Large Bulk Modulus Research Articles

The stability and properties of the PtFe2 Laves phases

The structural stability of PtFe2 Laves phase with cubic and hexagonal structures has been investigated with the help of the first-principles methods, followed by targeted calculations of electronic, magnetic, lattice-dynamics and mechanical properties. The formation enthalpies of different magnetic configurations are calculated and their ferromagnetic (FM) states are found to be thermodynamically stable. However, there is strong competition for the FM states of the three structures, although the cubic C15 structure has the lowest energy. These FM states are dynamically stable owing to the spontaneous magnetization. The total magnetic moments of the FM state for these three structures are close to each other at 0 GPa, and the spin-polarized electronic DOS is investigated to promote insight into the magnetic properties. The elastic constants and modulus show that all three structures with FM state are mechanically stable, and the hexagonal C14 structure has the largest bulk modulus, but its shear modulus and Young's modulus are the lowest. Finally, the elastic anisotropy was quantified by several anisotropy indexes. The results reported here reveal that the anisotropy is in a sequence of C15 > C14 > C36.

Transition metal-based halides double Cs2ZSbX6 (Z = Ag, Cu, and X = Cl, Br, I) perovskites: A mechanically stable and highly absorptive materials for photovoltaic devices

Cs2ZSbX6 (Z ​= ​Ag, Cu, and X ​= ​Cl, Br, I) are promising materials for photovoltaic applications based on their electronic and optical properties, whereas their suitable bandgap inspires us to move towards other substitutions. In the present investigation, when substitute Z with Ag and Cu, and X with Cl, Br, and I; the results suggested that these types of double perovskites have low bandgap and high absorption. The massive density of states and optical properties such as high dielectric constant of about 5.33 and 6.30 for Cs2AgSbI6 and Cs2CuSbI6 respectively, accepted these materials as a highly applicable double perovskite compounds. The large bulk modulus and maximum tolerance factor of Cs2CuSbCl6 among all six compounds make it an extremely reliable compound from structural stability standards. The obtained result shows that these materials are optically active in the visible and ultraviolet regions inclusive of ductile therefore they can be productively utilized for optoelectronic devices. The electronic transport properties have been calculated within the constant relaxation time approximation. Seebeck coefficient is noted to decreasing with increasing temperature and figure of merits are increasing with increasing temperature at a given electron and hole concentration (1018-1021 ​cm−3). The figure of merit signifies that these materials may also be used as thermoelectric devices. These computational observations hereby are of paramount importance for future integrated applications.

Exploring original properties of GaN-BN alloys using high-throughput ab initio computation

GaN alloyed with boron (B) is a potential candidate for wide band gap materials and for ultraviolet optoelectronic device applications. Using evolutionary algorithm and density functional theory, we investigate here the thermodynamic, mechanical, dynamical and optical properties of GaN-BN alloys. Three ordered BxGa1-xN structures: cubic (BGa3N4, and B3GaN4, space group P-43 m), and tetragonal (BGaN2, space group P-42 m) have been explored. The calculated relatively large mixing enthalpies point out that large miscibility gap exists in BGaN system (i.e., ΔH∼80 meV for x = 0.5). However, weak ΔH for x = 0.25 (ΔH∼24 meV) suggests that BGa3N4 compound is –thermodynamically- slightly unstable. Analysis of the computed elastic constants and phonon dispersion curves reveal that the three ordered structures are mechanically and dynamically stable. Our results show that BGa3N4 exhibits outstanding mechanical properties, such as a large bulk modulus (~328 GPa), important shear modulus (~248 GPa), and strong Vickers hardness of ~ 33 GPa. Moreover, the use of LDA-1/2 functional evidences that the incorporation of boron in GaN gives a huge optical band-gap bowing parameter of b∼11 eV for the entire range of composition, which is in agreement with experiment. In addition, we highlight the prevailing role of chemical and stress effects responsible of the giant disorder in this ternary system.

