Strain-assisted structural transformation and band gap tuning in BeO, MgTe, CdS and 2H-SiC: A hybrid density functional study

Zhu

et al. 2019

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The effects of hydrostatic pressure and biaxial strains on the elastic, electronic, and lattice vibrational properties of a new superhard material trigonal boron nitride (T‐BN) are studied by first‐principles calculation. The mechanical and dynamical stabilities of T‐BN are confirmed by elastic constants criteria and phonon dispersion curves. It is found that all elastic constants and elastic modulus increase (decrease) under pressure and compressive (tensile) strain ϵxx. The Vicker's hardness of every single chemical bond as well as the crystal is calculated by a microscopic model. The hardness of T‐BN 55.5 GPa is smaller than that in c‐BN (64 GPa) but increases significantly with increasing pressure and compressive ϵxx. The anisotropic index and anisotropic velocities, as well as the Debye temperature for T‐BN are also discussed. Moreover, the computational infrared absorption spectra exhibit significant blueshift under pressure and −5%<ϵxx<3%, while redshift of A3 and E4 modes can be observed when ϵxx>3%. Electronic calculation shows the T‐BN is an indirect band‐gap semiconductor, and energy gap decreases monotonically under computational conditions. Analysis based on density of states is conducted, clarifying the causes of bonding and bandgap changes in T‐BN.

ϵ_{x x} = ϵ_{y y} = true(α - α_{0} true) / α_{0} \times 100 %

, at the same time, uniaxial strain is

ϵ_{z z} = true(c - c_{0} true) / c_{0} \times 100 %

, where the a ( c ) and a 0 ( c 0 ) denote the lattice constant of T ‐BN in given and equilibrium condition, respectively. The

ϵ_{x x} < 0 true(ϵ_{x x} > 0 true)

represents compressive (tensile) in‐plane biaxial strains.…”

Section: Methods Of Calculationsmentioning

confidence: 99%

Effects of Hydrostatic Pressure and Biaxial Strains on the Elastic, Electronic and Lattice Vibrational Properties of Trigonal Boron Nitride

Zhu

et al. 2019

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“…The phonon dispersion curves and frequencies are performed by the linear‐response method using the Density‐functional Perturbation Theory (DFPT) . The biaxial strains are defined by

ϵ_{x x}

ϵ_{y y}

= ( a − a 0 )/ a 0 × 100% (in‐plane biaxial strain, paralleled to a ‐axis) and

ϵ_{z z}

= ( c − c 0 )/ c 0 × 100% (out‐of‐plane stress‐free strain, paralleled to c ‐axis), where the a ( c ) and a 0 ( c 0 ) correspond the lattice constants of crystal in equilibrium and strained conditions, respectively, and the

ϵ_{x x} > 0

(

ϵ_{x x} < 0

) denotes to the tensile (compressive) in‐plane biaxial strains. The relaxation is performed with the lattice constants a and b set, then the lattice constant c is optimized freely until the ionic Hellmann‐Feynaman forces had converged to less than 0.01 eV Å −1 and all the stress components are less than 0.02 GPa .…”

Section: Methodsmentioning

confidence: 99%

Elastic and Bandgap Modulations of Hexagonal BC₂N From First‐Principles Calculations

Zhu

et al. 2019

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The structural, elastic, and electronic properties of a potential superhard material h‐BC2N as a function of external forces, including hydrostatic pressure and biaxial strains, are investigated using first‐principles calculations. For both types of external forces employed, the well qualified elastic criteria and positive phonon frequencies confirm its mechanical and dynamical stability. Considerable elastic constants and elastic modulus, especially Youngs modulus (965 GPa) and shear modulus (444 GPa), are obtained at equilibrium condition. All the elastic constants and elastic modulus increase (decrease) with increasing pressure and compressive (tensile) ϵxx along with a better (worse) ductile behavior. The theoretical Vickers hardness of h‐BC2N is 70.34 GPa, which is calculated from a microscopic determined model and increases notably with pressure and compressive strain. Debye temperature and elastic anisotropy of h‐BC2N are also included in this work, a sizeable Debye temperature of 2047K is obtained at ambient temperature. Besides, h‐BC2N is an indirect bandgap semiconductor with Eg = 3.84 eV which can be slightly modulated by external forces.

“…Raman spectra are obtained from Born effective charges, dielectric susceptibilities, and phonons at zone‐center

normalΓ

point. The in‐plane strain (paralleled to a ‐axis) is defined by

ϵ_{x x} = ϵ_{y y} = true(a - a_{0} true) / a_{0} \times 100 %

, where

a_{0} true(a true)

denotes the lattice constants of the equilibrium (strained) crystal, and

ϵ_{x x} > 0

(

ϵ_{x x} < 0

) represents tensile (compressive) biaxial strain . The strain

ε_{x x}

imposed on the ab plane varies from −8% (−6%) to 10% (8%) for HAT (HIT) in our calculation.…”

Section: Methodsmentioning

confidence: 99%

Effects of Biaxial Strains and High Pressure on the Structural, Electronic, and Vibrational Properties of DC‐HgM₂Te₄ (M = Al, In)

Zhang

Huang

et al. 2018

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First-principles calculations are performed to study the effects of biaxial strains and high pressure on the structural, electronic, and lattice dynamical properties of defect chalcopyrite HgM 2 Te 4 (M ¼ Al, In) semiconductors. The evolutions of optimized structural parameters, band-gap energy, crystal-field splitting energy, and Γ-point phonon frequencies with the biaxial strain and pressure have been analyzed. The optimized lattice parameters are reasonable compared with the existing experimental results. Both compounds undergo an indirect to direct band gap transition under tensile e xx , however, the semiconductors retain their indirect band gap character under pressure. The increasing pressure pushes the crystal-field-splitting hole (CH) band downwards; in contrast, the increasing biaxial strain makes the CH band shift upwards, which results in that the crystal-field splitting energies Δ cf decrease from positive to negative. The pressure-induced softness of low-frequency modes around the X point suggests that both compounds become dynamically instable. Atomic displacement patterns show that the high frequency modes are mainly determined by the vibrations of group-III cations.