Energy-volume Equations Of State Research Articles

Ultra-hard rhombohedral carbon by crystal chemistry and ab initio investigations

A new ultra-hard rhombohedral carbon rh-C4 (or hexagonal h-C12) is reported as derived from 3R graphite through crystal chemistry construction and ground state energy within the density functional theory. A resulting cohesive extended hexagonal three-dimensional network of h-C12 is identified as formed of C4 tetrahedra like in cubic and hexagonal diamond. The electronic band structure of rh-C4 is characteristic of insulator with Egap ​= ​4 ​eV similarly to diamond. From the energy-volume equations of state and the set of elastic constants a larger value of bulk modulus versus lonsdaleite, and the largest Vickers hardness (HV) versus both forms of diamond were revealed. The phonon band structure showing only real wave numbers further confirms the stability of the new carbon phase.

Nitrided FeNi: Chemical versus magnetovolume effects from ab initio

Abstract Magnetic, magneto-volume properties and bonding analyses of binary intermetallic FeNi and its nitride FeNiN have been investigated within density functional theory DFT. From total energy calculations and establishing the energy-volume equations of states (EOS) the ground state of both compounds is ferromagnetic with a reduced magnetization of the nitrided intermetallic due to spin pairing between Fe/Ni and N. The volume expansion indicates that iron shows itinerant magnetism, whereas Ni exhibits a localized behavior. Important magneto-volume effects are observed whereby saturation of the magnetic moment is reached in FeNi but not in FeNiN. The chemical bonding shows Ni-N and Fe-N contributions to the stability of the nitride systems. However, this bonding, which reduces magnetization around experimental volume, loses its effect at large volumes where magneto-volume influence prevails again as in FeNi.

First principles investigations of Fe, Co, Ni in model honeycomb carbon networks

The behaviors of ferromagnetic transition metals of the first period: Fe, Co and Ni are examined within density functional theory calculations in two dimensional carbon extended networks using model structure LiC6. Around geometry optimized structures, the energy-volume equations of states considering non magnetic and spin polarized configurations established ferromagnetic ground states with magnetizations –reduced with respect to the metals’– of 2 μB for FeC6 and 1 μB for CoC6 while no magnetic solution could be identified for NiC6. In the D6h point group of the P6/mmm space group lm decomposition of the d states results with increasing energy into doublet state E1g with d(x2-y2) and d(xy); singlet state A1g d(z2) and doublet state E2g d(xz) and d(yz) lying on EF and responsible of the onset of magnetic moments. This was mirrored via molecular orbital approach with a construct of Fe embedded between two extended carbon networks thus validating the model structure proposed for TC6 compounds. The 100% polarization in one spin channel allows proposing potential uses in spintronics applications.

First-Principles Study on Polymorphs of AgVO3: Assessing to Structural Stabilities and Pressure-Induced Transitions

In this paper, we present a comprehensive theoretical study, based on density-functional theory calculations, and which focuses on the structural and electronic properties of silver vanadium oxide (AgVO3) in the monoclinic [Cm (β-AgVO3), C2/c (α-AgVO3), and Cc], orthorhombic (Amm2), and cubic (Pm3̅m) phases from 0–30 GPa. The structural and electronic properties, the stability of different phases, and the pressure-induced solid–solid phase transitions of AgVO3 have been previously studied. The effects of pressure on the band structures, energy–gap values, density of states, and vibrational frequencies are also studied. Numerical and analytical calculations are conducted to obtain the lattice parameters, the bulk modulus K and their pressure derivative K′, and the energy-volume equations of state. The influence of different parametrizations of the exchange-correlation functional (B3LYP, HSE06, and PBE) on the investigated properties is analyzed, and the results are compared to available experimental data. For the first time, a complex and unexpected structural and chemical behavior as a function of pressure is reported. The β-phase is the most stable and the first phase transition between the monoclinic β-AgVO3 and Cc phase takes place at 5 GPa (B3LYP), 3 GPa (HSE06), and 2 GPa (PBE). There are pressure-induced transitions among the β-, α-monoclinic, and cubic structures, and the corresponding values for the pressure transitions are dependent on the functional used. Two new polymorphs, monoclinic Cc and orthorhombic (Amm2), have been characterized for the first time, and their contrasting structural stabilities as well as their transition mechanisms can be understood from the intrinsic characteristics of the crystal lattices. The Badger’s rule is fulfilled for Cm, Amm2, and Pm3̅m polymorphs, while it is invalid for the C2/c and Cc phases. Theoretical results show that the studied reactive channels from β-AgVO3 toward binary oxides, Ag2O and V2O5, AgO and VO2; the elements Ag, V and O2; silver pyrovanadate, Ag4V2O7 and V2O5, as well as Ag2V4O11 and Ag2O are not thermodynamic favorable processes at pressures up to 30 GPa. These results contribute to the understanding of the pressure behavior of AgVO3-based compounds. In addition, it would be interesting to determine whether further measurements and calculations would confirm the predicted structural and thermodynamic properties as well as the solid-state transformations of AgVO3 polymorphs, which have not yet been experimentally shown.

Ab-initio Studies of the Electronic Structures of the Hexavalent Uranium Compounds K2UO4 and Na4UO5

Band theoretical results within the density functional theory are reported for the uranates K2UO4 and Na4UO5. The two structures are differentiated respectively by uranyl and reverse uranyl-type short U-O distances characterizing the UO6 octahedra, as also reflected by larger compressibility and stability of the potassium-based uranate derived from energy-volume equations of states and the cohesive energy. A larger ionic character of Na4UO5 is inferred from comparative Bader charge analyses. The two uranates are insulators with a larger band gap and less extended valence and conduction bands for Na4UO5. Chemical bonding shows major interactions for U-O versus Na,K-O and is further differentiated as a function of the distances. The band structure results confirm the hexavalent character of the uranium atoms.

Changes in electronic, magnetic and bonding properties from Zr2FeH5 to Zr3FeH7 addressed from ab initio

Potential hydrogen storage ternaries Zr3FeH7 and Zr2FeH5, are studied from ab initio with the purpose of identifying changes in electronic structures and bonding properties. Cohesive energy trends: Ecoh. (ZrH2) > Ecoh. (Zr2FeH5) > Ecoh. (Zr3FeH7) > Ecoh. (hypothetic-FeH) indicate a progressive destabilization of the binary hydride ZrH2 through adjoined Fe so that Zr3FeH7 is found less cohesive than Zr2FeH5. From the energy volume equations of states EOS the volume increase upon hydriding the intermetallics leads to higher bulk moduli B0 explained by the Zr/Fe–H bonding. Fe–H bond in Zr2FeH5 leads to annihilate magnetic polarization on Fe whereas Fe magnetic moment develops in Zr3FeH7 identified as ferromagnetic in the ground state. These differences in magnetic behaviors are due to the weakly ferromagnetic Fe largely affected by lattice environment, as opposed to strongly ferromagnetic Co. Hydrogenation of the binary intermetallics weakens the inter-metal bonding and favors the metal–hydrogen bonds leading to more cohesive hydrides as with respect to the pristine binaries. Charge analyses point to covalent like Fe versus ionic Zr and hydrogen charges ranging from covalent H−0.27 to more ionic H−0.5.