compounds

The structural, magnetic, elastic, dynamical and thermodynamic properties of the SrC and SrN compounds were studied using density functional theory. We found that the CsCl and NaCl phases have the lowest energy configurations as a function of volume for SrC and SrN compounds considered respectively. The preferred ground state configuration for the SrN and SrC compounds is the ferromagnetic configuration with the exception of the CsCl and tetragonal phases in the SrC compound. From the elastic properties calculation, we obtained that only the NaCl, ZB and WZ phases are elastically stable. The dynamical properties calculation of the elastically stable SrN and SrC compounds showed that the NaCl and ZB phases are thermodynamically stable. The stable phases were found to have half-metallic (HM) magnetic nature attributed to the spin-polarized 2p orbitals of the nitrogen and carbon atoms in the SrN and SrC compounds respectively. The calculated magnetic moments for the SrN and SrC compounds are 1 μB and 2 μB per formula unit, respectively. The calculated HM gaps for the SrN and SrC compounds using the GGA-PBEsol (mBJ-GGA-PBEsol) are 0.27 eV (1.54 eV) and 0.27 eV (1.44 eV) in the NaCl phase, and 0.57 eV (1.77eV) and 0.77eV (1.86 eV) in the ZB phase.

Prediction of half-metallicity in the NaS, NaSe and NaTe alkali-metal chalcogenides using first principles

First-principles full-potential linearized augmented plane-wave method based on density functional theory is used to investigate the structural, electronic and magnetic properties of NaS, NaSe and NaTe alkali-metal chalcogenides binary compounds. These compounds in different crystalline phases: NaCl (B1), CsCl (B2), ZB (B3), NiAs (B81), WZ (B4) and Pnma were calculated within the generalized gradient approximation (GGA-PBE) and the modified Becke–Johnson approach (mBJ-GGA-PBE) for the exchange-correlation energy and potential. We found that the most stable phase for the NaX binary compounds is the nonmagnetic Pnma phase. The calculated lattice parameters, bulk moduli, their first-pressure derivatives and internal parameters are in good agreement with the other theoretical data. The electronic band structure and density of states show that half-metallic and magnetic character arises, which can be attributed to the presence of spin-polarized [Formula: see text] orbitals in the group VI elements. The NaS, NaSe and NaTe binary compounds show half-metallic character in ZB and WZ phases, with an integer magnetic moment of 1 [Formula: see text] per formula unit and half-metallic gaps.

Electronic and mechanical properties of MgN compound: Prediction of stable half-metallic ferromagnet in NaCl and ZB phases

First-principles method within the generalized gradient approximation (GGA-PBEsol) and the modified Becke-Johnson approach (mBJ-GGA-PBEsol) for the exchange-correlation energy and potential were used to investigate the structural, elastic, mechanical and thermal properties of MgN compound in different phases. Various phases of MgN compound were considered, which are NaCl, CsCl, ZB, WZ, NiAs, PtH4I, and Pnma phases. We found that the NaCl phase is the lowest energy phase as a function of the volume. The ferromagnetic phase is energetically favored with respect to the non-magnetic phases, except for the CsCl and PtH4I phases. The calculated elastic properties for the NaCl, ZB and WB phases showed that they are elastically stable. Considering the phonon dynamics of MgN in the NaCl, ZB and WB phases, we observed that the MgN compound in the NaCl and ZB phases is dynamically stable. From electronic band structure and density of states, MgN compound shows half-metallic behavior in NaCl and ZB phases. The NaCl phase was found to have a robust half-metallic character with respect to the lattice compression and expansion. The half-metallic and magnetic character found in MgN compound is attributed to the presence of spin polarized 2p orbitals of the nitrogen atom. We found that the MgN compound in the NaCl and ZB phases is a half-metallic ferromagnet with magnetic moment of 1 μB per formula unit and half-metallic gaps of 0.01 eV (1.34 eV) and 0.57 eV (1.85 eV) within the GGA-PBEsol (mBJ-GGA-PBEsol) for the NaCl and ZB phases respectively.