High Pressure Effects on the Structural Properties of GaN Compound Using Equations of State

The present study is a theoretical calculation for the effects of high pressure on thermodynamic properties on GaN up to 40Gpa at room temperature. Volume compression ratio (V0/Vp), lattice constant (a) and elastic bulk modulus(B) have been established. Furthermore, lattice frequencies and disruptions function by analyzing phonon frequency spectrum (PFS) at (0 K). The entire calculations rely on using of two equation of state (EOS) "Birch-Murnaghan and modified Lennard-Jones" equation of state and with the integration of Grüneisen approximation theory. From the considered equations of state, formulation of bulk modulus was derived, that predicts a rising trend of bulk modulus. The large bulk modulus value of GaN has made a small fraction of change in volume (less than 15%) of the material even under an extreme pressure up to 45Gpa. It was also found that the results of phonon frequency spectrum obtained from Birch-Murnaghan equation of state in a better agreement with the experimental data than that of modified Lennard-Jones equation of state. Given that the Birch-Murnaghan equation of state developed according to Eulerian strain theory accounted as a universal equation of state. Moreover, good agreement between theoretically present calculations and experiment data of phonon frequency spectrum, reveals the validity of the equations of state used in the present study.

The new stable phases of Fe2Pd crystal alloy and their properties

This work presents a combination of particle swarm optimization crystal structure search method and first-principles calculations to investigate new stable crystal structures of Fe2Pd. Three structurally stable tetragonal phases (with space groups I4/mmm, P4/mmm, P4/nmm) and a structurally stable cubic phase (C15 Laves phase (Fd3¯m)) are predicted at 0 GPa. The enthalpy of formation reveals that these four phases are thermodynamically stable in the ferromagnetic state, and the tetragonal P4/nmm phase has the highest stability. Calculated phonon spectra indicates that the remaining three predicted structures are dynamically stable except for I4/mmm structure. From the elastic constant, all of the predicted structures are mechanically stable phases. Among these studied structures, the three tetragonal phases with similar values of bulk modulus have high uniaxial magneto-crystalline anisotropy, high thermal expansion coefficient, large bulk modulus, and good ductility. However, the cubic Fd3¯m phase is characterized by low magneto-crystalline anisotropy, small thermal expansion coefficient, high shear and Young’s modulus, and brittleness. Finally, the high-pressure stability of these four structures is investigated. The theoretical prediction indicates a structural transition from tetragonal P4/nmm phase to ductile Fd3¯m structure at 26.8 GPa, while the I4/mmm is the most stable above 80 GPa.

First-principles investigations of tricarbon: From the isolated C3 molecule to a novel ultra-hard anisotropic solid

Based on crystal chemistry considerations and quantum density functional theory ground state calculations, rhombohedral rh-C3 and hexagonal h-C6 carbon allotropes are proposed and energetically calculated as new stable ultra-hard phases likewise lonsdaleite. Along the two kinds of carbon in linear C2-C1-C2 lattice, distorted tetrahedra C2(sp3) with an angle of 106.17{\deg} (smaller than ideal 109.4{\deg}) and C1(sp)-like hybridizations are inferred from charge density projections. The calculated elastic constants point to a strong anisotropy of mechanical properties of rh-C3 and h-C6 with an exceptionally large C33 values (1636 GPa and 1610 GPa, respectively), exceeding that of lonsdaleite (1380 GPa), due to the presence of aligned tricarbon units along the hexagonal c-axis. Both phases are characterized by large bulk moduli and high hardness values that are slightly less than those of lonsdaleite and diamond. Weak metallic behavior of both new phases is identified from electronic band structure calculations.

