Crystal chemistry and DFT investigation of an ultrahard novel triclinic allotrope C 8 with a structure close to that of diamond

Prediction of new 212 M2AB2 borides as a promising candidate for future engineering: DFT calculations

In this Letter a novel triclinic tri-C8 with low symmetric space group P-1 No. 2 is proposed from crystal engineering of corner sharing C4 tetrahedra stacking. DFT-based calculations of the ground state structure show that tri-C8 belongs to “sie; deh3” topology characterizing base-center orthorhombic space group C16 allotrope and considered thereupon as a distorted structure from higher symmetry. Energy-derived properties such as the cohesive energy, the mechanical, dynamical, thermodynamic and electrons band structure show close relationship with Diamond especially regarding the high density of 3.44 g/cm3, the ultra-hard behavior with Vickers hardness HV=83 GPa and the largely insulating electronic system

We have studied the crystal chemistry and magnetic, thermodynamic, and transport properties of $R{\mathrm{Ni}}_{9}{\mathrm{Si}}_{4}$ with $R=\mathrm{La}$ and Ce. These compounds crystallize in a fully ordered tetragonal (space group $I4/mcm)$ variant of the cubic ${\mathrm{NaZn}}_{13}$ type. The low-temperature properties characterize ${\mathrm{CeNi}}_{9}{\mathrm{Si}}_{4}$ as a Kondo lattice with a large Sommerfeld value $\ensuremath{\gamma}=155(5){\mathrm{m}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{}\mathrm{K}}^{2}$ as compared to $\ensuremath{\gamma}=33{\mathrm{m}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{}\mathrm{K}}^{2}$ of Pauli paramagnetic ${\mathrm{LaNi}}_{9}{\mathrm{Si}}_{4}.$ The temperature dependencies of the specific heat and susceptibility are well described by the degenerate $(J=5/2)$ Coqblin-Schrieffer model with a characteristic temperature ${T}_{0}\ensuremath{\simeq}180\mathrm{K}.$ The large Ce-Ce spacing in ${\mathrm{CeNi}}_{9}{\mathrm{Si}}_{4}$ ${(d}_{\mathrm{C}\mathrm{e}\ensuremath{-}\mathrm{C}\mathrm{e}}\ensuremath{\approx}5.6\mathrm{\AA{}})$ implies very weak Ce-Ce intersite exchange interactions which is corroborated by the thermoelectric power $S(T)$ showing close agreement with theoretical results of the degenerate Anderson lattice without intersite interactions. ${\mathrm{CeNi}}_{9}{\mathrm{Si}}_{4}$ appears to be a model type Kondo lattice system with ${T}_{0}g{\ensuremath{\Delta}}_{\mathrm{CEF}}\ensuremath{\gg}{T}_{\mathrm{RKKY}}$ where ${T}_{0},$ ${\ensuremath{\Delta}}_{\mathrm{CEF}}$ (crystalline electric field), and ${T}_{\mathrm{RKKY}}$ (Ruderman-Kittel-Kasuya-Yosida) are the characteristic energy scales of the Kondo interaction, crystal field splitting, and Ce-Ce intersite exchange coupling, respectively. ${\mathrm{CeNi}}_{9}{\mathrm{Si}}_{4}$ shows a remarkably low ratio $A/{\ensuremath{\gamma}}^{2}=0.83(8)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}\ensuremath{\mu}\ensuremath{\Omega}\mathrm{cm}(\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{K}/\mathrm{m}\mathrm{J}{)}^{2}$ which is one order-of-magnitude smaller than the usual Kadowaki-Woods ratio of heavy-fermion systems.

Aiming to study the electronic structures, mechanical properties, and thermodynamic characteristics within framework of density function theory for scandium carbide (ScxCy) compounds, first-principles calculations were performed. The calculated cohesive energy and formation enthalpy indicate that the compounds have the thermodynamically stable structures. Their electronic characteristics indicate that ScC, Sc2C, Sc2C3, and Sc3C4 present metallic character but Sc4C3 presents semiconductor character. According to the stress-strain method, the elastic constants were calculated. In a framework of the Voigte-Reusse-Hill approximation, bulk modulus (B), shear modulus (G), Young's modulus (E), Poisson's ratio (σ), and Vickers hardness ( $${{H}_{{v}}}$$ ) were analyzed. The largest value of B, G, E are found from Sc2C3. Both ScC and Sc2C3 are ductile whereas other compounds are brittle. Mechanical anisotropy analysis indicates that Sc2C3 has the highest anisotropy because its percent anisotropy AG (0.038) is the largest one. Furthermore, the relationship of thermodynamic properties such as specific heat capacity, entropy, Helmholtz free energy and internal energy with temperature were analyzed.

Modified extended Finnis Sinclair potential for cubic crystal metal

Tailoring body center tetragonal allotropes Cx (x = 8, 12, 16, 20) with C(sp3) and hybrid C(sp3)/C(sp2): Crystal chemistry and ab initio investigations of the physical properties

By applying ab initio calculation within density functional theory (DFT), we study the structure parameters, electronic band structure, elastic coefficients, polycrystalline elastic properties, anisotropy factors and Debye temperature of ferroelectric and paraelectric phases of LiTaO3 within the generalised gradient approximation at ambient pressure. The atomic structure in both phases is fully relaxed and the lattice constant, angle and atomic positions are well consistent with experimental values. The computed single-crystal elastic coefficients indicate that mechanical stability of LiTaO3 in both phases is confirmed using the generalised Born criteria. The shear, bulk and Young’s modulus, Poisson’s ratio, and Vickers hardness were computed according to theoretical elastic constants by Voight–Reuss–Hill method. Several anisotropy factors and indexes are computed to illustrate mechanical anisotropy. Both phases are shown to be weakly anisotropic. The Debye temperature is estimated using the longitude and transverse elastic wave velocity of the ideal polycrystalline LiTaO3 aggregates. We have found that LiTaO3 in both phases has an indirect energy band gap. The differences in the electronic structure and density of states for both phases are quite small. Our results indicate that the mechanical and bonding properties of both phases are very similar. The obtained results were compared with the available experimental and theoretical values.

