First-Principles Investigation on Delithiation Mechanisms in a Li-Rich Monoclinic Li2MoO3 Cathode Material for Li-Ion Batteries.

Li2MoO3 is a promising cathode material for high-capacity Li-ion batteries. However, during cycling, migration of Mo to Li sites results in capacity fading. The present study analyzed structural, electronic, electrochemical, and mechanical characteristics of ordered monoclinic C2/m-Li2MoO3 and found that this phase has improved electrochemical properties compared to the rhombohedral R3̅m phase. Nudged elastic band calculations showed that Mo migration to the Li site is less probable in C2/m-Li2MoO3. The charge and chemical bonding analyses during delithiation showed Mo4+/Mo6+ oxidation and partial oxygen oxidation, but no spontaneous oxygen release occurred. The voltage profile calculated using the SCAN + U method exhibits high voltage, and partial W substitution at Mo sites suppresses intralayer Mo migration to the Li site and improves the voltage characteristics. These findings suggest that monoclinic Li2MoO3 is a potential cathode material for high-capacity Li-ion batteries with reduced Mo migration and maintained Mo4+/Mo6+ oxidation and oxygen stability. Moreover, partial W substitution at Mo sites further enhances the electrochemical properties of C2/m-Li2MoO3.

Two-Dimensional CdX/C2N (X = S, Se) Heterostructures as Potential Photocatalysts for Water Splitting: A DFT Study.

Global environmental issues, in addition to limited fossil fuel resources, are being addressed by quests in search of efficient visible-light-driven water splitting catalysts for hydrogen production. The photocatalytic water splitting activities of CdX/C2N (X = S, Se) heterostructures have been investigated here using hybrid density functional theory calculations. The calculated band gaps of CdS/C2N and CdSe/C2N heterostructures are 1.48 and 2.12 eV, respectively. These are ideal band gap values that make possible harvesting of more visible light from the solar spectrum, which will result in high solar to energy conversion efficiencies. Charge density difference analysis shows that the charge redistributions mainly occur in the interface regions and that the charges transfer from the C2N to CdX layers. It is interesting to note that the CdX/C2N heterostructures possess a type-II band alignment, where the relative band alignment of the C2N and CdX monolayers promotes a spatial separation of the electrons (that resides in C2N) and holes (that resides in CdX). Importantly, the band edges of the heterostructures straddle the water redox potential under different pH conditions. This study demonstrates that the CdS/C2N and CdSe/C2N heterostructures are suitable materials to split water (from various sources) in different ranges of pH values.

Giant Magnetoelectric Coupling in Multiferroic PbTi1-x V x O3 from Density Functional Calculations.

Giant magnetoelectric coupling is a very rare phenomenon that has gained much attention in the past few decades due to fundamental interest as well as practical applications. Here, we have successfully achieved giant magnetoelectric coupling in PbTi1-x V x O3 (x = 0-1) using a series of generalized gradient-corrected GGA (generalized gradient approximation), including on-site Coulomb repulsion (U)-corrected spin-polarized calculations based on accurate density functional theory. Our total energy calculations show that PbTi1-x V x O3 stabilizes in C-type antiferromagnetic ground state for x > 0.123. With the substitution of V into PbTiO3, the tetragonal distortion is highly enhanced accompanied by a linear increase in polarization. In addition, our band structure analysis shows that for lower x values, the tendency to form two-dimensional magnetism of PbTi1-x V x O3 decreases. The orbital magnetic polarization was calculated with self-consistent field method by including orbital polarization correction in the calculation as well as from the computed X-ray magnetic dichroism spectra. A nonmagnetic metallic ground state is observed for the paraelectric phase for V concentration (x) = 1 competing with a volume change of 10% showing a large magnetovolume effect. Our orbital-projected density of states as well as orbital ordering analysis suggest that the orbital ordering plays a major role in the magnetic-to-nonmagnetic transition when going from ferroelectric to paraelectric phase. The calculated magnetic anisotropic energy shows that the direction [110] is the easy axis of magnetization for x = 1 composition. The partial polarization analysis shows that the Ti/V-O hybridization majorly contributes to the total electrical polarization. The present study adds a new series of compounds to the magnetoelectric family with rarely existing giant coupling between electric- and magnetic-order parameters. These results show that such kind of materials can be used for novel practical applications where one can change the magnetic properties drastically (magnetic to nonmagnetic, as shown here) with external electric field and vice versa.

Large Negative Thermal Expansion Induced by Synergistic Effects of Ferroelectrostriction and Spin Crossover in PbTiO3-Based Perovskites.

