Bulk Modulus B0 Research Articles

Theoretical aspects of structure, electronic, optical and elastic properties of Ca(1-x)CxSe mixed alloys in binary and ternary phases

The structural, optoelectronic, and elastic properties of Carbon doped Calcium Selenide binary and ternary semiconductor alloys with the general form of Ca(1-x)CxSe, 0 ≤ x ≤ 1 in B1 and B3 phases have been scrutinized adopting augmented plane wave plus local orbital methods within density functional theory applying different approximations like the Local density approximation (LDA), Wu-Cohen-Generalized Gradients Approximation (WC-GGA) and modified Becke-Johnson (mBJ). Structural properties summarize the crystal structure, relevant atomic positions, lattice constant, Bulk modulus B0, minimum energy E0, and minimum volume V0. Elastic constants confirm the structure stability of the B1 and B3 phases. The band gap was modified from a wider for appropriate optoelectronic devices application to narrow that enhanced the range of applicability of these binary and ternary semiconductor compounds for optoelectronic device applications. Whereas optical properties which include the study of zero frequency limit of static dielectric constant ε0(ω) with its real and imaginary parts, static refractive index n(0), optical reflectivity R(ω), optical absorption coefficient α(ω), optical conductivity σ(ω), and loss function L(ω) studied comprehensively to get real electrical and optical properties of these materials and give semiconductor market best alternative candidate which shows its effectiveness in the visible, ultraviolet and infrared region of the electromagnetic spectrum. Obtained results compared together and with the available experimental along with theoretical work on these binary and ternary Carbon-doped Alkaline earth Selenide.

FP-LAPW calculations of the structural, and optoelectronic properties of vanadium silicide compound for optoelectronic applications

Optoelectronic devices play an essential part in our daily lives by seamlessly integrating optics and electronics, leading to technologies such as smartphones, solar cells, and optical communication. In this study, the structural and optoelectronic characteristics of the hexagonal vanadium silicide are determined using the density functional theory (DFT). The structural parameters such as the zero-pressure bulk modulus B0, its pressure derivative B0′; equilibrium energy, and the volume of the primitive cell at equilibrium V0 are compared with experiment results. The metallic character of VSi2 is confirmed by calculating the density of states and band structure. Moreover, the optical properties like the dielectric function, reflectivity, energy loss function, refractive index, absorption coefficient, and optical conductivity are studied deeply, using the Drude model, within the energy interval from 0 to 14 eV and they are compared with experiment results of a polycrystalline and single crystal. However, the phonon dispersion spectrum shows the presence of imaginary modes, which indicates instability in the structure of vanadium silicide. The results indicate that the VSi2 material has the potential to be used in many applications, including mirrors, glass coatings, optoelectronic devices, optical networks, electronics, optical communications, and waveguide components.

Pressure-induced metallization in the absence of a structural transition in the layered transition-metal dichalcogenide ZrSe2.

First-principles calculations were carried out on the ZrSe2 compound, which has been of interest owing to its technologically important physical properties. The structural, electronic and optical properties of this compound were investigated under pressure through the plane wave pseudopotential approach within the framework of density functional theory. A comparison between the computed crystal structure parameters and the corresponding experimental counterparts shows a very good agreement between them. Fitting the pressure-volume data using the third-order Birch-Murnaghan equation of state yielded a bulk modulus B0 = 38.17 GPa and a pressure derivative of bulk modulus B'0 = 8.2 for hexagonal ZrSe2. The relationship between the band structure and pressure is revealed. We calculated the total density of state (TDOS) under different pressures and partial density of state (PDOS) from 0 to 10 GPa. According to our calculations, metallization of hexagonal ZrSe2 is predicted to occur at around 10 GPa and pressure-induced band-gap engineering reveals the transformation of the indirect to direct band gap with increasing pressure. Furthermore, optical properties, such as the complex dielectric function, refractive index and reflectivity spectra of this compound, were studied for incident electromagnetic waves in an energy range up to 45 eV. The contributions to various transition peaks in the optical spectra are analyzed and discussed with the help of the energy-dependent imaginary part of the dielectric function.

Structural, Mechanical, Elastic, Electronic, Magnetic and Optical Properties of Spinel Compounds ATi2S4 (A=Ca, Sr and Ba): AB Initio Study

Abstract This research explored the crystal structure, mechanical and elastic, electronic, magnetic, and optical properties of spinel compoundsATi2S4 (A=Ca, Sr and Ba). The ferromagnetic phase of the compounds was found to be stable, and FP-LAPW method within density functional theory (DFT) was used to calculate and compare the lattice constant a, bulk modulus B0, and its pressure derivative B'. The exchange-correlation potential was also treated with both the GGA and TB-mBJ potential approximation. The results of the study showed that the examined spinels exhibit half-metallic behavior, which makes them a potential material for use in spintronic and optoelectronic devices.

