A first principle comparative study of the ionic diffusivity in LiAlO2 and NaAlO2 polymorphs for solid-state battery applications.

Lithium aluminates are attracting increasing attention as battery materials. They are typically used for the coating of electrodes. The diffusive properties of the equilibrium tetragonal phase (γ) are well understood from both an experimental and a theoretical perspective, and the major diffusive mechanism is recognised to be vacancy hopping between neighbouring sites. Investigations of this type are however not reported for other, high-pressure LiAlO2 phases. Moreover, the ionic diffusivity of the Na-based aluminates, manifesting a similar polymorphism to LiAlO2, has never been studied using atomistic techniques. In this work, we address these points, by presenting a comparative density functional theory-based study of these materials, describing the structural properties of the various phases, and evaluating the activation energies for single vacancy hops. While LiAlO2 exhibits a poor ionic conductivity due to a significant degree of covalency of the Al-O bonding, orthorhombic ß-NaAlO2 exhibits a significantly lower diffusion barrier. This feature cannot be explained only in terms of the larger equilibrium volume, as the same trend is not observed for the high-pressure trigonal α-LiAlO2 and α-NaAlO2. We utilise here various electronic-structure tools to verify the lower degree of covalency of the Na-O bonds. The electron localisation function, in particular, is shown to be intrinsically correlated to the diffusion pathways of Li and Na ions, and its variation along the path is a qualitative measure of ionic conductivity.

First-principles study of the structural stability and electrochemical properties of Na2MSiO4 (M = Mn, Fe, Co and Ni) polymorphs.

Sodium orthosilicates Na2MSiO4 (M = Mn, Fe, Co and Ni) have attracted much attention due to the possibility of exchanging two electrons per formula unit. They are also found to exhibit great structural stability due to a diamond-like arrangement of tetrahedral groups. In this work, we have systematically studied the possible polymorphism of these compounds by means of density functional theory, optimising the structure of a number of systems with different group symmetries. The ground state is found to be Pc-symmetric for all the considered M = Mn, Fe, Co, Ni, and several similar structures exhibiting different symmetries coexist within a 0.3 eV energy window from this structural minimum. The intercalation/deintercalation potential is calculated for varying transition metal atoms M. Iron sodium orthosilicates, attractive due to the natural abundance of both materials, exhibit a low voltage, which can be enhanced by doping with nickel. The diffusion pathways for Na atoms are discussed, and the relevant barriers are calculated using the nudged elastic band method on top of DFT calculations. Also in this case, nickel impurities would improve the material performances by lowering the barrier heights. Notably, the ionic conductivity is found to be systematically larger with respect to the case of lithium orthosilicates, due to a larger spacing between atomic layers and to the non-directional bonding between Na and the neighbouring atoms. Overall, the great structural stability of the material together with the low barriers for Na diffusion indicates this class of materials as good candidates for modern battery technologies.

Phonon, IR, and Raman spectra, NMR parameters, and elastic constant calculations for AlH3 polymorphs.

The electronic structure, lattice dynamics, and mechanical properties of AlH(3) phases have been studied by density functional calculations. The chemical bonding in different polymorphs of AlH(3) are evaluated on the basis of electronic structures, charge density analysis, and atomic charges, as well as bond overlap population analysis and the Born effective charges. The phonon dispersion relations and phonon density of states of all the polymorphs of AlH(3) are calculated by direct force-constant method. Application of pressure induces seqauence of phase transitions in ß-AlH(3) which are understood from the phonon dispersive curves of the involved phases. The previously predicted phases (Chem. Mater. 2008, 20, 5997) are found to be dynamically stable. The calculated single crystal elastic constants reveal that all the studied AlH(3) polymorphs are easily compressible. The chemical bonding of these polymorphs have noticeable covalent character (except the hp2 phase) according to the present chemical bonding analyses. For all these polymorphs, the NMR-related parameters, such as isotropic chemical shielding, quadrupolar coupling constant, and quadrupolar asymmetry, are also calculated. All IR- and Raman-active phonon frequencies, as well as the corresponding intensities, are calculated for all the AlH(3) polymorphs and are compared with available experimental results.

