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.

Theoretical and experimental investigation on structural, electronic and magnetic properties of layered Mn5O8.

We have investigated the crystal, electronic, and magnetic structures of Mn5O8 by means of state-of-the-art density functional theory calculations and neutron powder diffraction (NPD) measurements. This compound stabilizes in the monoclinic structure with space group C2/m where the Mn ions are in the distorted octahedral and trigonal prismatic coordinations with oxygen atoms. The calculated structural parameters based on total energy calculations are found to be in excellent agreement with low temperature NPD measurements when we accounted for the correct magnetic structure and Coulomb correlation effect in the computation. Using fully relativistic generalized-gradient approximation with Hubbard U (GGA+U) we found that the magnetic ordering in Mn5O8 is A-type antiferromagnetic and the direction of the easy axis is [1 0 0] in agreement with susceptibility and NPD measurements. However, the calculation without the inclusion of Hubbard U leads to ferrimagnetic half metal as a ground state contradictory to experimental findings, indicating the presence of a strong Coulomb correlation effect in this material. The GGA calculation without the Coulomb correction effect itself is sufficient to reproduce the experimentally observed magnetic moments in various Mn sites. We found that Mn in this material exhibits mixed valence behavior with 2+ and 4+ oxidation states reflecting different magnetic moments in the Mn sites. We explored the electronic band characteristics using total, site-, and orbital-projected density of states which emphasized the mixed-valent nature of Mn. A dominant Mn 3d character of the density of states at Fermi energy is the origin for the metallic behavior of Mn5O8. The bond strength analysis based on the crystal orbital Hamiltonian population between constituents indicates strong anisotropy in the bonding behavior which results from the layered nature of its crystal structure. Our bonding analysis shows that there is a noticeable covalent bond between Mn 3d-O 2p states which stabilizes the observed low symmetric structure. Our experimental findings and theoretical predictions suggest that Mn5O8 can be classified as a strongly correlated mixed valent antiferromagnetic metal.

Two new Cu(ii) and La(iii) 2D coordination polymers, synthesis and in situ structural analysis by X-ray diffraction.

Two new coordination polymers were synthesized solvothermally using 4,4'-dimethoxy-3,3'-biphenyldicarboxylic acid (H2dmbpdc), and di- and trivalent metal salts (Cu(NO3)2·2.5H2O and La(NO3)3·6H2O). Their structures were determined by single-crystal X-ray diffraction analysis, and their thermal stability was evaluated by thermogravimetric analysis. The copper compound Cu(dmbpdc)(DMF; N,N-dimethylformamide), CPO-71-Cu, is based on the well known copper acetate paddlewheel secondary building unit. The asymmetric unit comprises one copper cation with one DMF molecule and one linker molecule coordinated. The lanthanum compound La2(dmbpdc)3(DMF)(H2O)3, CPO-72-La, is formed from a dimer of nine-coordinate, edge sharing lanthanum cations. To this dimer, three water molecules and one DMF molecule are coordinated in an ordered fashion. In addition, the asymmetric unit contains three crystallographically unique linker molecules. Both CPO-71-Cu and CPO-72-La form two-dimensional layered structures, and topological analyses reveal sql topologies with point symbol 4(4)·6(2) and vertex symbol 4·4·4·4·6(2)·6(2). The thermal behavior of CPO-71-Cu was investigated in an in situ structural analysis by variable temperature powder- and single-crystal X-ray diffraction.

Observation of direct magneto-dielectric behaviour in Lu3Fe5O12-δ above room-temperature.

