Inorganic functional materials: optimization of properties by structural and compositional control

A review is given of the strategies used to dope inorganic solids and the consequences for properties such as ionic and electronic conductivity. Doping mechanisms involve either substitution of foreign ions onto lattice sites, creation of vacancies on either cation or anion sites, or population of normally empty interstitial sites by either anions or cations. Mechanisms for charge compensation associated with aliovalent doping are reviewed and examples are given in the fields of solid state ionics and high-temperature superconductivity. The strategies used for targeting materials with new properties are reviewed, including a surprising number of cases where startling new properties are encountered in well-known materials. Specific examples discussed include MgB2 superconductor, Na beta-alumina sodium ion conductor, Ca12Al14O33 oxide ion conductor, LiCoMnO4 lithium battery cathode, doped Li4SiO4 tunable lithium ion conductor, and La-doped BaTiO3 ferroelectric, which can be either semiconducting or insulating. Examples are also given of a curious observation that extraordinary properties are often encountered in materials that are on the edge of stability, either structurally or compositionally or at the crossover between different property types.     

Structure determination of the 4d metal diborides: a quantum mechanical study

Metal diborides (MB(2)) often have interesting thermal, mechanical, and superconducting properties. MgB(2) was put into focus some years ago for its high transition temperature (39 K) in combination with its simple AlB(2) structure. The boron structure in MB(2) is assumed to be dependent on the electron transfer from the nearby positioned metal atoms. An electronic and structural comparison has been performed here for various initially planar and puckered transition-metal borides, using quantum mechanical density functional theory (DFT) calculations under periodic boundary conditions. In comparison to MgB(2), the experimentally planar transition-metal diborides (ZrB(2), NbB(2), and MoB(2)) and the experimentally puckered ones (TcB(2), RuB(2), RhB(2), and PdB(2)) have been examined. The results indicate that the energetic stability generally follows the experimentally obtained results. The metals that are less electronegative than boron donate electrons to boron, which in turn induce planar boron structures (graphitic-like). The metals that prefer to be planar donate more than one electron, while the trend for metals which favor puckered B structures is that they donate less than one electron per metal atom. Two donated electrons per metal atom (or very close to) will result in the most stable AlB(2) structure.     

Electronic structure of HgBa2Ca(n-1)Cu(n)O(2n+2) (n = 1, 2, 3) superconductor parent compounds from periodic hybrid density functional theory

The electronic structure of HgBa(2)Ca(n) (-1)Cu(n)O(2n+2) (n = 1, 2, and 3) high T(c) superconductor parent compounds has been investigated by means of periodic hybrid density functional theory. Similar to other cuprates, these materials are predicted to exhibit an antiferromagnetic ground state with well localized S = 1/2 magnetic centers at the Cu(2+) sites. However, the presence of the HgO(2) structural units largely defines the nature of states dominating the energy range around Fermi energy. This results in a complex charge transfer character of the insulating gap which decreases when increasing the number of CuO(2) planes in the unit cell, to the point that in the HgBa(2)Ca(2)Cu(3)O(8) compound it becomes so small that one can claim that the resulting material is metallic. Nevertheless, the metallic character arises from the HgO(2) structural units and coexists with the antiferromagnetic order arising from the localized spins at the Cu(2+) sites.     

Comparative study of the electronic structure of alkaline-earth borides (MeB2; Me=Mg, Al, Zr, Nb, and Ta) and their normal-state conductivity

By means of density functional theory the electronic structure of the MgB₂ superconductor was characterized and compared with that of the related iso-structural systems: AlB₂, ZrB₂, NbB₂, and TaB₂. Using the full potential-linearized augmented plane wave (FP-LAPW) method and the generalized gradient approximation, the electronic density distribution, density of states, and band structures were obtained for these compounds. The electrical conductivity, which cannot be easily measured in the c-direction, was calculated, in the relaxation time approximation using band structure results. It was found that the two-dimensional (2D) crystal structure character of these metallic diborides is also reflected in the electronic charge distribution. This 2D pattern is not reproduced in the electrical conductivity as it is, for instance, in the superconductor high T_c cuprates. The calculations indicate a bulk, yet anisotropic, conductivity for all these compounds.     