The discovery of unusual negative thermal expansion (NTE) provides the opportunity to control the common but much desired property of thermal expansion, which is valuable not only in scientific interests but also in practical applications. However, most of the available NTE materials are limited to a narrow temperature range, and the NTE effect is generally weakened by various modifications. Here, we report an enhanced NTE effect that occurs over a wide temperature range α‾V=-5.24×10-5∘C-1,25-575∘C, and this NTE effect is accompanied by an abnormal enhanced tetragonality, a large spontaneous polarization, and a G-type antiferromagnetic ordering in the present perovskite-type ferroelectric of (1-x)PbTiO3-xBiCoO3. Specifically, for the composition of 0.5PbTiO3-0.5BiCoO3, an extensive volumetric contraction of ~4.8 % has been observed near the Curie temperature of 700 °C, which represents the highest level in PbTiO3-based ferroelectrics. According to our experimental and theoretical results, the large NTE originates from a synergistic effect of the ferroelectrostriction and spin crossover of cobalt on the crystal lattice. The actual NTE mechanism is contrasted with previous functional NTE materials, in which the NTE is simply coupled with one ordering such as electronic, magnetic, or ferroelectric ordering. The present study sheds light on the understanding of NTE mechanisms, and it attests that NTE could be simultaneously coupled with different orderings, which will pave a new way toward the design of large NTE materials.

Electronic and Magnetic Structures of Hole Doped Trilayer La4-xSrxNi3O8 from First-Principles Calculations.

The magnetic and electronic properties of trilayer La4Ni3O8, similar to hole-doped cuprates, are investigated by performing full-potential linearized augmented plane wave method-based spin-polarized calculations with LDA and GGA functionals including Hubbard U parameters to account for strong correlation effects. On the basis of these calculations, we found that La4Ni3O8 is a C-type anti-ferromagnetic (C-AFM) Mott insulator in agreement with previous experimental and theoretical observations. Our calculations suggest that the two crystallographically nonequivalent nickel atoms Ni1 and Ni2 are found to be in high-spin state with an average valency of +1.33. Intermediate band-gap states are originated from dz2 electrons of both types of Ni ions after including the strong correlation effects. To understand the role of hole doping on electronic structure, phase stability, and magnetic properties of La4Ni3O8, similar calculations were performed for La4-xSrxNi3O8 as a function of x, using the supercell approach. We found that the hole doping brings insulator-to-metal transition without changing the C-AFM ordering, though the magnetic moment is enhanced at both Ni sites. Moreover, these Ni atoms are always in an average valence state irrespective of hole doping or volume change. So the electronic properties of hole-doped La4Ni3O8 cannot be compared with hole-doped cuprates that are high-TC superconductors.

Solid-state structure and calculated electronic structure, formation energy, chemical bonding, and optical properties of Zn4O(FMA)3 and its heavier congener Cd4O(FMA)3.

The equilibrium solid-state structure of the experimentally synthesized but incompletely characterized Zn4O(FMA)3 is revised with the help of density functional theory computational methods. The electronic structure, formation energy, chemical bonding, and optical properties of Zn4O(FMA)3 and its heavier congener Cd4O(FMA)3 have been systematically investigated. The calculated bulk moduli for Zn4O(FMA)3 and Cd4O(FMA)3 are similarly small (and slightly smaller than the previously reported values for MOF-5), indicative of relatively soft materials. Their estimated band-gap values are ca. 3.2 eV (somewhat lower than that of MOF-5, 3.4-3.5 eV), indicating semiconducting character. The optical properties including dielectric function ε(ω), refractive index n(ω), absorption coefficient α(ω), optical conductivity σ(ω), reflectivity R(ω), and electron energy-loss spectrum L(ω) of M4O(FMA)3 (M = Zn, Cd) were systematically studied. Analysis of chemical bonding reveals that the M-O bonds are largely ionic, with an increase in ionicity from Zn to Cd. The total energy calculations establish that compounds M4O(FMA)3 have large negative formation energies, ca. -80 and -70 kJ·mol(-1) for Zn and Cd, respectively. Whereas Zn4O(FMA)3 has already been synthesized, the results suggest that the heavier congener Cd4O(FMA)3 might be experimentally accessible.

Properties of IRMOF-14 and its analogues M-IRMOF-14 (M = Cd, alkaline earth metals): electronic structure, structural stability, chemical bonding, and optical properties.

