Zr 4d States Research Articles

Newly synthesized Pb-based 312 MAX phases M3PbC2 (M = Zr and Hf): A First-principles study

The MAX phases' unique mixture of ceramic and metallic features makes them appealing for various technological uses. Zr3PbC2 and Hf3PbC2, two Pb-based MAX phases, were recently synthesized experimentally. The physical properties, such as structural properties as well as electronic, mechanical, hardness, thermal, and lastly, the optical properties of M3PbC2 (M = Zr and Hf) have been investigated using density functional theory (DFT) and analogized with those of other 312 MAX phases. The optimized cell volume and computed lattice constants match the experimental values. The calculated band structure certifies the metallic character, and DOS calculations confirmed the dominant contribution to conductivity from the Zr-4d and Hf-5d states. The stiffness constant (Cij) proved the mechanical stability of these compounds. The mechanical behavior of the studied compounds was explored by calculating the elastic moduli, hardness parameters, and fracture toughness, which are also compared to those of the 312 MAX phases. The tightly bound M–C covalent bonds within the crystal give these compounds high stiffness. The Mulliken population (both atomic and bond) was calculated to explore the bonding characteristics within them and the Vickers hardness of the titled phases. The degree of elastic anisotropy present within phases has been analyzed by calculating different indices. The phonon dispersion curves and phonon DOS (PHDOS) were computed to check the dynamical stability of the phases. Thermodynamic potential functions were calculated from the PHDOS. The thermal parameters were also studied, including the Debye temperature, melting temperature, Grüneisen parameter, and minimum thermal conductivity (Kmin). A thorough computation and analysis of the vital optical constants was done to explore their potential in diverse fields. The titled compounds are suitable for use as thermal barrier coating (TBC) materials and coating materials to prevent solar heating.

Realizing ferromagnetic semiconductors in SrRu1−xZrxO3 alloys

Using first principle calculations, we study the structural, electric and magnetic properties of SrRu1−xZrxO3 (0 ≤ x ≤ 1). The spin-polarization calculations present that SrRu1−xZrxO3 is a ferromagnetic metal at x = 0, a ferromagnetic semiconductor at x = 0.125, 0.25, an antiferromagnetic semiconductor at x = 0.5 and a nonmagnetic insulator at x = 1, which is in agreement with available experiments. As increasing Zr contents, the lattice parameters and band gaps of SrRu1−xZrxO3 increase while the energy difference between antiferromagnetic and ferromagnetic states decreases. Through Ru-O and Zr-O bonds, hybridization between Ru 4d and Zr 4d states near the Fermi level becomes strong. As a result, Ru 4d states split and then metal-insulator transition occurs at x = 0.125 due to Zr. Ferromagnetic semiconductors are first predicted in SrRu1−xZrxO3 alloys, which may have potential applications in spintronic devices.

Influence of vacancy on helium interaction with α-Zirconium

The first-principle calculations of the interaction between helium and zirconium have been carried out. The main feature of studying such systems is the localization of a He atom in a region near the vacancy in Zr. It has shown that the location of a helium atom in a vacancy vicinity leads to lower formation energy. The calculated density of electron states curves revealed shifts of He 1s state by ~ 0.5 eV towards higher binding energies while located the vacancy vicinity against He-in-vacancy position. Moreover, He 2s states are observed in a region of Zr 4d states from –1.2 to –0.1 eV suggest the hybridization between these states. The crystal orbital Hamilton populations curves have been analyzed to reveal the features of the Zr-He chemical interaction due to the hybridization of He 2s and Zr 4d states.