The coupling of dielectric and magnetic order is highly nontrivial and seldom observed in rare-earth iron garnets. Careful investigations on polycrystalline Lu3Fe5O12-δ, prepared by the solid state route, establish a direct correlation between the magnetic and dielectric order parameters. A dielectric anomaly at the magnetic ordering temperature supports this correlation. The dielectric permittivity at various magnetic fields is measured using an indigenously developed connector setup. Lu3Fe5O12-δ exhibits a magneto-dielectric coupling of ∼6% at room temperature, which is significant in the case of a single-phase magneto-dielectric material at low fields. Rietveld refinement of the X-ray diffraction pattern, bond valence sum method, Mössbauer spectroscopy, and X-ray photoelectron spectroscopy indicate two different oxidation states of Fe. Complete structural optimizations performed using the density functional theory establish a ferrimagnetic ground state and provide structural parameters in agreement with experimental values. Electronic structure analysis shows that Lu3Fe5O12 exhibits insulating behavior both in ferromagnetic and ferrimagnetic configurations. The capability of Lu3Fe5O12-δ to exhibit room temperature magneto-dielectric response is a key factor in designing and fabricating various electronic devices and sensors.

Atomic layer deposition of sodium and potassium oxides: evaluation of precursors and deposition of thin films.

Thin films of sodium and potassium oxides have for the first time been deposited using atomic layer deposition. Sodium and potassium complexes of tert-butanol, trimethylsilanol and hexamethyldisilazide have been evaluated as precursors by characterising their thermal properties as well as tested in applications for thin film depositions. Out of these, sodium and potassium tert-butoxide and sodium trimethylsilanolate and hexamethyldisilazide were further tested as precursors together with the Al(CH3)3 + H2O/O3 process to form aluminates and together with ozone to form silicates. Sodium and potassium tert-butoxide and sodium trimethylsilanolate showed self-limiting growth and proved useable at deposition temperatures from 225 to 375 or 300 °C, respectively. The crystal structures of NaO(t)Bu and KO(t)Bu were determined by single crystal diffraction revealing hexamer- and tetramer structures, respectively. The current work demonstrates the suitability of the ALD technique to deposit thin films containing alkaline elements even at 8'' wafer scale.

Guidance of growth mode and structural character in organic-inorganic hybrid materials - a comparative study.

A main goal in the construction of thin films is to control film growth in all aspects. Accurate control of the building blocks and their reaction sites is one way to achieve that. This is a key feature of the atomic layer deposition (ALD) technique. The aim of this study is to achieve such growth control of organic-inorganic thin films. The organic building blocks consist of the linear carboxylic acids: glutaric, tricarballylic, and trans-aconitic acid and the amino acid l-glutamic acid. All of these are based on five carbon long backbones. The acids were linked by aluminium using trimethylaluminum (TMA). These precursors made it possible to study the effect of the functionality of the organic acid backbone on growth rate, reaction modes, and the material properties of the deposited materials. The growth dynamics were investigated by in situ characterization using a quartz crystal microbalance (QCM). QCM revealed that all systems are of a self-limiting ALD-type. Ideal ALD growth was found for the tricarballylic acid-TMA system. For the other systems, the growth rate decreased with increasing temperature. The growth rates ranged from 0.05 to 2 nm per cycle. Analysis by Fourier transform infrared spectroscopy (FTIR) verified the hybrid character of the films and the presence of two different growth modes. The films were X-ray amorphous as deposited, with the exception of the l-glutamic-TMA system. Surface roughness and topography of the films was investigated by atomic force microscopy (AFM). Optical and surface wetting properties of the films were investigated by UV-Vis spectroscopy and the goniometer method for sessile drops, respectively. All films were stable in contact with water and generally had very low surface roughness. The present work has shown that the ALD technique can offer controlled growth of functionalized hybrid materials. It is likely that specifically chosen functionalized precursors can be employed to obtain specific structural designs and properties. The first sign of this was found for the l-glutamic-TMA system. The diffraction features of the as-deposited films of this system indicate the presence of sheet-like ordering within the material. This is one of the first observations of this kind by ALD.

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.

Density functional theory studies of spin, charge, and orbital ordering in YBaT2O5 (T = Mn, Fe, Co).