The chemical bonding, electronic structure, and optical properties of the experimentally available metal-organic framework IRMOF-14 and its metal-substituted analogues M-IRMOF-14 (M = Zn, Cd, Be, Mg, Ca, Sr, Ba), which contain a pyrene-2,7-dicarboxylate linker group, have been systematically investigated using DFT calculations. The unit cell volume and atomic positions were optimized with the Perdew-Burke-Ernzerhof (PBE) functional and showed good agreement between experimental and theoretical equilibrium structural parameters for Zn-IRMOF-14. The calculated bulk moduli indicate that the whole M-IRMOF-14 series are soft materials. The estimated band gap from DOS calculations for the M-IRMOF-14 series is ca. 2.5 eV, essentially independent of the metal ion and indicative of nonmetallic character. The band gap value is distinctly different from those calculated previously for the M-IRMOF-1 (benzene-1,4-dicarboxylate linker; ca. 3.5 eV) and M-IRMOF-10 (biphenyl-4,4'-dicarboxylate linker; ca. 3.0 eV) series and this confirms that the identity of the linker is a key parameter to control band gaps in an isoreticular series of main-group MOFs. In view of potential uses of MOFs in organic semiconducting devices such as field-effect transistors, solar cells, and organic light-emitting devices, the linear optical properties of these materials were also investigated. Comparisons are made with the M-IRMOF-1 and M-IRMOF-10 series.

Revisiting isoreticular MOFs of alkaline earth metals: a comprehensive study on phase stability, electronic structure, chemical bonding, and optical properties of A-IRMOF-1 (A = Be, Mg, Ca, Sr, Ba).

Formation energies, chemical bonding, electronic structure, and optical properties of metal-organic frameworks of alkaline earth metals, A-IRMOF-1 (where A = Be, Mg, Ca, Sr, or Ba), have been systemically investigated with DFT methods. The unit cell volumes and atomic positions were fully optimized with the Perdew-Burke-Ernzerhof functional. By fitting the E-V data into the Murnaghan, Birch and Universal equation of states (UEOS), the bulk modulus and its pressure derivative were estimated and provided almost identical results. The data indicate that the A-IRMOF-1 series are soft materials. The estimated bandgap values are all ca. 3.5 eV, indicating a nonmetallic behavior which is essentially metal independent within this A-IRMOF-1 series. The calculated formation energies for the A-IRMOF-1 series are -61.69 (Be), -62.53 (Mg), -66.56 (Ca), -65.34 (Sr), and -64.12 (Ba) kJ mol(-1) and are substantially more negative than that of Zn-based IRMOF-1 (MOF-5) at -46.02 kJ mol(-1). From the thermodynamic point of view, the A-IRMOF-1 compounds are therefore even more stable than the well-known MOF-5. The linear optical properties of the A-IRMOF-1 series were systematically investigated. The detailed analysis of chemical bonding in the A-IRMOF-1 series reveals the nature of the A-O, O-C, H-C, and C-C bonds, i.e., A-O is a mainly ionic interaction with a metal dependent degree of covalency. The O-C, H-C, and C-C bonding interactions are as anticipated mainly covalent in character. Furthermore it is found that the geometry and electronic structures of the presently considered MOFs are not very sensitive to the k-point mesh involved in the calculations. Importantly, this suggests that sampling with Γ-point only will give reliable structural properties for MOFs. Thus, computational simulations should be readily extended to even more complicated MOF systems.

Theoretical investigations on the chemical bonding, electronic structure, and optical properties of the metal-organic framework MOF-5.

The chemical bonding, electronic structure, and optical properties of metal-organic framework-5 (MOF-5) were systematically investigated using ab initio density functional calculations. The unit cell volume and atomic positions were optimized with the Perdew-Burke-Ernzerhof (PBE) functional leading to a good agreement between the experimental and the theoretical equilibrium structural parameters. The calculated bulk modulus indicates that MOF-5 is a soft material. The estimated band gap from a density of state (DOS) calculation for MOF-5 is about 3.4 eV, indicating a nonmetallic character. As MOFs are considered as potential materials for photocatalysts, active components in hybrid solar cells, and electroluminescence cells, the optical properties of this material were investigated. The detailed analysis of chemical bonding in MOF-5 reveals the nature of the Zn-O, O-C, H-C, and C-C bonds, that is, Zn-O having mainly ionic interaction whereas O-C, H-C, and C-C exhibit mainly covalent interactions. The findings in this paper may contribute to a comprehensive understanding about this kind of material and shed insight into the synthesis and application of novel and stable MOFs.