Probing the physical and superconducting properties of hexagonal ZrRuAs: A first-principles calculation

In the presented work, ab initio pseudopotential calculations have been realized on structural, electronic, elastic, mechanical, phonon and electron-phonon interaction properties for ZrRuAs with the hexagonal ZrNiAl-type layer crystal structure. In the vicinity of the Fermi level, Ru 4d and Zr 4d states dominate and mainly contribute to the conduction properties of ZrRuAs. A critical assessment of elastic and mechanical properties for ZrRuAs reveals that, this superconductor is soft (ductile) due to remarkable metallic bonding. Phonon anomalies have been observed in the phonon spectrum of ZrRuAs for the lowest transverse acoustic branch and low-frequency transverse optical branches along the Γ-K and Γ-M-K symmetry directions. Our electron-phonon interaction calculations indicated that ZrRuAs is a conventional phonon-mediated superconductor with strong electron-phonon coupling parameter of 1.32. The strong electron-phonon coupling parameter of ZrRuAs produces its superconducting transition temperature to be 11.12 K which accords nicely with its experimental values of 12 K.

Recently synthesized (Zr1-xTix)2AlC (0 ≤ x ≤ 1) solid solutions: Theoretical study of the effects of M mixing on physical properties

The effects of M atomic species mixing on the structural, elastic, electronic, and thermodynamic properties of newly synthesized MAX phase (Zr1-xTix)2AlC (0 ≤ x ≤ 1) solid solutions have been studied for the first time by means of density functional theory (DFT) based first principles calculations. The lattice constants are in good accord with the experimental results and found to decrease with Ti contents. The elastic constants, Cij, and the other polycrystalline elastic moduli have been calculated. The elastic constants satisfy the mechanical stability conditions of these solid solutions. The constants, C11, C33 and C44, are found to increase with Ti contents up to x = 0.67, thereafter they decrease slightly. A reverse trend is roughly followed by C12 and C13. The elastic moduli are also found to increase up to x = 0.67, beyond which these moduli go down slightly. Pugh's ratio and Poisson's ratio both confirm the brittleness of (Zr1-xTix)2AlC. Different anisotropy factors revealed the mechanically anisotropic character of these solid solutions. A non-vanishing value of the electronic energy density of states (EDOS) at the Fermi level suggests that (Zr1-xTix)2AlC are metallic in nature. A mixture of covalent, ionic and metallic bonding has been found from the electronic structure with dominant covalent bonding due to hybridization of Zr-4d states and C-2p states. This is also supported by the charge density distribution. The variation of elastic stiffness and elastic parameters with x is seen to be correlated with the partial DOS (PDOS) and charge density distribution. The calculated Debye temperature and minimum thermal conductivity are found to increase with Ti contents, while melting temperature is the highest for x = 0.67. The solid solution with x = 0.67 shows improved mechanical and thermal properties compared to that of the two end members Zr2AlC and Ti2AlC.

Syntheses, crystal and electronic structures of ternary rare-earth zirconium sulfides, RE2ZrS5 (RE = Y, Ce and Pr)

Three ternary rare-earth zirconium sulfides, Y2ZrS5 (1), Ce2ZrS5 (2) and Pr2ZrS5 (3), have been synthesized by high-temperature solid-state reactions. They crystallize in the space group Pnma of the orthorhombic system, belonging to the Y2HfS5 structure type. There are two types of building units in their structures, RES8 bicapped trigonal prisms and ZrS7 monocapped trigonal prisms. Their structures feature polyanionic {[ZrS5]6−}n chains and cubane-like {[RE4Zr2S6]8+}n double chains along the b direction, and RE atoms occupying the polyhedral interspaces between the former chains. The lanthanide contraction phenomenon in the series of RE2ZrS5 (RE=La–Nd, Sm and Gd) is discussed. The electronic band structure and density of states calculations using Material Studio indicate that the direct optical energy gap of 1 is 1.39eV and the optical absorption is mainly ascribed to the charge transition from S-3p to Y-4d and Zr-4d states.

