The atomistic mechanisms of Li(+) ion mobility/conductivity in Li(7-x)PS(6-x)I(x) argyrodites are explored from both experimental and theoretical viewpoints. Ionic conductivity in the title compound is associated with a solid-solid phase transition, which was characterised by low-temperature differential scanning calorimetry, (7)Li and (127)I NMR investigations, impedance measurements and molecular dynamics simulations. The NMR signals of both isotopes are dominated by anisotropic interactions at low temperatures. A significant narrowing of the NMR signal indicates a motional averaging of the anisotropic interactions above 177+/-2 K. The activation energy to ionic conductivity was assessed from both impedance spectroscopy and molecular dynamics simulations. The latter revealed that a series of interstitial sites become accessible to the Li(+) ions, whilst the remaining ions stay at their respective sites in the argyrodite lattice. The interstitial positions each correspond to the centres of tetrahedra of S/I atoms, and differ only in terms of their common corners, edges, or faces with adjacent PS(4) tetrahedra. From connectivity analyses and free-energy rankings, a specific tetrahedron is identified as the key restriction to ionic conductivity, and is clearly differentiated from local mobility, which follows a different mechanism with much lower activation energy. Interpolation of the lattice parameters as derived from X-ray diffraction experiments indicates a homogeneity range for Li(7-x)PS(6-x)I(x) with 0.97 < or = x < or = 1.00. Within this range, molecular dynamics simulations predict Li(+) conductivity at ambient conditions to vary considerably.

Na 4Si4, monoclinic, C12/c1 (no. 15), a = 12.1536(5) Å, b = 6.5452(5) Å, c = 11.1323(6) Å, b = 118.9(1)°, V = 775.3 Å 3 , Z = 4, Rgt(F) = 0.028, wRref(F 2 ) = 0.064, T = 293 K. Source of materialThe Zintl phase Na 4Si4 was prepared from the elements with an excess of alkali metal (Na, ingot, 99.9 %, ChemPur; Si, pieces, 99.999 %, Alfa) in a closed Ta container which was sealed into an evacuated quartz glass ampoule. The reaction mixture was heated up to 750°C within 1 hour, annealed for 3 hours and afterwards cooled down to ambient temperature within 4 days. Well shaped needle-like crystals were obtained by removing the excess of sodium using vacuum distillation at 230°C and 5 × 10 −6 mbar [1,2]. Due to sensitivity of the compound to air and humidity, the investigated single crystal was sealed into a glass capillary under purified argon. Experimental detailsLattice parameters were refined from synchrotron powder diffraction data (l = 0.39987 Å), using 58 reflections between 4°# 2q # 14.5°, and applying LaB 6 as internal standard (NIST, a = 4.15692 Å). DiscussionThe crystal structure of Na 4 Si 4 was first determined by Witte, von Schnering and Klemm from Weißenberg film data on single crystals [1]. Na 4 Si 4 was re-investigated in order to elucidate the chemical bonding by quantum chemical calculations as well as solid state nuclear magnetic resonance measurements. For these investigations, high quality crystallographic data are crucial. The present re-determination confirms structural information obtained in Ref.[1] and provides data of the crystal structure with higher accuracy. Two crystallographic non-equivalent sites for both Si and Na are occupied in the crystal structure. The Si atoms form dis-

The Zintl phase Ba 3 Si 4 has been synthesized from the elements at 1273 K as a single phase. No homogeneity range has been found. The compound decomposes peritectically at 1307(5) K to BaSi 2 and melt. The butterfly-shaped Si 4 6Ϫ Zintl anion in the crystal structure of Ba 3 Si 4 (Pearson symbol tP28, space group P4 2 /mnm, a ϭ 8.5233(3) Å , c ϭ 11.8322(6) Å ) shows only slightly different Si-Si bond lengths of d(SiϪSi) ϭ 2.4183(6) Å (1ϫ) and 2.4254(3) Å (4ϫ). The compound is diamagnetic with χ ഠ Ϫ50 ϫ 10 Ϫ6 cm 3 mol Ϫ1 . DC resistivity measurements show a high electrical resistivity (ρ(300 K) ഠ 1.2 ϫ 10 Ϫ3 Ω m) with positive temperature gradient dρ/dT. The temperature dependence of the isotropic signal shift and the spin-lattice relaxation times in 29 Si 1651 NMR spectroscopy confirms the metallic behavior. The experimental results are in accordance with the calculated electronic band structure, which indicates a metal with a low density of states at the Fermi level. The electron localization function (ELF) is used for analysis of chemical bonding. The reaction of solid Ba 3 Si 4 with gaseous HCl leads to the oxidation of the Si 4 6Ϫ Zintl anion and yields nanoporous silicon.

The Zintl phase Rb 7 NaSi 8 was synthesized as single-phase material. Powder and single-crystal X-ray diffraction, thermal analysis, chemical analysis, measurement of the magnetic susceptibility, and 23 Na, 29 Si, and 87 Rb nuclear magnetic resonance (NMR) spectroscopy were employed to characterize the material. Rb 7 NaSi 8 crystallizes in the Rb 7 NaGe 8 type of structure forming trigonal pyramidal anions Si 4 4-. Two unique environments of the cations are observed, the linear arrangement [Na(Si 4 ) 2 ] 7with short Na-Si distances of 2.94 Å and the Rb2 atom coordinated by six Si 4 4anions with long Rb-Si distances of 4.09 Å. The environment of the Rb1 atom is similar to the coordination of the rubidium atoms in Rb 4 Si 4 . The chemical bonding was investigated by quantum mechanical calculations of the electron localizability indicator (ELI), the electronic density of states (DOS), and NMR coupling 1982 parameters. Good agreement of theoretically calculated and experimentally determined NMR coupling parameters was obtained for both chemical shielding and quadrupole coupling. The anisotropy of chemical shielding (CSA) indicates an anisotropic bonding situation of the silicon atoms. This is confirmed by the observation of a lone-pair-like feature and three two-center Si-Si bonds for each silicon atom using the ELI. The electric field gradients (EFG) of 23 Na and 87 Rb indicate the anisotropy of the charge distribution of the cations. In particular, the linear arrangement [Na(Si 4 ) 2 ] 7and the Rb1 atom feature anisotropic charge distributions of the cations. This is confirmed by calculating the anisotropy ratio defined as Δn p = n(p z )/(½[n(p x )+n(p y )]) with n(p i ) corresponding to the integrated DOS of the respective state. The long distances of Rb2 to the six coordinating Si 4 4ions are reflected by the small EFG.

The Zintl phases M 4 Si 4 with M = Na, K, Rb, Cs, and Ba 2 Si 4 feature a common structural unit, the Si 4 4anion. The coordination of the anions by the cations varies significantly. This allows a systematic investigation of the bonding situation of the anions by 29 Si NMR spectroscopy. The compounds were characterized by powder X-ray diffraction, differential thermal analysis, magnetic susceptibility measurements, 23 Na, 29 Si, 87 Rb, 133 Cs NMR spectroscopy, and quantum mechanical calculation of the NMR coupling parameter. The chemical bonding was investigated by quantum mechanical calculations of the electron localizability indicator (ELI). Synthesis of the compounds results for all of them in single phase material. A systematic increase of * Prof. Dr. F. Haarmann