Crystal structures and mechanical properties of tungsten monocarbide predicted by first-principles investigations

The crystal structure of tungsten monocarbide (WC) is researched from 0 to 650 GPa through first principles calculations. The results verify that the experimental structure (hP2-WC) with the space group P6¯m2 is the most stable phase in a wide range of pressure. Above 231 GPa, a new stable structure (space group P63/mmc, hP4-WC) is found to be the most stable phase, and it will transform to a CsCl-type phase (cF8-WC) around 582 GPa. Phonon calculations reveal that the hP4-WC phase is dynamically stable and may be a metastable structure at ambient conditions. The cF8-WC phase possesses dynamical stability above 20 GPa. The hP4-WC phase is a low compressible material with a large bulk modulus of 377 GPa at zero pressure. The hardness values of hP2-WC and hP4-WC at zero pressure are 32 and 21 GPa, respectively, while the cF8-WC phase possesses a hardness of 21 GPa at 20 GPa, implying that these phases are potential hard materials. The temperature–pressure phase boundary of WC is obtained by means of the quasi-harmonic approximation method. As the temperature increases, the transition pressure from hP2-WC to hP4-WC remained nearly unchanged. The transition pressure between hP4-WC and cF8-WC decreases with the increasing temperature.

Crystal chemistry rationale and ab initio investigation of ultra-hard dense rhombohedral carbon and boron nitride

Rhombohedral dense forms of carbon, rh-C2 (or hexagonal h-C6), and boron nitride, rh-BN (or hexagonal h-B3N3), are derived from rhombohedral 3R graphite based on original crystal chemistry scheme backed with full cell geometry optimization to minimal energy ground state computations within the quantum density functional theory. Considering throughout hexagonal settings featuring extended lattices, the calculation of the hexagonal set of elastic constants provide results of large bulk moduli with B0 (rh-C2) = 438 GPa close to that of diamond, and B0 (rh-BN) = 369 GPa close to that of cubic BN. The hardness assessment in the framework of three contemporary models enables both phases to be considered as ultra-hard. From the electronic band structures calculated in the hexagonal Brillouin zones, 3R graphite is a small-gap semiconductor, oppositely to rh-C2 that is characterized by a large band gap of 6 eV, and rh-BN is a large band gap insulator with Egap = 5 eV.

Crystal chemistry and ab initio prediction of ultrahard rhombohedral B2N2 and BC2N

New ultrahard rhombohedral B2N2 and BC2N – or hexagonal B6N6 and B3C6N3 – are derived from 3R graphite based on crystal chemistry rationale schematizing a mechanism for 2D → 3D transformation. Full unconstrained geometry optimizations leading to ground state energy structures and energy derived quantities as energy-volume equation of states (EOS) and the elastic constants were based on computations within the density functional theory (DFT) with generalized gradient approximation (GGA) for exchange-correlation (XC) effects. The new binary and ternary phases are characterized by tetrahedral stacking alike diamond, visualized with charge density representations, and illustrating ion characters. Atom averaged total energies are similar between cubic BN and rh-B2N2 on one hand, and larger stabilization of rhombohedral BC2N versus cubic and orthorhombic forms (in literature) assigned to favorable C–C and B–N bonding, on the other hand. The electronic band structures are characteristic of insulators with Egap ~5 eV. Both phases are characterized by large bulk and shear moduli and high hardness values.

Properties of Polyhedral Oligomeric Silsesquioxane-Modified Cellulose Insulation Paper with Different Number of Phenyls

Polyhedral oligomeric silsesquioxane (POSS) has a total of eight substituents. The number of substituents determines the modification effect of POSS. Through simulation of molecular dynamics, this paper explores how the number of substituents affects the POSS-modified cellulose insulation paper. Specifically, the mechanical properties, thermal stability and polarizability were calculated for cellulose models with 1-phenyl POSS, 2-phenyl POSS, 3-phenyl POSS, 4-phenyl POSS, 5-phenyl POSS, 6-phenyl POSS, 7-phenyl POSS, and 8-phenyl POSS, respectively. The results show that the cellulose model modified by 6-phenyl POSS achieved better effect than the other seven models, as evidenced by its distinctively large bulk modulus, shear modulus, and elastic modulus. Besides, the cellulose model modified by 6-phenyl POSS had the most stable and the minimum mean square displacement (MSD). This is because the mechanical properties of the insulation paper are improved, as the motion of cellulose chains is inhibited by the nano-size effect of POSS and the effect of phenyls. Further, the cellulose model modified by 6-phenyl POSS also had the smallest polarizability among the modified models. Therefore, the 6-phenyl POSS-modified cellulose model can effectively reduce the polarizability of cellulose, uniform the electric field distribution in oil-paper insulation system, and thus enhance the insulation property of transformers.