The detailed crystal and electronic structures of the cotunnite-type ZrO2

Abstract The detailed crystal and orbital-decomposed electronic structures of cotunnite-type ZrO2 have been investigated by using the first-principles projector augmented wave (PAW) potential within the generalized gradient approximation as well as taking into account on-site Coulomb repulsive interaction (GGA+U). The optimized structure shows that the O I and O II anions are surrounded by an arbitrary tetrahedron of four Zr cations and an arbitrary pentahedron of five Zr cations, respectively, in turn, the Zr cation is surrounded by an arbitrary tetrakaidecahedron formed by nine oxygen ligands. Although one more Zr cation is coordinated to O II , the larger bond lengths between O II and its adjacent five Zr cations ( d O II − Zr ) than those between O I and its adjacent four Zr cations ( d O I − Zr ) makes density of states (DOS) of s and three p ( p x , p y and p z ) states of the O II anion driving down in lower energy region and driving up in higher energy region. No crystal-field splitting is observed between three p ( p x , p y and p z ) states of anions O I and O II (between three p ( p x , p y and p z ) states and five d ( d xy , d yz , d xz , d z 2 and d x 2 - y 2 ) states of cation Zr) is resulted from the arrangements of the surrounding cations (anions) do not have any symmetry. The additional covalent character upon Zr–O ionic bonds is attributed to the hybridization of itinerant Zr(5s) and less filled Zr(4d) states to the separated O(2s) and O(2p) states.

Toward Optimizing the Performance of Self-Regenerating Pt-Based Perovskite Catalysts

Self-regenerating automotive catalysts owe their remarkable performance to the repeated motion of the precious metal atoms in and out of the perovskite lattice under fluctuating oxidizing and reducing conditions, preventing coalescence of the metal nanoparticles. Here we use resonant inelastic X-ray scattering to characterize the occupied and unoccupied Pt 5d states in two self-regenerating Pt-perovskite catalysts, CaTi0.95Pt0.05O3 and CaZr0.95Pt0.05O3. Upon reduction, the element and symmetry-specific charge excitation spectra reveal a sizable hybridization between the Pt 5d and the Ti 3d or Zr 4d states at the interface between the nanoparticles and the perovskite, which involves the occupied states and is thus invisible in X-ray absorption spectra. A correlation is found between the strength of this d-band hybridization and the proportion of Pt nanoparticles that remain buried below the surface during reduction, indicating that the motion of the Pt atoms toward the surface is hindered by this hybridization specifically, rather than by the Pt–O bonding. These results provide direct evidence that the strength of the metal–metal d-band hybridization plays a pivotal role in determining the efficiency of self-regeneration in perovskite catalysts.

A Theoretical Investigation on the Electron Structures of Al-Based Intermetallic Compounds

Abstract Theoretical investigations were performed to study on alloying stability, and electronic structure of (AlCu3, AlCu2Zr and AlZr3). The results show that the lattice parameters obtained after full relaxation of crystalline cells are consistent with experimental data, and these intermetallics have a strong alloying ability and structural stability due to the negative formation energies and the cohesive energies. The further analysis find out that single-crystal elastic constants at zero-pressure satisfy the requirement of mechanical stability for cubic crystals. The calculations on Poisson’s ratio show that AlCu3 is much more anisotropic than the other two intermetallics. In addition, calculations on densities of states indicates that the valence bonds of these intermetallics are attributed to the valence electrons of Cu 3d states for AlCu3, Cu 3d and Zr 4d states for AlCu2Zr, and Al 3s, Zr 5s and 4d states for AlZr3, respectively; in particular, the electronic structure of the AlZr3 shows the strongest hybridization.

Elastic Properties, Mechanical Stability, and State Densities of Aluminnides

First-principles calculations were performed to study on alloying stability, electronic structure, and mechanical properties of Al-based intermetallic compounds. The results show that the lattice parameters obtained after full relaxation of crystalline cells are consistent with experimental data. The calculation of cohesive energies indicated that the structure stability of these Al-based intermetallics will become higher with increasing Zr element in crystal. The calculations of formation energies showed that AlCu2Zr has the strongest alloying ability, followed by AlZr3 and nally the AlCu3. Further analysis nds out that single-crystal elastic constants at zero-pressure satisfy the requirement of mechanical stability for cubic crystals. The calculations on the ratio of bulk modulus to shear modulus reveal that AlCu2Zr can exhibit a good ductility, followed by AlCu3, whereas AlZr3 can have a poor ductility; however, for sti ness, these intermetallics show a converse order. The calculations on Poisson's ratio show that AlCu3 is much more anisotropic than the other two intermetallics. In addition, calculations on densities of states indicates that the valence bonds of these intermetallics are attributed to the valence electrons of Cu 3d states for AlCu3, Cu 3d and Zr 4d states for AlCu2Zr, and Al 3s, Zr 5s and 4d states for AlZr3, respectively; in particular, the electronic structure of the AlZr3 shows the strongest hybridization, leading to the worst ductility.