Spin, charge, and orbital orderings are influenced by electron/hole doping, cation radii, oxygen stoichiometry, temperature, magnetic field, and so on. In order to understand the role of electron/hole doping, we have studied variations in spin, charge, and orbital ordering in terms of d-band filling for YBaT 2O 5 (T = Mn, Fe, Co). The calculations were performed using density functional theory as implemented in the full-potential linearized augmented-plane-wave method. We have carried out calculations for nonmagnetic, ferromagnetic, and antiferromagnetic configurations. A ferrimagnetic ground state was established for YBaMn 2O 5, whereas YBaFe 2O 5 and YBaCo 2O 5 have antiferromagnetic ground states; all of these results are in agreement with experimental findings. The effects of spin-orbit coupling, the Hubbard U parameter, and orbital polarization on the magnetic properties were also analyzed. The electronic band characteristics were analyzed using total as well as site- and orbital-projected densities of states. Inclusion of spin-orbit coupling and Coulomb correlation effects in the calculations was found to be important in order to reproduce the experimentally established semiconducting behaviors of YBaFe 2O 5 and YBaCo 2O 5. In order to quantify the charges at each atomic site, we made use of the Bader "atom-in-molecule" concept and Born effective-charge (BEC) analyses. The structural optimizations and BEC tensor calculations were performed using the VASP-PAW method. The different types of charge and orbital orderings in these compounds were visualized using the energy-projected density matrices of the d electrons. Substantial differences in ordering patterns with respect to d-band filling emerged. Ordering of the d z (2) orbital of Mn in YBaMn 2O 5 gave rise to G-type ferrimagnetic spin ordering along the c direction and checkerboard-type charge ordering, whereas ordering of the d x (2) - y (2) orbital of Fe in YBaFe 2O 5 caused Wollan-Koehler G-type antiferromagnetic spin ordering along the b direction and stripe-type charge ordering. Similarly, a complex pattern of orbital ordering in YBaCo 2O 5 activated spin and charge orderings similar to those in YBaFe 2O 5.

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.

Coordination preference of Ga in hydrides.

Aluminum and gallium show some interesting differences in their coordination chemistry. Solid GaH3 is unknown, in contrast to solid AlH3. Ga equivalents of Li3AlH6, Na3AlH6, and other hydrides whose structure contain AlH(3-)6 ions, are unknown. We relate these differences to an instability of the hexacoordinated gallium moiety.

The crystal structure of Zr2NiD4.5.

The crystal structure of Zr2NiD4.5 has been determined by a combination of synchrotron radiation powder X-ray diffraction, electron diffraction and powder neutron diffraction data. Deuterium ordering results in a triclinic supercell given by asuper=6.81560 (7), bsuper=8.85137 (9), csuper=8.88007 (10) A, alphasuper=79.8337 (8), betasuper=90.0987 (9), gammasuper=90.3634 (9) degrees, which relates to the non-super unit cell as asuper=-a, bsuper=-b-c, csuper=-b+c. The centrosymmetric and fully ordered deuterium sublattice was determined by simulated annealing and Rietveld refinement. Deuterium was found to occupy three types of tetrahedral sites: two that are coordinated by four Zr atoms and one that is coordinated by three Zr atoms and one Ni atom. All D-D distances are longer than 2 A. The feasibility of the crystal structure was supported by density functional theory calculations.

Tailor-made electronic and magnetic properties in one-dimensional pure and Y-substituted Ca3Co2O6.

Full-potential density-functional calculations show that the electronic structure of one-dimensional ferrimagnetic Ca3Co2O6 varies from metal to half metal to insulator as its magnetic ordering changes from the ferrimagnetic through the ferromagnetic to the paramagnetic state. The present Letter is the first to establish the occurrence of half metallicity in one-dimensional oxides. Moreover, the electronic and magnetic properties of this material can be tuned by substitution of Y for Ca, as shown by our detailed study on Ca(3-x)YxCo2O6 (x = 0, 0.3, 0.75, and 1). The Co ions are in two different valence states [Co4+ (low-spin) and Co2+ (high-spin)], and hence the occurrence of charge ordering in addition to spin ordering is established. For specific Y concentrations we predict a rarely seen combination of ferromagnetic and insulating behavior.

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.