Probing impact of molecular structure on bulk modulus and impact sensitivity of energetic materials by machine learning methods

Machine learning (ML) can map the complex relationship between structures and properties, thus exhibiting great promise in various fields. The bulk modulus (mechanical property) and the impact sensitivity are crucial for energetic compounds. However, relationships have not been elucidated between the molecular structure and the bulk modulus and between the two important properties. Thus, artificial neural network (ANN) and sure independence screening and sparsifying operator (SISSO) methods coupled with selected molecular descriptors were introduced to explore their relationship models for 240 nitroaromatic compounds. In total, six ML models are constructed. The coefficient of determination, root mean squared error and the t-test from 10-fold cross-validation indicate that ANN significantly outperforms SISSO for the bulk modulus and the impact sensitivity models. However, SISSO exhibits comparable prediction accuracy with ANN for the relationship model between the impact sensitivity and the bulk modulus. Besides, the feature evaluation reveals that the average molecular weight, the number of unsaturated and oxygen-containing groups, hydrophilicity and three-dimensional structure play major roles in influencing the bulk modulus while main contributions to the impact sensitivity come from the structure features involving oxygen-containing group, distribution of atomic properties and hydrophilicity. Furthermore, the SISSO model explicitly gives an inverse-like function relationship for the two important properties. It indicates that the impact sensitivity sharply decreases with increasing bulk modulus when the bulk modulus is small, while an opposite trend occurs in the case of relatively large bulk modulus. These findings could provide useful structural information for improving comprehensive properties of energetic materials.

Stability, electronic structure, mechanical properties and lattice thermal conductivity of FeS and FeS2 polymorphs

First-principles calculations were used to investigate the stability, electronic structure, elastic and lattice thermal conductivity of FeS and FeS2 polymorphs ([Formula: see text]-FeS, [Formula: see text]-FeS, [Formula: see text]-FeS, [Formula: see text]-FeS2, [Formula: see text]-FeS2). The calculated lattice parameters were in agreement with experimental results. The results showed that these Fe-S binary compounds are thermodynamically and mechanically stable. The elastic anisotropies of Fe-S binary compounds were exhibited by 3D modulus ball and 2D projections. Among all the five compounds, [Formula: see text]-FeS2 compound has the largest bulk modulus and [Formula: see text]-FeS2 has the largest Young’s modulus and hardness. Furthermore, [Formula: see text]-FeS, [Formula: see text]-FeS and [Formula: see text]-FeS compounds can be regarded as ductile material according to [Formula: see text] and Poisson’s ratio. The FeS compounds show metallic character and FeS2 compounds show semiconductor character through analyzing their bandgap and density of states (DOS). The [Formula: see text]-FeS2 has the largest thermal conductivity according to the Clarke model, and the [Formula: see text]-FeS shows the strongest thermal conductivity anisotropy among the five compounds.

Prediction of scandium tetraboride from first-principles calculations: Crystal structures, phase stability, mechanical properties, and hardness* *Project supported by the Young Scientists Fund of the National Natural Science Foundation of China (Grant Nos. 11704170 and 61705097) and the Natural Science Foundation of Shandong Province, China (Grant Nos. ZR2016AP02 and ZR2016EMP01).

Using the evolutionary methodology for crystal structure prediction, we have predicted the orthorhombic Cmcm and Pnma phases for ScB4. The earlier proposed CrB4-, FeB4-, MnB4-, and ReP4-type structures for ScB4 are excluded. It is first discovered that the Cmcm phase transforms to the Pnma phase at about 18 GPa. Moreover, both phases are dynamically and mechanically stable. The large bulk modulus, shear modulus, and Young’s modulus of the two phases make it an optimistic low compressible material. Moreover, the strong covalent bonding nature of ScB4 is confirmed by the ELF analysis. The strong covalent bonding contributes greatly to its stability.