Review: Luttinger or fermi liquids versus topological superconductivity

Superconductors, classified by materials, embrace at least four broad groups: (i) BCS metals and alloys; (ii) heavy Fermion materials; (iii) high- T c cuprates and (some) organic compounds, and (iv) fullerides. Broadly speaking, in classes (i) and (iv), with (i) possibly embracing the recent discovery of superconductivity in MgB 2 with T c ～ 40 K, electron liquids flow through essentially non-magnetic lattices and the electron-phonon interaction is a key component of the mechanism for Cooper pairing. In classes (ii) and (iii), plus the low- T c material Sr 2 RuO 4 , electron or hole liquids flow through assemblies with magnetic spin fluctuations. The nature of the normal state in class (iii) is not yet universally agreed, both Fermi or Luttinger liquids remaining viable to date, the former, however, with the formation of precursor 2 e Bosons somewhat above T c . Our own studies reveal some common ground between classes (ii) and (iii), involving coherence lengths and effective masses, as well as non- s -wave pairing, though the interactions leading to pairing almost certainly have different physical origins in these two groups. Finally, topological superconductivity is reviewed. It is argued that such a treatment of a topological superfluid could eventually deepen the understanding of the class (iv) fullerides. Resonating valence bond theory, used by Anderson and co-workers as an, of course, approximate strongly-correlated electron technique for high- T c cuprates, can itself be re-written in the form of topological superconductivity, as discussed especially by Wiegmann and collaborators.     

Li2B12Si2: the first ternary compound in the system Li/B/Si: synthesis, crystal structure, hardness, spectroscopic investigations, and electronic structure

We present the synthesis, crystal structure, hardness, IR/Raman and UV/Vis spectra, and FP-LAPW calculations of the electronic structure of Li(2)B(12)Si(2), the first ternary compound in the system Li/B/Si. Yellow, transparent single crystals were synthesized from the elements in tin as solvent at 1500 degrees C in h-BN crucibles in arc-welded Ta ampoules. Li(2)B(12)Si(2) crystallizes orthorhombic in the space group Cmce (no. 64) with a=6.1060(6), b=10.9794(14), c=8.4050(8) A, and Z=4. The crystal structure is characterized by a covalent network of B(12) icosahedra connected by Si atoms and Li atoms located in interstitial spaces. The structure is closely related to that of MgB(12)Si(2) and fulfils the electron-counting rules of Wade and Longuet-Higgins. Measurements of Vickers (H(V)=20.3 GPa) and Knoop microhardness (H(K)=20.4 GPa) revealed that Li(2)B(12)Si(2) is a hard material. The band gap was determined experimentally and calculated by theoretical means. UV/Vis spectra revealed a band gap of 2.27 eV, with which the calculated value of 2.1 eV agrees well. The IR and Raman spectra show the expected oscillations of icosahedral networks. Theoretical investigations of bonding in this structure were carried out with the FP-LAPW method. The results confirm the applicability of simple electron-counting rules and enable some structural specialties to be explained in more detail.     

A comparative first-principles study of orbital hybridization in two-dimensional C, Si, and Ge

Information on orbital hybridization is very important to understand the structural, physical, and chemical properties of a material. Results of a comparative first-principles study on the behaviours of orbital hybridization in the two-dimensional single-element phases by carbon, silicon, and germanium are presented. From the well-known three-dimensional hexagonal lonsdaleite structure, in which the atoms are in ideal sp(3)-bonding, the layer spacing along c-axis is gradually stretched to simulate the evolutions of structural and electronic properties from three-dimensional to two-dimensional lattice configurations in the three materials. A turning point of the total system energy due to the sp(3) to sp(2) transition is observed during this process in carbon. In contrast, no such phenomenon is found in silicon and germanium. The differences in electronic structure and bonding behaviour are further examined through comparative investigation of atomic angular-momentum projected density of states and electronic energy band spectrums of these materials. We demonstrate that the valence electronic orbital in the two-dimensional hexagonal crystals of Si and Ge shows sp(3)-like behaviour for the partial hybridization of s and p(z), which leads to their different lattice configurations to graphene. The role of π-bonds in stabilizing the flat configuration of graphene is also discussed.     