Luminescence spectroscopy and electronic structure of ZrP2O7 and KZr2(PO4)3 crystals

The photoluminescence (PL) of ZrP2O7 and KZr2(PO4)3 phosphate crystalline micro-powders grown by spontaneous crystallization method is studied under vacuum ultra-violet (VUV) synchrotron radiation excitations (4–20 eV region of excitation photon energies) in 8–300 K temperature region. The electronic structures (partial densities of states) and optical absorbance spectra of the crystals are calculated by the Full-Potential Linear Augmented Plane Wave Method. Both phosphate crystals reveal PL emission band in the UV spectral region peaking near 300 and 295 nm for ZrP2O7 and KZr2(PO4)3 respectively. The spectral profile of the band weakly depends on temperature. The excitation spectra of the UV emission in each crystal contain intensive excitation band peaking at 189 and 182 nm for ZrP2O7 and KZr2(PO4)3 respectively. The excitation band of the UV emission is related to band-to-band electronic transitions with charge transfer from O 2p to Zr 4d states. The energy band gaps Eg of ZrP2O7 and KZr2(PO4)3 are estimated as 6.7 and 6.6 eV respectively.

Structural, Electronic, and Lattice Dynamics of PbTiO3, SnTiO3, and SnZrO3: A Comparative First-Principles Study

The properties of cubic (Pm3m, 221 space group) perovskite prototypes PbTiO3 (PTO), SnTiO3 (STO), and SnZrO3 (SZO) were investigated via first-principles calculation using the density functional theory as implemented in CASTEP computer code. The lattice parameters of PbTiO3 (as the reference compound) were calculated. The accuracy values of the calculation functional (GGA-PBEsol) were acceptable relative to the experimental values with typical error of approximately 0.6% underestimate. The independent elastic constants (C11, C12, and C44) and bulk modulus, B, were obtained and analyzed. The density of state studies indicated hybridizations among anion O 2p, cation Pb 6s/Sn 5s and the Ti 3d/Zr 4d states of PTO, STO, and SZO. An indirect band gap was respectively obtained for both PTO and STO at the X-G point. A direct band gap was attained for SZO at the X-X point along the high-symmetry direction in the Brillouin zone. The born effective charge values of PTO, STO, and SZO were attributed to the responses of the bond charges to the displacement caused by the strong covalency between the cation orbital and O 2p (strong covalency A-O and B-O bonding). Results also reveal that anion O 2p, cation Pb 6s/Sn 5s, and Ti 3d/ Zr 4d states have played an important role in the instability of perovskite oxide. Comparative results from the PTO, STO, and SZO prototypes showed the calculated theoretical and experimental values were in good agreement.

The detailed geometrical and electronic structures of monoclinic zirconia

The detailed geometrical and orbital-decomposed electronic structures of monoclinic ZrO2 have been investigated by using the first-principle projector augmented wave (PAW) potential within the generalized gradient approximation as well as taking into account on-site Coulomb repulsive interaction (GGA+U). The optimized structure shows the OI atom locates slightly outside the plane of an arbitrary triangle formed by three coordinated Zr atoms and the OII atom is inside an arbitrary tetrahedron formed by four coordinated Zr atoms; in turn, the Zr atom is surrounded by an arbitrary decahedron formed by seven oxygen ligands. Compared to OI atom coordinated to three Zr atoms in an almost planar environment, one more coordinated Zr atom and thus an arbitrary tetrahedron environment of total four coordinated Zr atoms makes density of states (DOS) of s and three p (px, py and pz) states of the OII atom not only shift toward lower energy region but also drive down in higher energy region and drive up in lower energy region. No crystal-field splitting is observed among three p (px, py and pz) states of anions OI and OII as well as among three p (px, py and pz) states and five d (dxy, dyz, dxz, dz2 and dx2−y2) states of cation Zr as the arrangements of the surrounding cations (anions) do not have any symmetry. The additional covalent character of Zr–O ionic bonds is attributed to the hybridization of itinerant Zr(5s) and less filled Zr(4d) states to the separated O(2s) and O(2p) states.

