R. Nesper

Structure and chemical bonding in zintl-phases containing lithium

5 Zint l Phases 2.1 T h e Basic Z i n t l K l e m m C o n c e p t . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 E x t e n s i o n s of the Z i n t l K l a m m C o n c e p t . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 W h y Does t he Z i n t l K l e m m C o n c e p t Work? . . . . . . . . . . . . . . . . . . . . . . 2.4 P r e p a r a t i o n . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Nodal surfaces of Fourier series : fundamental invariants of structured matter

Periodic Nodal Surfaces (PNS) of Fourier series are derived and classified as fundamental invariants of structured matter. Relationships to periodic minimal surfaces PMS and to periodic zero potential surfaces (POPS) are given. A basic set of cubic PNS is represented in arithmetic form. The special importance of the invariance of the zeros to the type of the potential is stressed.

Li21Si5, a Zintl phase as well as a Hume-Rothery phase

Li21Si5 is the most lithium-rich phase in the binaryLiSi system and substitutes the so-called Li22Si5. This was proved by a careful X-ray structure analysis (F4¯3m-T2d; a = 1871.0pm; Z = 16; R = 0.052; 397hkl). The reduced Li content is due to ordered vacancies in the huge 6 × 6 × 6 superstructure of bcc (cF416). This gives rise to the formation ofM26 clusters typical for γ-brass structures. Two different cluster types, namely Li22Si4 and Li20Si6, set up a complex linkage of diamond, zinc blende, and NaCl type partial structures, in which short SiSi distances are avoided. Li21Si5 is on one hand a representative consistent only of main group elements of the Hume-Rothery phases for structural and electronic reasons (VEC=20.5/13= 1.58). On the other hand the twoM26 clusters fulfill electronically an extended Zintl phase formalism, according to [Li22Si44+ and [Li20Si64−. The latter anion follows classical valence rules (Si4− formation), while the former one carries a cage orbital in addition to the filled Si states, which is delocalized over the Li sites. The two quantum mechanically stable units act as donor-acceptor pairs, which is also reflected in the experimental valence electron density. In a new topological description the whole structure can be understood as being built ofM14 cluster nuclei and a continuous periodic curved surface formed by the network of all remaining Li atoms.

Trilithium tetradecaboride Li3B14: Synthesis, structure, and properties

Lithium boron compounds are of interest for technical applications (battery technology, neutron scattering techniques). Binary lithium borides have been investigated by several authors, but none of these phases is well characterized: LiB/sub 10.85/ LiB/sub 3/, LiB, Li/sub /approximately/1/B, Li/sub 7/B/sub 6/, Li/sub 5/B/sub 4/, and Li/sub 2/B. Not one single-crystal structure investigation is available and the only reasonable structural model is that of Li/sub 5/B/sub 4/ which was proposed from X-ray powder data. These unreliable results and the questionable characterization of the lithium borides stimulated their reinvestigation of the binary Li-B system. Three new phases have been identified on the boron-rich side, but none of the older results were confirmed. A careful comparison of all experimental data reported earlier with current results show that some so-called compounds re obviously phase mixtures. The boron richest compound Li/sub 3/B/sub 14/ = LiB/sub 4.67/ is described to date.

The lithium sodium silicide Li3NaSi6 and the formation of allo-silicon

Abstract Metallic grey Li3NaSi6 is formed by heating the elements in stoichiometric amounts. The compound is the only stable ternary phase in the Liue5f8Naue5f8Si system, and does not belong to the tetrasilatetrahedrane derivatives. The novel complex layer structure is characterized by two-dimensional infinite 2∞[Si64−] polyanions showing polymerized tube-like structural units which are known from Hittorfs violet phosphorus and GeAs2 respectively. The alkali metal atoms are inserted between the respective polyanionic layers (Pnma (62); a = 1797.2 pm, b = 378.8 pm, c = 1029.9 pm; 808 hkl; R = 0.025; d(Si-Si) = 237.2 – 244.9 pm; d(Li-Si) = 254.1 – 296.6 pm; d(Na-Si) = 312.0 – 347.1 pm). Li3NaSi6 is a diamagnetic semiconductor with a molar susceptibility χmol of −243 × 10−6 emu mol−1, R(300 K ) = 50 Ω and R(2 K ) = 3 × 10 3 Ω . Li3NaSi6 reacts with protic solvents as well as with benzophenone (in tetrahydrofuran) topotactically yielding a new metastable silicon modification, namely allo-Si. The polyanionic block structure is decharged and is polymerized to a three-dimensional network (χmol = −38 × 10−6 emu atom−1). According to reflection electron microscopy, larger crystals show a pronounced lamellar structure which is responsible for the graphite-like mechanical behaviour. At 800 K, allo-Si transforms irreversibly into α-Si.