Pressure-induced phase transition in titanium alloys

The structural, elastic, magnetic and electronic properties of titanium-based ferromagnetic (FM) TiX (X = Fe, Co, Ni) alloys are investigated by the first principles calculations based on density functional theory using the Vienna ab initio simulation code. At ambient pressure, all the three alloys TiFe, TiCo and TiNi are highly stable in CsCl structure. The calculated lattice parameters and ground state properties are in good agreement with the available theoretical and experimental results. The density of states explains that these alloys possess the metallic nature at normal and high pressures. A pressure-induced structural phase transitions from CsCl to NaCl phase at 46 GPa and NaCl to ZB phase at 49 GPa in TiFe, CsCl to ZB phase in TiCo at 52 GPa, CsCl to hexagonal phase at 22 GPa and hexagonal to ZB phase at 66 GPa in TiNi are observed. The calculated Debye temperatures of TiX (X = Fe, Co, Ni) alloys are in good agreement with earlier reports. Binding energy shows that the TiCo is the most stable alloy. The magnetic property of TiX (X = Fe, Co, Ni) alloys reveals that TiFe is stable in nonmagnetic phase and the other two alloys, TiCo and TiNi, are stable in FM phase at normal pressure.

DFT study on phase transition behavior and mechanical properties of HgSe polymorphs under high pressure

Total energy calculations based on density functional theory are performed for HgSe in the ambient and pressure induced phases. The electronic exchange and correlation effects are described by employing the generalized gradient approximation together with the projector augmented wave potentials. The equation of state parameters and the phase transition paths are obtained from the energy-volume landscape and enthalpy versus pressure data, correspondingly. The observed phase transition sequence of HgSe as ZB → Cinnabar → RS → Cmcm and the final transition from Cmcm to CsCl structure predicted by the other theoretical study are confirmed by our calculations. The pressure dependence of elastic constants of the structures considered in this study is reported. Bulk modulus, shear modulus, Young's modulus, Poisson's ratio, and Debye temperature as isotropic mechanical properties are evaluated for HgSe polymorphs. Our calculations of energy band structure reveal the inverted band structure evoking that HgSe in ZB phase is a semimetal.

Optical, electronic and elastic properties of ScAlN alloys in WZ and ZB phases: Prospective material for optoelectronics and solar cell applications

To comprehend merits and demerits of Sc0.25Al0.75N alloys in WZ and ZB phases for optoelectronics and solar cell technology, we performed a comparative study of the elastic, electronic and optical properties of these alloys using first-principles methodology with density functional theory. The elastic constants (Cij, B, G, E, ν and B/G), longitudinal and shear wave velocities (vs, vl) and Debye temperature θD of ZB and WZ-Sc0.25Al0.75N alloys have been calculated for the first time. The calculated results show that ZB and WZ-Sc0.25Al0.75N alloys have the direct band gap and the ZB-Sc0.25Al0.75N alloy exhibit comparatively narrower energy gap than WZ. The density of states of W-Al0.75 Sc0.25 N and ZB-Sc0.25Al0.75N were analyzed. The peaks of imaginary dielectric function and absorption curves are at higher energy point for ZB-Sc0.25Al0.75N. The highest absorption of ZB-Sc0.25Al0.75N is 3.65 × 105/cm at 14.26 eV, which is larger than wurtzite of 3.32 × 105/cm at 11.22 eV. The average reflectivity of WZ-Sc0.25Al0.75N is lower than that of ZB-Sc0.25Al0.75N. Moreover wide energy gap of WZ-Sc0.25Al0.75N results in smaller values of the static refractive indices compared to ZB-Sc0.25Al0.75N. The comparison of the electronic and optical characteristics of the ZB-Sc0.25Al0.75N with those of WZ-Sc0.25Al0.75N endorses that ZB-Sc0.25Al0.75N can be a material of choice for optoelectronics and solar cell applications besides the WZ-Sc0.25Al0.75N.

Crystal Structure Induced Preferential Surface Alloying of Sb on Wurtzite/Zinc Blende GaAs Nanowires.

We study the surface diffusion and alloying of Sb into GaAs nanowires (NWs) with controlled axial stacking of wurtzite (Wz) and zinc blende (Zb) crystal phases. Using atomically resolved scanning tunneling microscopy, we find that Sb preferentially incorporates into the surface layer of the {110}-terminated Zb segments rather than the {112̅0}-terminated Wz segments. Density functional theory calculations verify the higher surface incorporation rate into the Zb phase and find that it is related to differences in the energy barrier of the Sb-for-As exchange reaction on the two surfaces. These findings demonstrate a simple processing-free route to compositional engineering at the monolayer level along NWs.