Piezoelectric properties of triply periodic minimum surface structures

Piezoelectric ceramic-polymer composites have attracted substantial interest owing to their distinct piezoelectric performance. This paper investigates the dependence of their output voltage on the volume fraction and structure of the ceramic component, together with the type of stimulus, using finite element analysis. When ceramic parts of piezocomposites are shaped into structures with a topology of triply periodic minimum surface such as Schwarz Primitive surface, Gyroid surface, and Neovius surface, they exhibit much better piezoelectric performance than existing piezocomposites under both the compressive strain and the shear strain. Compared to a piezocomposite with three intersecting ceramic cuboids, Schwarz piezocomposite with the same volume fraction of 50% can increase output voltage by approximately 50% under compressive strains 2%–8%. With 16% ceramic material and under a compressive strain of 8%, Neovius piezocomposite demonstrates ~17-fold and ~6,000-fold enhancement of output voltage than that of the piezocomposite in the 3-3 mode (connected and irregularly-shaped ceramic component) and in the 0–3 mode (disconnected ceramic particles), respectively. Under simple shear, performance superiority of Neovius piezocomposite to that of the 3-3 mode piezocomposite becomes more significant as output voltage can be enhanced up to approximately 30-fold. Computational analysis shows that high von Mises stress helps to enlarge the difference between positive and negative electrical potential, and therefore enhance output voltage. The findings in this work also reveal output voltage is inversely proportional to strain energy stored in piezocomposites. Because Schwarz piezocomposite has the largest bulk modulus with minimum strain energy under compression, it has the maximum output voltage.

An ultra-incompressible Mn3N compound predicted by first-principles genetic algorithm

Using genetic algorithms for an unbiased structure search and first-principles total-energy calculations, a stable manganese nitride, Mn3N, is discovered. Mn3N is a nonmagnetic metal and isostructural to superhard Re3N. Mn3N exhibits a large bulk modulus and incompressibility comparable to that of the ultra-incompressible OsB. We show that the large bulk modulus can be attributed to the strong covalent bonding in this system. Phonon calculations and analysis confirm the dynamical stability of the Mn3N compound. We also show that weak electron–phonon coupling leads to a small superconducting transition temperature for Mn3N.

A density functional theory study on the interface stability between CsPbBr3 and CuI

This paper assesses the interface stability of the perovskite CsPbBr3 and transport layer CuI using density functional theory and band offset calculations. As a low-cost, more stable alternative to current hole transport materials, CuI may be used to template the epitaxial growth of perovskites such as CsPbBr3 owing to a 1% lattice constant mismatch and larger bulk modulus. We compare all eight atomic terminations of the interfaces between the (100) low-energy facet for both CsPbBr3 and CuI, increasing material thickness to consider charge density redistribution and bonding characteristics between surface and bulk-like regions. A low energy atomic termination is found to exist between these materials where alternating charge accumulation and depletion regions stabilize bonds at the interface. Band offset calculations reveal a type I straddling gap offset in the bulk shifting to a type II staggered gap offset as the thickness of the materials is increased, where the built-in potential changes as layer thickness increases, indicating the tunability of charge separation at the interface. CuI may, thus, be used as an alternative hole transport layer material in CsPbBr3 optoelectronic devices.