First principles calculation on structural and lattice dynamic of SnTiO3 and SnZrO3

The geometry optimization, density of state (DOS) and lattice dynamic (born effective charges (BEC) and phonon dispersion) of SnTiO3 (STO) and SnZrO3 (SZO) cubic are investigated via first-principles computational methods. The results of DOS show that effect of different B-site cation–(Ti 3d, Zr 4d) states play an important role in chemical interaction particularly in changing of covalency between O2p and cation Sn-5p in STO and SZO, respectively. The BEC calculation shows the anomalous contribution to the Sn charge for both compound STO and SZO beyond the nominal charge of +2. Moreover, the replacement of Ti in STO by Zr strongly modifies the B–O interaction, suppressing the involvement change in nature of the unstable modes of SnZrO3. The results were compared and showed in good agreement with other calculated values using different methods.

Electronic structures and effective masses of photogenerated carriers of CaZrTi2O7 photocatalyst: First-principles calculations

The band structures, density of states and effective masses of photogenerated carriers for CaZrTi2O7 photocatalyst were performed using first principles method with the virtual crystal approximation. The results indicated that CaZrTi2O7 has an indirect band gap of about 3.25eV. The upper valence bands of CaZrTi2O7 are formed by O 2p states mixed with Ti 3d states, Zr 4d, 4p and 5s states, while the conduction bands are dominated by Ti 3d states, Zr 4d states and O 2p states. The calculated valence bands maximum (VBM) potential is located at 2.60V (vs. normal hydrogen electrode (NHE)), while the conduction bands minimum (CBM) potential at −0.65V. Therefore, CaZrTi2O7 has the ability to split water to hydrogen and oxygen under UV light irradiation. The calculated minimum effective mass of electron in CBM is about 1.35m0, and the minimum effective mass of hole in VBM is about 1.23m0. The lighter effective masses facilitate the migration of photogenerated carriers and improve photocatalytic performance.

First-principles study of phase stability and electronic properties of RhZr

First-principles calculations were carried out to investigate the structural stabilities and electronic properties of RhZr. The plane wave based pseudopotential method was used, in which both the local density approximation (LDA) and the generalized gradient approximation (GGA) implanted in the CASTEP code were employed. The internal positions of atoms in the unit cell were optimized and the ground state properties such as lattice parameter, density of state, cohesive energies and enthalpies of formation of ortho-RhZr and cubic-RhZr were calculated. The calculation results indicate that ortho-RhZr can form more easily than cubic-RhZr and the ortho-RhZr is more stable than cubic-RhZr. The density of states (DOS) reveals that the strong bonding in the Rh-Zr and Rh-Rh or Zr-Zr interaction chains accounts for the structural stability of ortho-RhZr and the hybridization between Rh-4d states and Zr-4d states is strong.

First-principles study of structural, electronic and elastic properties of single crystal CuZr

Investigations into the structural, electronic and elastic properties of intermetallic CuZr compound had been conducted by the plane-wave pseudopotential method. The calculated lattice constant was consistent well with the experimental value. The absence of band gap and finite value of the density of states (DOS) at the Fermi level reveal the metallic behavior of CuZr crystal, and Zr 4d states give rise to the electrical conductivity. The calculated elastic constants for single crystal CuZr at zero pressure obey the cubic mechanical stability condition, which indicates that the cubic CuZr crystal is mechanical stable at zero pressure. By analyzing the ratio between the bulk and shear moduli, we conclude that CuZr crystal is ductile in nature. The present theoretical investigations might give prediction to polycrystalline CuZr system.