Nano-phases of NaBH4 and KBH4.

Phase stability and chemical bonding of beta-NaBH4 and beta-KBH4 derived nano-structures and possible low energy surfaces of them from thin film geometry have been investigated using ab initio projected augmented plane wave method. Structural optimizations based on total energy calculations predicted that, for beta-NaBH4 and beta-KBH4 phases, the (011) and (101) surfaces are more stable among the possible low energy surfaces. The predicted critical size of the nano-cluster for beta-NaBH4 and beta-KBH4 is 1.35 and 1.8 nm, respectively. The corresponding critical diameter for the nano-whisker is 2.6 and 2.8 nm respectively for beta-NaBH4 and beta-KBH4. Structural optimization based on total energy calculations show that the bond distances in the surfaces of nano-whisker are found to be higher than that in the bulk material and the calculated H site energies and bond overlap population analysis suggesting that it is considerably easier to remove hydrogen from the surface of the clusters and nano-whiskers than that from the bulk crystals.

Structural investigation and thermodynamical properties of alkali calcium trihydrides.

The ground-state structure, equilibrium structural parameters, electronic structure, and thermodynamical properties of MCaH(3) (M=Li, Na, K, Rb, and Cs) phases have been investigated. From the 104 structural models used as inputs for structural optimization calculations, the ground-state crystal structures of MCaH(3) phases have been predicted. At ambient condition, LiCaH(3), NaCaH(3), and KCaH(3) crystallize in hexagonal, monoclinic, and orthorhombic structures, respectively. The remaining phases RbCaH(3) and CsCaH(3) crystallize in a cubic structure. The calculated phonon spectra indicate that all the predicted phases are dynamically stable. The formation energy for the MCaH(3) phases have been calculated along different reaction pathways. The electronic structures reveal that all these phases are insulators with an estimated band gap varying between 2.5 and 3.3 eV.

Nanostructures of LiBH4: a density-functional study.

The phase stability and electronic structure of alpha- LiBH(4)-derived nanostructures and possible low energy surfaces of thin films have been investigated using the ab initio projected augmented plane wave method. Structural optimizations based on total energy calculations predicted that, for the alpha- LiBH(4) phase, the (010) surface is the most stable of the possible low-energy surfaces. The predicted critical sizes of the nano-cluster and nano-whisker for alpha- LiBH(4) are 1.75 and 1.5 nm, respectively. Similarly, the bond distances in the surfaces of a nano-whisker are found to be higher than that in the bulk material. The calculated hydrogen site energies suggest that it is relatively easier to remove hydrogen from the surface of the clusters and nano-whiskers than from bulk crystals.

Phase stability and pressure-induced structural transitions at zero temperature in ZnSiO(3) and Zn(2)SiO(4).

Using density functional total energy calculations the structural phase stability and pressure-induced structural transition in different polymorphs of ZnSiO(3) and Zn(2)SiO(4) have been studied. Among the considered monoclinic phase with space groups (P 2(1)/c) and (C 2/c), rhombohedral [Formula: see text] and orthorhombic (Pbca) modifications the monoclinic phase (P 2(1)/c) of ZnSiO(3) is found to be the most stable one. At high pressure monoclinic ZnSiO(3) (C 2/c) can co-exist with orthorhombic (Pbca) modification. Differences in equilibrium volume and total energy of these two polymorphs are very small, which indicates that it is relatively easier to transform between these two phases by temperature, pressure or chemical composition. It can also explain the experimentally established result of metastability of the orthorhombic phase under all conditions. The following sequence of pressure-induced structural phase transitions is found for ZnSiO(3) polymorphs: monoclinic [Formula: see text] monoclinic [Formula: see text] rhombohedral [Formula: see text]. Among the rhombohedral ([Formula: see text]), tetragonal [Formula: see text], orthorhombic (Pbca), orthorhombic (Imma), cubic [Formula: see text] and orthorhombic (Pbnm) modifications of Zn(2)SiO(4), the rhombohedral phase is found to be the ground state. For this chemical composition of zinc silicate the following sequence of structural phase transitions is found: rhombohedral [Formula: see text] tetragonal [Formula: see text] orthorhombic [Formula: see text] orthorhombic (Imma) [Formula: see text] cubic [Formula: see text] orthorhombic (Pbnm). Based on the analogy of crystal structures of magnesium and zinc silicates and using the lattice and positional parameters of Mg(2)SiO(4) as input, structural properties of spinel Zn(2)SiO(4) have also been studied.