Structure and bonding of zinc antimonides: complex frameworks and narrow band gaps

We investigated crystal structure relationships, phase stability and chemical bonding of the thermoelectric materials ZnSb, alpha-Zn4Sb3, and beta-Zn4Sb3 by means of first principles calculations. The structures of these materials are difficult to rationalise. This is especially true for beta-Zn4Sb3 because of the presence of vacancies and interstitial atoms. We recognised rhomboid rings Zn2Sb2 as central structural building units present in all materials. Importantly, these rings enable to establish a clear relationship between disordered beta-Zn4Sb3 and ordered low-temperature alpha-Zn4Sb3. Concerning the phase stability of Zn4Sb3 we identified a peculiar situation: alpha-Zn4Sb3 is metastable and beta-Zn4Sb3 can only be thermodynamically stable when its structural disorder accounts for a large entropy contribution to free energy. According to their electronic structure zinc antimonides represent heteroatomic framework structures with a modest polarity. The peculiar electronic structure of Zn/Sb systems can also be modelled by Al/Si systems. The high coordination numbers in the frameworks implies the presence of multicentre bonding. We developed a simple bonding picture for these frameworks where multicentre bonding is confined to rhomboid rings Zn2Sb2.     

Proximity-induced superconductivity in carbon nanotubes

Single wall carbon nanotubes (SWNT) are model systems for the study of electronic transport in one-dimensional conductors. They are expected to exhibit strong electronic correlations and non-Fermi liquid behavior as suggested by recent experiments. The possibility to induce supercurrents through such molecular wires is a challenging question both for experimentalists and theoreticians. In this paper we show experimental evidence of induced superconductivity in a SWNT. This proximity effect is observed in a single 1 nm diameter SWNT, in individual cristalline ropes containing about 100 nanotubes and also on multiwalled tubes. These samples are suspended as strings between two superconducting electrodes (double layer Au-Re, Au-Ta or Sn film) at a distance varying between 100 and 2 000 nm. This allows their structural study in a transmission electron microscope. When their resistance is low enough, SWNT become superconducting with surprisingly high critical currents (in the micro-Ampere range for a single tube of normal state resistance 25 kΩ). This critical current, extensively studied as function of temperature and magnetic field, exhibits unusual features which are not observed in conventional Superconducting-Normal-Superconducting junctions and can be related to the strong 1D character of these samples. We also show evidence of a huge sensitivity of dc transport properties of the tubes to electromagnetic radiation in the radio-frequency range.     

Computational investigation of tuning the electronic ability and featured for heterofullerene based dye sensitized solar cells

The structural characteristics of the heteroatom substituted fullerene to improve its physical and chemical properties are discussed in this work, highlighting possible applications in aromaticity, photocatalysis, solar cells, and superconducting materials. The energy gap of doped fullerene lowers significantly, making C₃₁Nb a more reactive material and transforming it into an efficient superconductor. The molecular structure, energy and relative stabilities of the heterofullerene were examined and evaluated to determine the material's identification. According to the results analysis, the extra niobium atom and substituted carbon atom improve the electronic stability of heterofullerene. Using ¹³C NMR nuclear independent chemical shift, the stability of each fullerene and aromatic found in nature is discovered. Furthermore, the simulated infrared spectra of fullerene are reviewed, and the major distinctive peaks are given to different functional groups. NBO study which shows the intermolecular charge transfer from the donor to the acceptor in doped fullerene, demonstrates that the strong intermolecular contact between carbon and niobium atoms makes this material a notable material for NLO property.     

Absence of superconductivity in LiCu2P2

We successfully synthesized the copper-based pnictide LiCu(2)P(2), which was reported as a superconductor with T(c) = 3.7 K before. The temperature dependence of resistivity and DC magnetization was measured on both polycrystalline and single-crystalline LiCu(2)P(2). However, our repeatable synthesizing and measurements showed no superconducting transition either in resistivity or DC magnetization above 2 K. A metallic behavior can be seen in resistivity, and a Curie-Weiss behavior was observed in DC magnetization from 2 to 300 K. We have also carried out the Hall effect and MR measurements on the sample, from which we conclude that the LiCu(2)P(2) has a single-band character. We also synthesized the polycrystalline Li(1-x)Cu(2)P(2), LiCu(2-x)P(2), and Li(1+x)Cu(2-x)P(2) with different stoichiometries, and observed no superconductivity in all the samples.     