Cluster structure of ultrahard fullerite revealed by Raman spectroscopy

We have discovered Raman peculiarities of sp3-bonded carbon nanoclusters forming ultrahard fullerite synthesized at pressure up to 85 GPa. The nanoclusters differ in the values of bulk modulus B0 ranging from 580 to 730 GPa. We found that the Raman frequency, corresponding to the C-C stretching mode of the nanoclusters, varies linearly from 1485 to 1640 cm−1, depending on the excitation wavelength used, which ranges from 785 nm to 257 nm. In particular, the nanoclusters with higher B0 have a higher Raman frequency. We also observed a resonant Raman spectrum when excited at a wavelength of 1064 nm. This showed a plateau from 1400 to 1720 cm−1, covering all Raman band positions of the nanoclusters from 1485 to 1640 cm−1. In addition, two Raman bands at 1285 and 1600 cm−1 appear at the resonance. The presence of several types of carbon sp3-bonded nanoclusters is confirmed by high-resolution transmission electron microscopy. Using moment tensor potentials, a class of machine learning interatomic potentials, we calculated the phonon density of states of the nanoclusters formed during the 3D polymerization of C60. The presence of high frequency modes in the resonant Raman spectra suggests the existence of bonds with force constants higher than those in a single crystal diamond. In addition, we observed a stiffening effect when analyzing the dependence of the mechanical stiffness on the cluster size, with smaller clusters exhibiting a more rigid structure.

High-pressure study of hydrogen storage material ethylenediamine bisborane from first-principle calculations

Ethylenediamine bisborane (EDAB), as a hydrogen storage material, is one of the most promising possible candidates. This work reports the effects of pressure on the structural, mechanical, electronic, and vibrational behaviors of EDAB using density functional theory calculations. The predicted lattice parameters, bulk modulus B0 and its first pressure derivative B′ are in accord with available experimental data. From the unit-cell constants as a function of pressure, crystal EDAB is found to present an anisotropic compressibility. The calculated elastic properties show that the ambient phase EDAB is mechanical unstable at 3 GPa. Furthermore, the discontinuous behavior was found in the pressure dependence of crystal and electronic structure, suggesting a structural transformation occurred around 3 GPa. Finally, vibrational spectra are reported together with their infrared active modes assignment. The NH and BH stretching modes soften under compression, indicating the strengthening of hydrogen bonding, which leads to the occurrence of pressure-induced structural transformation.

Superconducting and structural properties of the noncentrosymmetric Re6Hf superconductor under high pressure

We report the effect of high pressure on the superconducting, vortex pinning, and structural properties of a polycrystalline non-centrosymmetric superconductor Re6Hf. The superconducting transition temperature, Tc, reveals a modest decrease as pressure P increases with a slope -0.046 K/GPa (-0.065 K/GPa) estimated from resistivity measurements up to 8 GPa (magnetization measurement ~ 1.1 GPa). Structural analysis up to ~18 GPa reveals monotonic decreases of lattice constant without undergoing any structural transition and a high value of bulk modulus B0= 333.63 GPa, indicating the stability of the structure. Furthermore, the upper critical field and lower critical field at absolute temperature (Hc2(0) & Hc1(0)) decreases slightly from the ambient pressure value as pressure increases up to 2.5 GPa. In addition, up to P ~ 2.5 GPa using thermally activated flux flow of vortices revealed a double linearity field dependence of activation energy of vortices, confirming the coexistence of single and collective pinning vortex states. Moreover, analysis of critical current density using the collective pinning theory showed the transformation of {\delta}Tc to {\delta}l pinning as pressure increases, possibly due to migration of grain boundaries. Besides, the band structure calculations using density functional theory show that density of states decreases modestly with pressure, which may be a possible reason for such a small decrease in Tc by pressure.