Structural phase stability studies on MBeH3 (M = Li, Na, K, Rb, Cs) from density functional calculations.

Density functional theory calculations within the generalized-gradient approximation are used to establish the ground-state structure, equilibrium structural parameters, and electronic structure for MBeH(3) phases. From the 24 structural arrangements used as inputs for structural optimization calculations, the ground-state crystal structures of MBeH(3) phases have been predicted. At ambient conditions, LiBeH(3) and NaBeH(3) crystallize with perovskite-related orthorhombic and cubic structures, respectively. The remaining phases KBeH(3), RbBeH(3), and CsBeH(3) crystalize in a monoclinic structure. In the predicted phases one can store up to 15.93 wt % of hydrogen. The formation energy for the MBeH(3) phases have been investigated along different reaction pathways. The electronic structures reveal that all these phases are insulators with estimated band gaps varying between 1.79 and 3.44 eV.

Theoretical investigations on low energy surfaces and nanowires of MgH(2).

The phase stability, chemical bonding, and electronic structure of MgH(2) nanowires and possible low energy surfaces of α-MgH(2) thin films have been investigated using the ab initio projected augmented plane-wave method. Structural optimizations based on total energy calculations predicted that, for the α-MgH(2) phase, the (101) surface is more stable among the possible low energy surfaces. The electronic structure study reveals that the nanowires also have nonmetallic character similar to that of the bulk and thin film phases. Bonding analysis shows that the character of chemical bonding in nanowires has been considerably changed compared with that in bulk phases. Similarly, the bond distances in the surfaces of nanowires are found to be higher than in the bulk material, suggesting that it is possible to remove hydrogen from the nanowires considerably more easily than from bulk crystals.

Pressure-induced structural transitions in MgH2.

The stability of MgH2 has been studied up to 20 GPa using density-functional total-energy calculations. At ambient pressure alpha-MgH2 takes a TiO2-rutile-type structure. alpha-MgH2 is predicted to transform into gamma-MgH2 at 0.39 GPa. The calculated structural data for alpha- and gamma-MgH2 are in very good agreement with experimental values. At equilibrium the energy difference between these modifications is very small, and as a result both phases coexist in a certain volume and pressure field. Above 3.84 GPa gamma-MgH2 transforms into beta-MgH2, consistent with experimental findings. Two further transformations have been identified at still higher pressure: (i) beta- to delta-MgH2 at 6.73 GPa and (ii) delta- to epsilon-MgH2 at 10.26 GPa.

Violation of the minimum h-h separation "Rule" for metal hydrides.

Using gradient-corrected, full-potential, density-functional calculations, including structural relaxations, it is found that the metal hydrides RTInH1.333 (R=La, Ce, Pr, or Nd; T= Ni, Pd, or Pt) possess unusually short H-H separations. The most extreme value (1.454 A) ever obtained for metal hydrides occurs for LaPtInH1.333. This finding violates the empirical rule for metal hydrides, which states that the minimum H-H separation is 2 A. The paired, localized, and bosonic nature of the electron distribution at the H site are polarized towards La and In which reduces the repulsive interaction between negatively charged H atoms. Also, R-R interactions contribute to shielding of the repulsive interactions between the H atoms.