Li8MgSi6, a novel Zintl compound containing quasi-aromatic Si5 rings

Abstract Li 8 MgSi 6 is the compound with the highest silicon content in the ternary system Li/Mg/Si. The gray compound forms columnlike crystals with metallic lustre and is very sensitive to moisture. It reacts spontaneously with water to silanes and amorphous silicon. Li 8 MgSi 6 is a diamagnetic semiconductor with E g = 0.72 eV, ϱ(292 K) = 1.3 × 10 3 Ω cm − . The compound is monoclinic and crystallizes in space group P2 1 m , a = 12.701, b = 4.347, c = 10.507 A, β = 107.58°, Z = 2. The structure of Li 8 MgSi 6 contains isolated silicon atoms and planar five-membered Si 5 rings which form 1 ∞ [LiSi 5 ] sandwich stacks. Semiempirical SCF calculations are in accordance with the physical properties and support a description of the five-membered silicon rings as quasi-aromatic 26 electron systems. A generalization of the electron counting rules of Zintl and Klemm is proposed. A remark on the ambiguous Li 11 Ge 6 is given.

Dilithium Hexaboride, Li2B6

Li2B6 is formed from the elements as transparent red microcrystalline compound (Liu200a:u200aBu200a=u200a1u200a:u200a3; Mo crucible in closed Nb ampoule; 1723u200aK; 4u200ah). Single crystals are grown from a lithium silicide melt with large Li excess at 1923u200aK. Li2B6 is a semiconductor with electron as well as Li+ ionic conductivity which dominates above 600u200aK. Microcrystalline samples react with H2O liberating gases and forming a brownish amorphous product, but larger crystals are not very sensitive. – Li2B6 crystallizes tetragonally in a new tP16 structure type which is a variant of the CaB6 structure (au200a=u200a5.975u200aA, cu200a=u200a4.189u200aA; Zu200a=u200a2; space group P4/mbm). The [B62–] net of the polymeric octahedro-anion is slightly distorted to give space for the insertion of a (32434) net ofxa0the Li+ cations in the cavities (d(B–B)endou200a=u200a1.766u200aA; d(B–B)exou200a=u200a1.720u200aA; d(Li–B)u200a=u200a2.363u200aA; d(Li–Li)u200a=u200a3.094u200aA). The incomplete occupancy of the Li position (80%) and the electron density at a further position (20%) indicate the mobility of the Li+ cations. n n n nDilithiumhexaborid, Li2B6 n n n nLi2B6 bildet sich aus den Elementen als transparent-rote, mikrokristalline Verbindung (Liu200a:u200aBu200a=u200a1u200a:u200a3; Mo-Tiegel in geschlossener Nb-Ampulle; 1723u200aK; 4u200ah). Einkristalle erhalt man aus einer Lithiumsilicidschmelze mit hohem Li-Gehalt bei 1923u200aK. Li2B6 ist ein Halbleiter mit Elektronenleitung und Li+-Ionenleitung, die oberhalb 600u200aK dominiert. Die mikrokristalline Verbindung reagiert mit H2O unter Gasentwicklung zu einem braunen, amorphen Produkt. Grosere Kristalle sind nicht sehr empfindlich. Li2B6 kristallisiert tetragonal in einem neuen tP16-Strukturtyp, einer Variante der CaB6 Struktur (au200a=u200a5.975u200aA, cu200a=u200a4.189u200aA; Zu200a=u200a2; Raumgruppe P4/mbm). Das [B62–]-Netz des polymeren octahedro-Anions ist leicht verzerrt und erlaubt damit die Einlagerung eines (32434)-Netzes der Li+-Kationen in den Kavernen (d(B–B)endou200a=u200a1.766u200aA; d(B–B)exou200a=u200a1.720u200aA; d(Li–B)u200a=u200a2.363u200aA; d(Li–Li)u200a=u200a3.094u200aA). Die unvollstandige Besetzung der Li-Position (80%) und die Elektronendichte auf einer weiteren Position (20%) weisen auf die Mobilitat der Li+-Kationen hin.