Compressibility and Phase Stability of Iron-Rich Ankerite

The structure of the naturally occurring, iron-rich mineral Ca1.08(6)Mg0.24(2)Fe0.64(4)Mn0.04(1)(CO3)2 ankerite was studied in a joint experimental and computational study. Synchrotron X-ray powder diffraction measurements up to 20 GPa were complemented by density functional theory calculations. The rhombohedral ankerite structure is stable under compression up to 12 GPa. A third-order Birch–Murnaghan equation of state yields V0 = 328.2(3) Å3, bulk modulus B0 = 89(4) GPa, and its first-pressure derivative B’0 = 5.3(8)—values which are in good agreement with those obtained in our calculations for an ideal CaFe(CO3)2 ankerite composition. At 12 GPa, the iron-rich ankerite structure undergoes a reversible phase transition that could be a consequence of increasingly non-hydrostatic conditions above 10 GPa. The high-pressure phase could not be characterized. DFT calculations were used to explore the relative stability of several potential high-pressure phases (dolomite-II-, dolomite-III- and dolomite-V-type structures), and suggest that the dolomite-V phase is the thermodynamically stable phase above 5 GPa. A novel high-pressure polymorph more stable than the dolomite-III-type phase for ideal CaFe(CO3)2 ankerite was also proposed. This high-pressure phase consists of Fe and Ca atoms in sevenfold and ninefold coordination, respectively, while carbonate groups remain in a trigonal planar configuration. This phase could be a candidate structure for dense carbonates in other compositional systems.

Phase transformations of zircon-type DyVO4 at high pressures up to 36.4 GPa: X-ray diffraction measurements

The compression behavior of zircon-type DyVO4 has been investigated in a diamond anvil cell by angle-dispersive X-ray diffraction (ADXRD) at high pressures up to 36.4 GPa. Under quasi-hydrostatic compression with argon as the pressure transmitting medium, DyVO4 underwent a sluggish zircon-to-scheelite phase transformation that started at approximately 8.0 GPa and was completed at 15.3 ± 0.9 GPa. The scheelite-type phase remained stable at pressures up to 25.3 GPa and during the process of decompression to ambient conditions, implying that the zircon-to-scheelite transformation is irreversible. Fitting the pressure-volume data using the third-order Birch-Murnaghan equation of state yielded a bulk modulus B0 = 129 (7) GPa and a pressure derivative of bulk modulus B′ = 3.4 (4) for the zircon-type phase, and B0 = 184 (10) GPa and B′ = 5.3 (8) for the scheelite-type phase. When compressed in a methanol-ethanol (4:1) pressure transmitting medium, DyVO4 was transformed into the scheelite-type phase at a pressure of 8.8 ± 1.1 GPa and then transformed into a fergusonite-type phase at 21.3 ± 1.6 GPa. The scheelite-to-fergusonite transformation was reversible during decompression. Our study indicates that DyVO4 under compression experiences a sequential zircon-scheelite-fergusonite phase transformation and that the scheelite-fergusonite transformation is sensitive to nonhydrostatic conditions.

DFT-Study of Structural, Mechanical, and Electronic Properties of L-leucine under Pressure

In this paper, the study of the properties of L-leucine crystal is carried out using the density functional theory with PBE exchange-correlation functional with consideration of the van der Waals forces according to the Grimme scheme in option D3 (BJ). The calculations are based on the linear combination method of atomic orbitals and performed using the CRYSTAL’17 software package. The structural and electronic properties of L-leucine crystal under pressure are investigated, and the equation of state for the pressure ranges from 0 to 4 GPa is obtained. The constructed equation of state shows that anomalies are observed due to changes in the unit cell parameter a directed across the layers of molecules, as well as due to the angle p. It has been suggested that a possible complication of the structure of L-shaped amino acid molecules by functional groups is associated with the bulk modulus B0 and its first derivative B1. Using the calculated compressibility modulus as an example, it is shown that the highest compressibility is observed along the c axis, which directed along the orientation of the molecules in the unit cell of the L-leucine crystal. The observed compressibility is four and five times higher than the compressibility in L-alanine and L-threonine, respectively. An analysis of the dependence of electronic properties on pressure, such as the effective charges of functional groups, the band gap, and the location of the peaks of the density of states, demonstrates that the above properties do not undergo noticeable changes in the pressure range from 0 to 4 GPa.

Crystal Growth and Investigation of High-Pressure Physical Properties of Fe2As

We reported the growth of Fe2As single crystals and the study of its physical properties via comprehensive measurements, such as transport properties under pressure and high-pressure synchrotron radiation X-ray diffraction. Fe2As is an antiferromagnetic metal with TN ~ 355 K. Within the pressure range of 100 GPa, no superconductivity was observed above 2 K. The abrupt drop in resistance from 21 to 31.7 GPa suggests a high-pressure phase transition happens. The high-pressure X-ray experiments indicate a new high-pressure phase appears, starting from 27.13 GPa. After the refinement of the high-pressure X-ray data, the pressure dependence of lattice constants of Fe2As (P4/nmm phase) was plotted and the bulk modulus B0 was obtained to be 168.6 GPa.