First-principles study on alloying stability, electronic structure, and mechanical properties of Al-based intermetallics

First-principles calculations were performed to study on alloying stability, electronic structure, and mechanical properties of Al-based intermetallic compounds (AlCu 3, AlCu 2Zr, and AlZr 3). The calculated results show that the lattice parameters obtained after full relaxation of crystalline cells are consistent with experimental data. The calculation of cohesive energies indicated that the structure stability of these Al-based intermetallics will become higher with increasing Zr element in crystal. The calculations of formation energies showed that AlCu 2Zr has the strongest alloying ability, followed by AlZr 3 and finally the AlCu 3. The further analysis find out that single-crystal elastic constants at zero-pressure satisfy the requirement of mechanical stability for cubic crystals. The calculations on the ratio of bulk modulus to shear modulus reveal that AlCu 2Zr can exhibit a good ductility, followed by AlCu 3, whereas AlZr 3 can have a poor ductility; however, for stiffness, these intermetallics show a converse order. The calculations on Poisson's ratio show that AlCu 3 is much more anisotropic than the other two intermetallics. In addition, calculations on densities of states indicate that the valence bonds of these intermetallics are attributed to the valence electrons of Cu 3d states for AlCu 3, Cu 3d, and Zr 4d states for AlCu 2Zr, and Al 3s, Zr 5s and 4d states for AlZr 3, respectively; in particular, the electronic structure of the AlZr 3 shows the strongest hybridization, leading to the worst ductility.

Electronic structure of ZrTiO 4 and HfTiO 4: Self-consistent cluster calculations and X-ray spectroscopy studies

Total and partial densities of states of the constituent atoms of ZrTiO 4 and HfTiO 4 titanates have been calculated using a self-consistent cluster method as incorporated in the FEFF8 code. The calculations reveal the similarity of the electronic structure of both titanates and indicate that the valence band of the compounds under consideration is dominated by contributions of O 2p states. These states contribute throughout the whole valence-band region; however their maximum contributions occur in the upper portion of the band. Other significant contributors in the valence-band region are Ti 3d and Zr 4d states in ZrTiO 4 and Ti 3d and Hf 5d states in HfTiO 4. All the above d-like states contribute throughout the whole valence-band region of the titanates; however maximum contributions of the Ti 3d states occur in the upper portion, whilst those of the Zr 4d (Hf 5d) states are in the central portions of the valence band. The FEFF8 calculations render that the bottom of the conduction band of ZrTiO 4 and HfTiO 4 is dominated by contributions of Ti 3d ⁎ states, with also smaller contributions of Zr 4d ⁎/Hf 5d ⁎ and O 2p ⁎ states. To verify the above FEFF8 data, the X-ray emission bands, representing the energy distributions of mainly O 2p, Ti 3d and Zr 4d states, were measured and compared on a common energy scale. These experimental data are found to be in agreement with the theoretical FEFF8 results for the electronic structure of ZrTiO 4 and HfTiO 4 titanates. Additionally, X-ray photoelectron valence-band and core-level spectra were recorded for the constituent atoms of the titanates under study.

First-principles study on electronic structure and elastic properties of hexagonal Zr 2Sc

An investigation into the equation of state (EOS), electronic and elastic properties of Zr 2SC has been conducted by first-principles pseudopotential calculations. The calculated EOS is well consistent with the recent experimental reports. The absence of band gap at the Fermi level and the finite value of the density of states at the Fermi energy reveal the metallic behavior of Zr 2SC. From the variations of elastic constants with pressure, we find that hexagonal Zr 2SC is most stable in the pressure range from 0 to 100 GPa, which is consistent with the experimental observations. The strong hybridization of Zr 4d states, S 3p states and C 2p states and the presence of pseudogap stabilize the structure of Zr 2SC. By analyzing the ratio between the bulk and shear moduli, we conclude that Zr 2SC is brittle in nature. The mechanism of brittleness of Zr 2SC originated from the large value of Zr atom occupying the internal parameter z.