Binary boron-rich borides of magnesium: single-crystal investigations and properties of MgB(7) and the new boride Mg(∼5)B(44)

Single crystals of dark-red MgB(7) were grown from the elements in a Cu-melt. The crystal structure (Pearson symbol oI64; space group Imma; a = 10.478(2) ?, b = 5.977(1) ?, c = 8.125(2) ?, 2842 reflns, 48 params, R(1)(F) = 0.018, R(2)(I) = 0.034) consists of a hexagonal-primitive packing of B(12)-icosahedra and B(2)-units in trigonal-prismatic voids. According to the UV-vis spectra and band structure calculations MgB(7) is semiconducting with an optical gap of 1.9 eV. The long B-B distance of 2.278 ? within the B(2)-unit can be seen as a weak bonding interaction. The new Mg(～5)B(44) occurs beside the well-known MgB(12) as a byproduct. Small fragments of the black crystals are dark-yellow and transparent. The crystal structure (Pearson symbol tP196, space group P4(1)2(1)2, a = 10.380(2) ?, c = 14.391(3) ?, 4080 reflns, 251 params, R(1)(F) = 0.025, R(2)(I) = 0.037) is closely related to tetragonal boron-II (t-B(192)). It consists of B(12)-icosahedra and B(19+1)-units. With a charge of -6 for the B(19+1)-units and a Mg-content of ～20 Mg-atoms per unit cell the observed Mg content in Mg(～5)B(44) is quite close to the expected value derived from simple electron counting rules. All compositions were confirmed by EDXS. The microhardness was measured on single crystals for MgB(7) (H(V) = 2125, H(K) = 2004) and MgB(12) (H(V) = 2360, H(K) = 2459).     

Structural, electronic, and magnetic properties of CaCNi3, SrCNi3, and BaCNi3 antiperovskites in comparison to superconducting MgCNi3

Within the first principle FLAPW-GGA band method we predict the structural, electronic, and magnetic properties of CaCNi₃, SrCNi₃, and BaCNi₃ hypothetical antiperovskites. The results are discussed in comparison to the MgCNi₃ isostructural superconductor.     

Possibility of superconductivity in intercalation compound related to MgB2

We investigated the electronic structure of MgB₂ and intercalation compounds, MgBX (X = Li, Be, C), with fully relaxed crystal structure by using density functional theory. The compounds MgBLi and MgBBe could be similar two‐band superconductors to MgB₂ because the Fermi surface crosses σ and π bands. Our results indicate that changing the lattice constants, hole or electron doping, and stacking of B? X effect the energy levels of the σ and π bands in MgBX compounds. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2004     

Theoretical investigation of the structural,elastic, electronic and optical properties of the ternary indium sulfide layered structures AInS2 (A = K,Rb and Cs)

Ab initio calculations were performed to investigate the structural, elastic, electronic and optical properties of the ternary layered systems AInS₂ (A = K, Rb and Cs). The calculated structural parameters are in good agreement with the existing experimental data. Analysis of the electronic band structure shows that the three studied materials are direct band-gap semiconductors. Density of states, charge transfers and charge density distribution maps were computed and analyzed. Numerical estimations of the elastic moduli and their related properties for single-crystal and polycrystalline aggregates were predicted. The optical properties were calculated for incident radiation polarized along the [100], [010] and [001] crystallographic directions. The studied materials exhibit a noticeable anisotropic behaviour in the elastic and optical properties, which is expected due to the symmetry and the layered nature of these compounds.     