High-pressure investigations on the isostructural phase transition and metallization in realgar with diamond anvil cells

The high-pressure structural, vibrational and electrical properties for realgar were investigated by in-situ Raman scattering and electrical conductivity experiments combined with first-principle calculations up to ~30.8 ​GPa. It was verified that realgar underwent an isostructural phase transition at ~6.3 ​GPa and a metallization at a higher pressure of ~23.5 ​GPa. The isostructural phase transition was well evidenced by the obvious variations of Raman peaks, electrical conductivity, crystal parameters and the As–S bond length. The phase transition of metallization was in closely associated with the closure of bandgap rather than caused by the structural phase transition. And furthermore, the metallic realgar exhibited a relatively low compressibility with the unit cell volume V0 ​= ​718.1.4 ​Å3 and bulk modulus B0 ​= ​36.1 ​GPa.

High-pressure Raman scattering and x-ray diffraction studies of MgTa2O6

High-pressure Raman scattering and angle dispersive synchrotron x-ray diffraction studies have been carried out on magnesium tantalate (MgTa2O6) up to 41 GPa at room temperature. A pressure-induced phase transition at pressures above 10 GPa accompanied by softening of the internal ν11 (Eg) modes has been observed. Another phase transition took place at 20.2 GPa, which indicates that the edge shared TaO6 octahedra are distorted. The equation of state for MgTa2O6 in low pressure phase I was obtained by the second-order Birch–Murnaghan method, yielding a zero-pressure bulk modulus B0 of 205.8 ± 8.4 GPa. The pressure dependencies of the lattice vibration mode were extracted, and it is suggested that the instability of the trirutile structure at high pressure is related to the strong deformation of the TaO6 octahedra. Decompression measurements suggest that the pressure-induced transformation is irreversible.

Synthesis and characterization of Pt(Cu0.67Sn0.33)

Pt(Cu0.67Sn0.33) has recently been found in a natural sample. In order to be able to characterize this new ternary compound, we synthesized it from the elements. Samples were characterized by X-ray powder diffraction, differential scanning calorimetry, thermal relaxation calorimetry, and scanning electron microscopy studies. Density functional theory-based model calculations complemented the experimental studies. Pt(Cu0.67Sn0.33) was already formed at a relatively low temperature of 773 K. Rietveld refinement of Pt(Cu0.67Sn0.33) has been carried out in CuAu-type or L10-type structure, space group P4∕mmm, with Pt on 0,0,0 and disordered Cu and Sn on 12, 12, 12 and Z = 1. The lattice parameters are a = 2.823(1) Å, c = 3.64(1) Å, and V = 29.00(4) Å 3; which are in good agreement with values obtained earlier on the natural sample and with the results of DFT calculations. The vibrational entropy for Pt(Cu0.67Sn0.33) is S298.15vib = 79.9(7) J mol−1 K−1. The pressure dependence up to 36(2) GPa of the unit-cell volume and the lattice parameters and unit-cell volume have been obtained by synchrotron based powder diffraction using a diamond anvil cell. A fit of a 3rd-order Birch–Murnaghan equation of state to the Pt(Cu0.67Sn0.33) (p,V)-data results in a bulk modulus of B0 = 215(27) GPa and B′ = 5(2).

Irreversible high pressure phase transformation of onion-like carbon due to shell confinement

Onion-like carbon (OLC) was investigated at high pressure (up to 48 GPa) in a shear diamond anvil cell (SDAC) equipped with in-situ Raman spectroscopy control. The OLC was obtained from natural gas by a highly productive and efficient partial oxidation method. Vast bulk modulus B0 = 486 ± 15 GPa and outstanding stability under high pressure were registered in the experiment. A phase transition was observed at 20 GPa under conditions of shear deformation. The transition is characterized by an appearance of a new Raman band at 1560 cm−1 accompanied with disappearance of G band of OLC. We attribute these features to sp2-sp3 transition in the carbon nanoclusters. It was found however that the concentric multi-shell onion-like structure remains intact after pressure release, which may witness that the outer shell of the OLC serves as a confinement and prevents structural collapse. X-ray photoelectron spectroscopy revealed however that the phase transition was not completely reversible because the OLC after pressure release showed the presence of sp3-hybridized carbon in significant amounts.