Hydrostatic Pressure Effect on Ga0.75Cr0.25As DMS: DFT Study

This paper studies the first-principles calculations of the effect of hydrostatic pressure on the structural, electronic and magnetic properties of Ga0.75Cr0.25As dilute magnetic semiconductor in zb (B3) phase. High-pressure behaviour of Ga0.75Cr0.25As has been investigated between 0 and 100 GPa. The calculations have been performed using DFT as implemented in code SIESTA using LDA + U as an exchange-correlation (XC) potential. The study of band structures shows half-metallic ferromagnetic nature with 100% spin polarization. Under application of external pressure, the valence band and conduction band are shifted from their positions which lead to modification of electronic structure.

Spatially resolved Raman spectroscopy study of uniform and tapered InAs micro‐nano wires: correlation of strain and polytypism

The asymmetric peak ~212–218 cm−1 occurring in InAs micro‐nanowires (MNWs: diameter: 2 μm – 400 nm) is investigated using spatially resolved Raman spectroscopy of uniform, bent and long tapered MNWs grown on a Si (001) substrate. It is attributed to superposition of E2h phonon (wurtzite: WZ) and TO phonon (zinc blende: ZB) of InAs. Polarized and wavelength‐dependent spatially resolved Raman spectroscopy establishes the presence of WZ and ZB phases in these MNWs. However, formation of WZ phase for larger diameter InAs MNWs is not commensurate with existing growth mapping studies. Further, study of these MNWs suggests that the fraction of WZ to ZB content in a MNW depends not only on the diameter but also seems to be governed by local growth seeding/conditions. This in turn leads to either tapered or uniform MNW growth under the same (external) growth conditions. The variation of these wavenumbers that of bulk value is correlated to the residual stress present in ZB and WZ phases due to the presence of the other (WZ/ZB) phase. Consistently, temperature‐dependent Raman data show that there is a measurable contribution of stress to dω/dT, a positive for ZB and negative for WZ phonons, due to differences in their thermal expansions. Further, effective thermal expansion coefficient of WZ InAs in the presence of ZB phase is calculated to vary in the range 10–19 × 10−6/K from base to tip of a MNW at ~80 K, which is not possible to determine otherwise. Copyright © 2017 John Wiley & Sons, Ltd.

DFT Study of Hydrostatic Pressure Effect on Cd 1 − x Z x X (Z = Cr, Mn; X = S, Se) DMSs

We have studied the first-principle calculations of the effect of hydrostatic pressure on the structural, electronic, and magnetic properties of Cd1−x Z x X (Z = Cr, Mn; X = S, Se) DMS in zb (B3) phase. High-pressure behavior of these compounds have been investigated between 0 and 100 GPa The calculations have been performed using density functional theory (DFT) as implemented in code SIESTA using LDA as exchange-correlation (XC) potential. Study of band structures shows half-metallic ferromagnetic nature with 100 % spin polarization. Under application of external pressure, the valence band and conduction band are shifted from their positions, which lead to modification of electronic structure.

Exploring Crystal Phase Switching in GaP Nanowires.

The growth of wurtzite/zincblende (WZ and ZB, respectively) superstructures opens new avenues for band structure engineering and holds the promise of digitally controlling the energy spectrum of quantum confined systems. Here, we study growth kinetics of pure and thus defect-free WZ/ZB homostructures in GaP nanowires with the aim to obtain monolayer control of the ZB and WZ segment lengths. We find that the Ga concentration and the supersaturation in the catalyst particle are the key parameters determining growth kinetics. These parameters can be tuned by the gallium partial pressure and the temperature. The formation of WZ and ZB can be understood with a model based on nucleation either at the triple phase line for the WZ phase or in the center of the solid-liquid interface for the ZB phase. Furthermore, the observed delay/offset time needed to induce WZ and ZB growth after growth of the other phase can be explained within this framework.

First principles study of structural, electronic and elastic properties of group IB metal nitrides TMNx (TM=Cu, Ag and Au: x=1, 2)

Abstract The structural, electronic and elastic properties of group IB transition metal nitrides (TMN x : TM = Cu, Ag, Au and x = 1, 2) are investigated by first principles calculation using the Vienna ab initio simulation package. The lattice constants, cohesive energy, bulk modulus, band structures and the density of states are obtained. The calculated lattice parameters are in good agreement with the available results. The electronic structure reveals that these materials exhibit metallic behaviour. A pressure-induced structural phase transition from NaCl to CsCl phase in AgN at 6 GPa, NaCl to ZB phase in AuN at 34.2 GPa and ZB to NaCl phase in CuN at 36.2 GPa is observed. In group IB metal di-nitrides, CaF 2 to AlB 2 phase transition is predicted. The computed elastic constants indicate that these nitrides are mechanically stable at ambient pressure. The calculated Debye temperature values are in good agreement with experimental and other theoretical results.