Electronic structure of francium

This article presents the first calculations of the electronic structure of francium for the bcc, fcc, and hcp structures, using the linearized augmented plane wave (LAPW) method. Both the local density approximation (LDA) and generalized gradient approximation (GGA) were used to calculate the electronic structure and total energy of francium (Fr). The GGA and LDA both found the total energy of the hcp structure to be slightly below that of the fcc and bcc structures, respectively. This is in agreement with similar results for the other alkali metals where the bcc structure is found not to be the ground state in contradiction to experiment. The equilibrium lattice constant, bulk modulus, and superconductivity parameters were calculated. Calculations of the enthalpy of the system suggest a structural transition from hcp to bcc under a pressure of 0.57 GPa. Using the McMillan‐Gaspari‐Gyorffy theories, we found that under further pressures, in the range of 3–14 GPa, Fr could be a superconductor with critical temperature up to 7 K. This is consistent with the other alkali metals and originates from an increase of the d‐like density of states at the Fermi level, which makes the alkali metals behave like transition metals. © 2013 Wiley Periodicals, Inc.     

Electronic structure and properties of isoreticular metal-organic frameworks: the case of M-IRMOF1 (M = Zn, Cd, Be, Mg, and Ca)

We investigate the possibility of tailoring the electronic properties of isoreticular metal-organic materials by replacing the metal atom in the metal-organic cluster and by doping. The electronic structure of M-IRMOF1, where IRMOF1 stands for isoreticular metal-organic framework 1 and M = Be, Mg, Ca, Zn, and Cd, was examined using density-functional theory. The results show that these materials have similar band gaps (ca. 3.5 eV) and a conduction band that is split into two bands, the lower of which has a width that varies with metal substitution. This variation prompted us to investigate whether doping with Al or Li could be used to tailor the electronic properties of the Zn-IRMOF1 and Be-IRMOF1 materials. It is shown that replacing one metal atom with Al can effectively be used to create IRMOFs with different metallic properties. On the other hand, adding Li produces structural changes that render this approach less suitable.     

Evidence of two-dimensional superconductivity in the single crystalline nanohybrid of organic-bismuth cuprate

A coordination compound of HgI(2)(pyridine)(2) can be successfully intercalated into a single crystalline Bi(2)Sr(2)CaCu(2)O(y) high-T(c) superconductor through an interlayer complexation reaction between pyridine molecules and bismuth cuprate pre-intercalated with mercuric iodide. X-ray diffraction and X-ray absorption spectroscopic results clearly demonstrate that the single crystalline nature of the pristine bismuth cuprate remains unchanged even after the intercalation of organic complex as well as those of iodine and mercuric iodide. According to the angle-dependent dc magnetization measurements, the intercalation of bulky organic molecules completely blocks superconductive currents along the c-axis, whereas a superconducting transition along the in-plane direction still occurs in the organic intercalate. In the case of the iodine or mercuric iodide intercalates with smaller lattice expansions, an out-of-plane diamagnetic transition is not wholly quenched but significantly depressed by the intercalation, confirming the reduction of interlayer interaction. The present finding can provide straightforward evidence of the two-dimensionality of high temperature superconductivity in the present cuprate-based nanohybrid material.     

Conjugated polymer surfaces and interfaces in polymer-based light-emitting diodes

Since the discovery of high electrical conductivity in doped polyacetylene in 1977, π-conjugated polymers have emerged as viable semiconducting electronic materials for numerous applications. In the context of polymer electronic devices, it is of critical importance to understand the nature of the electronic structure of the polymer surface and the interface with metals. It has been shown that, for conjugated polymers, photoelectron spectroscopy, especially in connection with quantum chemical modeling, provides a maximum amount of both chemical and electronic structural information in one type of measurement. There is no such thing as the ideal metal-on-polymer contact; there is always some chemistry that occurs at the interface. © 1998 John Wiley & Sons, Ltd.     

Mechanism of lithium intercalation in titanates

First principles calculations of Li insertion in a variety of titanate structures have revealed a common mechanism underlying the intercalation behavior of these materials. The mechanism is based on the accommodation of the electron density donated upon intercalation in particular orbitals of Ti ions and is governed by a strong coupling between the structural and electronic degrees of freedom. A new predictive model is developed which relates the local structure of TiO₂ polymorphs to their phase behavior upon Li intercalation.