Raman spectroscopy and X‐ray diffraction of pressure‐induced reversible structure change in K2OsO2(OH)4

AbstractHigh‐pressure structural evolution of K2OsO2(OH)4, a potassium osmate dihydrate, has been studied by high‐pressure Raman spectroscopy and angle dispersive X‐ray diffraction. A pressure‐induced reversible structure change is observed at approximately 8 GPa, evidenced by the splitting of Raman modes and the addition of X‐ray diffraction peaks. The splitting of phonon generally occurs in the Raman modes with smaller Grüneisen parameters. The pressure‐induced structural change is correlated with the relatively isolated arrangement of Os–O polyhedron. In addition, the zero‐pressure bulk modulus B0 = 46(1) GPa and zero‐pressure Grüneisen parameters γ of K2OsO2(OH)4 are given and discussed.

C60 structures: Structural, electronic and elastic properties

A systematic investigation, using density functional theory (DFT) methods, of the physical properties of one-dimensional (1D), two-dimensional (2D) and three-dimensional (3D) pressure-induced polymerized C60 structures, was performed. The optimized crystal structures, the electronic density of states (DOS) and the bulk moduli B0 were calculated for such polymerized structures, showing how these properties change with the degree of polymerization and providing an overview of the properties of this class of materials. The increasing number of intermolecular bonds, across the low-dimensional polymers, induces a decrease of the electronic bandgap, which in turn vanishes for the metallic 3D polymers. The compressibility behavior of these materials also shows a monotonous dependence on the degree of polymerization: the rise in the number of polymeric bonds induces an expected increase of the bulk modulus. From semiconducting to metallic and from soft to low-compressibility, this class of materials is shown to display an enormous range of electronic and elastic properties.

Effect of ball to powder ratio on the mechanosynthesis of Re2C and its compressibility

The mechanosynthesis of hexagonal rhenium carbide (Re2C) from the elements is explored as a function of the balls to powder ratio (BPR). Burgios's equation has been used to calculate the accumulated energy (ΔEaccum) and the transferred energy per hit (ΔEb). To get the complete mechanosynthesis of Re2C several conditions have to be met: BPR > 115:1, ΔEaccum > 500 kJg, and ΔEb > 2.656 × 10−14 kJ. The reaction is completed after 30 min of milling if a BPR of 230:1 is used. The pressure dependence of the unit-cell volume and the lattice parameters have been evaluated by synchrotron radiation diffraction using a diamond anvil cell up to 61(2) GPa. A fit of a 2nd-order Birch-Murnaghan equation of state results in a bulk modulus of B0 = 374(3) GPa.

Theoretical investigation of electronic and thermoelectric properties of spinel sulfides A2BS4 (A=Sc and Y; B= Cd and Zn)

Based on first principle methods, structural, electronic and thermoelectric properties of cubic structures of spinel sulfides A2BS4 with (A = Sc and Y; B=Cd and Zn) have been investigated. We have calculated the lattice parameter a, the bulk modulus B0, its pressure derivative B’0 and the density of states (DOS) by using the full potential-linearized augmented plane wave method within the density functional theory (DFT). The computations were done by treating the exchange-correlation potential with the generalized gradient approximation (GGA) and modified Becke-Johnson potential approximation (TB-mBJ) and by using new parameters of the original TB-mBJ proposed by Koller et al. This new parameterization yields band gaps in very good agreement with experiments. The four compounds treated were found semiconductors and they have all direct bandgaps at the center of Brillouin zone (Γ). Furthermore, the thermoelectric parameters represented by the Seebeck coefficient, electrical and thermal conductivities were calculated employing the Boltzmann transport equations resolved in respect to DFT eigenvalues. Based on the obtained results, the reported parameters recommend that these spinel sulfides materials A2BS4 (A = Sc and Y; B=Cd and Zn) are suitable candidates for thermoelectric applications.

Neutron diffraction study of the crystal structure of TlInSe2 at high pressure

We have investigated the crystal structure of strongly anisotropic semiconductor TlInSe2 by neutron diffraction method under high pressure upto P = 3.3 GPa. It was shown that the tetragonal phase of TlInSe2 crystal (the space group I4/mcm) is stable in the whole investigated range of pressure. The lattice parameters dependence of the pressure and the unit cell volume are obtained, the linear coefficients of compressibility and the bulk moduli are calculated. At the low pressure, obtained value of compressibility for the lattice parameter a is k[Formula: see text] = 14.23 × 10[Formula: see text] GPa[Formula: see text] and for c is k[Formula: see text] = 5.93 × 10[Formula: see text] GPa[Formula: see text]. Obtained values for bulk modulus B0 and its pressure derivative B[Formula: see text] in tetragonal phase are 30(7) GPa and 4(1), respectively.