Using the one-atom theory (OA) of pure metals, the atomic states of Ru metal with hcp structure, fcc structure, bcc structure and liquid state were determined as follows: [Kr](4d n )3.78(4d c )2.22(5s c )1.77(5s f )0.23, Ψ a (fcc-Ru)=[Kr](4d n )3.70(4d c )2.44(5s c )1.42(5s f )0.44, Ψ a (bcc-Ru)=[Kr](4d n )4.00(4d c )2.22(5s c )1.56(5s f )0.22, Ψ a (L-Ru)=[Kr](4d n )4.00(4d c )2.00(5s c )1.52 (5s f )0.48. The potential curve and physical properties as a function of temperature for hcp-Ru such as lattice constant, cohesive energy, linear thermal expansion coefficient, specific heat and Gibbs energy and so on were calculated quantitatively. The theoretical results are in excellent agreement with experimental value. The relationship between the atomic states and catalytic performance was explained qualitatively and these supplied the designation of Ru metal and relative materials with theoretical instruction and complete data.

All-atom models of liquid hydrocarbons (n-pentane, n-hexane, n-octane, neohexane, isooctane, cyclohexane, cyclohexene, toluene, benzene) are studied by the Monte Carlo method at atmospheric pressure and room temperature. The approach based on a simplified consideration of deformation energy of valence bonds and bond angles allows developing highly efficient supercomputer programs to study large biopolymer systems within the framework of classical physics. Structural and thermodynamic characteristics of hydrocarbons (molar heat of vaporization, density, specific heat (cP), isothermal compressibility, volumetric coefficient of thermal expansion) calculated within this approach agree well with experimental values.

The crystal chemistry of three Li-bearing mica-2 M 2 crystals from pegmatites has been studied by chemical analyses and single crystal X-ray diffraction; their belonging to the trilithionite-polylithionite join is highlighted by the following compositional ranges in atoms per formula unit [based on O 12-(x+y) (OH) x F y ]: 3.198 ≤ Si ≤ 3.538, 0.462 ≤ [IV] Al ≤ 0.811, 1.195 ≤ [VI] Al ≤ 1.390, 0.031 ≤ (Fe+Mn) ≤ 0.072, 1.522 ≤ Li ≤ 1.757, 0.872 ≤ K ≤ 0.906, 0.030 ≤ Na ≤ 0.073, 0.000 ≤ (Cs+Rb) ≤ 0.099, 1.541 ≤ F ≤ 1.722. The correlation between F and Li content is confirmed, as observed in Li-rich micas. Crystal structure refinements were carried out in space group C 2/ c ( R values vary between 0.030 and 0.031). The crystal chemistry is mostly influenced by tetrahedral chemical composition. Increasing [IV] Al content, α and ψ M1 parameters increase; Si content involves a lowering of the interlayer separation and tetrahedral thickness. Li content affects octahedral thickness. The stability of 2 M 2 polytype seems to be induced by a relative increase of Δz tetrahedral parameter, which reduces the repulsion between basal tetrahedral oxygen atoms. Unlike Li-bearing muscovite, trioctahedral Li-bearing mica crystals show an octahedral occupancy not related to octahedral charge.

From quartz (qtz) to diamond (dia) carbon topologies: Stepwise rationale from crystal chemistry and DFT investigations

Tetragonal and hexagonal hybrid sp3/sp2 carbon allotropes C5 were proposed based on crystal chemistry and subsequently used as template structures to identify new binary phases of the B-N system, specifically tetragonal and hexagonal boron nitrides, B2N3 and B3N3. The ground structures and energy-dependent quantities of the new phases were computed within the framework of quantum density functional theory (DFT). All four new boron nitrides were found to be cohesive and mechanically (elastic constants) stable. Vickers hardness (HV), evaluated by various models, qualified all new phases as superhard (HV > 40 GPa). Dynamically, all new boron nitrides were found to be stable from positive phonon frequencies. The electronic band structures revealed mainly conductive behavior due to the presence of π electrons of sp2-like hybrid atoms.

Shape and size dependent thermophysical properties of nanocrystals

In this paper, a new theoretical two-phase (solid–liquid) type model of melting temperature has developed based on the modified Gibbs–Thomson equation. Further, it is extended to derive other different size-dependent thermodynamic properties such as cohesive energy, Debye temperature, specific heat capacity, the thermal and electrical conductivity of metallic nanoparticles. Quantitative calculation of the effect of size on thermodynamic properties resulted in, varying linearly with the inverse of characteristic length of nanomaterials. The models are applied to Al, Pb, Ag, Sn, Mo, W, Co, Au and Cu nanoparticles of spherical shape. The melting temperature, Debye temperature, thermal and electrical conductivity are found to decrease with the decrease in particle size, whereas the cohesive energy and specific heat capacity are increased with the decrease in particle size. The present model is also compared with previous models and found consistent. The results obtained with this model validated with experimental and simulation results from several sources that show similar trends between the model and experimental results.Graphic abstract

Cohesive energy and energy fluctuation as a measure of stability of alloy phases