Extreme incompressibility and hardness of Re-N system

Synthesized hexagonal phases, Re3N and Re2N, have always been considered to be the ground-state structures of ruthenium-nitrides systems, whereas theoretical studies proposed cubic (F-43m), tetragonal (P42/mnm), orthorhombic (Pbca, Pbcn), hexagonal (P-6m2, P63/mmc), and monoclinc (C2/m) crystal phases as the stable ground state structure of Re-N systems. However, the present state-of-the-art evolutionary technique combined with first-principles theory calculations revealed that cubic Pm-3m ReN is the real ground-state-phase of ReN systems under ambient conditions. The proposed cubic-ReN is found thermodynamically, mechanically and dynamically stable. The present study evinces that ReN has huge elastic constants C11(∼830 GPa), large bulk (∼415 GPa), and shear modulus (∼222 GPa). Theoretical Vickers hardness is estimated to be ∼20 GPa and thus support that ReN is ultra incompressible and potential hard material. The analysis of the partial density of states reveals that the covalent-ionic bonding between 5d-rhenium and 2p-nitrogen states is the driving force of the fascinating mechanical properties of Re-N systems. This study could provide additional insight into the cubic Pm-3m crystal phase of Re-N systems that are not readily apparent from experiments.

Investigation on the stability, electronic, optical, and mechanical properties of novel calcium carbonate hydrates via first‐principles calculations

AbstractCalcium carbonate (CaCO3) is an inorganic compound which is widely used in industry, chemistry, construction, ocean acidification, and biomineralization due to its rich constituent on earth and excellent performance, in which calcium carbonate hydrates are important systems. In Zou et al's work (Science, 2019, 363, 396‐400), they found a novel calcium carbonate hemihydrate phase, but the structural stability, optical, and mechanical properties have not been studied. In this work, the stability, electronic, optical, and mechanical properties of novel calcium carbonate hydrates were investigated by using the first‐principles calculations using density functional theory. CaCO3·xH2O (x = 1/2, 1 and 6) are determined dynamically stable phases by phonon spectrum, but the Gibbs energy of reaction of CaCO3·1/2H2O is higher than other calcium carbonate hydrates. That is why CaCO3·1/2H2O is hard to synthesize in the experiments. In addition, the optical and mechanical properties of CaCO3·xH2O (x = 1/2, 1 and 6) are expounded in detail. It shows that the CaCO3·1/2H2O has the largest bulk modulus, shear modulus, and Young's modulus with the values 60.51 GPa, 36.56 GPa, and 91.28 GPa. This work will provide guidance for experiments and its applications, such as biomineralization, geology, and industrial processes.

Effect of Structural Phases on Mechanical Properties of Molybdenum Disulfide.

Molybdenum disulfide (MoS2) is a promising layer-structured material for use in many applications due to its tunable structural and electronic properties in terms of its structural phases. Its performance including efficiency and durability is often dependent on its mechanical properties. To understand the effects of the structural phase on its mechanical properties, a comparative study on the mechanical properties of bulk 2H, 3R, 1T, and 1T′ MoS2 was conducted using the first-principles density functional theory. Since considerable applications of MoS2 are developed through strain engineering, the impact of the external pressure on its mechanical properties was also considered. Our results suggest a strong relationship between the mechanical properties of MoS2 and the structural symmetry of its crystal. Accordingly, the impacts of the external pressure on the mechanical properties of MoS2 also greatly vary with respect to the structural phases. Among all of the considered phases, the 2H and 3R MoS2 have a larger bulk modulus, Young’s modulus, shear modulus, and microhardness due to their higher stability. Conversely, 1T and 1T′ MoS2 are less strong. As such, 1T and 1T′ MoS2 can be a better candidate for strain engineering.

Tuning to more compressible phase in TiZrHfNb high entropy alloy by pressure

In this work, the starting nominal Ti25Zr25Hf25Nb25 high entropy alloy (HEA) has two body centered cubic (BCC) phases with a volume percentage about 100:1, with the primary phase having a much larger bulk modulus (incompressible) than the uniform single-phase HEA. We found that these two phases merged into one single BCC phase at pressures beyond 36 GPa, whose bulk modulus dropped to that of the normal homogeneous HEA. After decompressing, the new phase can be sustained to ambient conditions. This abnormal pressure-induced softening was largely related to the lattice distortion evolution and interfacial energy during compression.