The solid-state electronic structure and the nature of the chemical bond of the ternary Zintl-phase Li8MgSi6. A tight-binding analysis

Abstract The electronic strcuture of the ternary Zintl-phase Li 8 MgSi 6 has been investigated in the computational framework of a semi-empirical crystal orbital (CO) formalism based on the tight-binding approximation. Li 8 MgSi 6 crystallizes in the space group P2 1 /m-C 2h 2 with a = 12.701 A, b = 4.347 A, c = 10.507 A and β = 107.58°. A self-consistent-field (SCF) Hartree-Fock (HF) INDO CO procedure has been employed for the numerical approach. In order to reduce the computational expenditure of the CO calculations we have adopted a one-dimensional (ID) model simulating the real solid. To allow for a clear theoretical analysis the ID system is divided into simpler subfragments (MgSi 2- , Li 3 MgSi + , Li 5 Si 5 − ); the solid-state electronic structures of these moieties can be rationalized in a straightforward way. The band structure properties, density of states distributions, net charges and atomic orbital populations of Li 8 MgSi 6 are interpreted. A forbidden band gap of 0.62 eV is calculated by the semi-empirical tight-binding scheme, a value that is in excellent agreement to the measured band gap of the semiconducting compound which amounts to = 0.7 eV. The nature of chemical bond in the Li 8 MgSi 6 phase is analyzed by fragmenting the net diatomic interaction energies between Siue5f8Si, Siue5f8Li and Mgue5f8Si pairs into covalent resonance elements as well as exchange and classical electrostatic (Coulomb) contributions. Partial coordination numbers (PCN) are defined for the various atomic species of the ternary phase that are labels of strongly stabilizing interactions (bonds) in the low-dimensional units. The calculated charge distributions show a striking 1:1 correspondence between the present CO results and the expectations derived on the basis of classical (Zintl-Klemm) electron-counting rules thus corroborating the utility of extended Zintl-Klemm conceptions in solids with atoms beyond the first two rows.

Electrical and magnetic properties of single crystalline EuAs3, β-EuP3 and their mixed crystals

The authors have investigated the electrical and magnetic properties of the mixed crystals Eu(As1-xPx)3 by measuring the temperature and magnetic field dependences of the electrical resistivity, magnetic susceptibility and specific heat. The variation from semimetallic (EuAs3) to semiconducting (EuP3) behaviour is explained in terms of a simple electron energy scheme. The antiferromagnetic ordering observed in the whole concentration range below about 10K is believed to be due to an Eu-Eu superexchange via two-bonded (2b)X- atoms (X=As, P). The exchange coupling constants for EuAs3 are given within a molecular-field approximation.

Dithorium undecaphosphide Th2P11, a polyphosphide with a one-dimensional superstructure generated by a periodic change in the covalent bonds

Abstract Thorium undecaphosphide (Th2P11) is the first polyphosphide of a tetravalent cation (M) with a ratio P/M greater than 2. The compound was synthesized from the elements at 770 K. The black crystals decompose to Th3P4 at 740 K in vacuo. Th2P11 is a diamagnetic semiconductor with Eg = 0.3 eV and ρ (298) = 5392 Ω cm . The compound was found by single-crystal methods to crystallize in the monoclinic space group solP21 c with a = 1738.4 pm, b = 1010.4 pm, c = 1919.3 pm, β = 117.62° andZ = 12 (4053 reflections hkl; R = 0.061). Th2Pn possesses a structure with puckered polyanionic layers formed by parallel oriented 1∞ [P16] bands with isolated P6 rings between them. The 1∞ [P16] bands are built up by the condensation of skewboat P6 rings and endo P8 rings. The thorium atoms are inserted in channels and have a ninefold coordination. The isolated P6 rings act as doubly tridentate ligands. A one-dimensional superstructure exists along the a axis. This results from a periodic sequence of open and closed P6 rings. The superstructure is discussed in terms of a valence-governed periodic modulation which is caused by a frozen-in SN2 reaction accompanied by a charge transfer.