Highly Hydrated Cations: Deficiency, Mobility, and Coordination of Water in Crystalline Nonahydrated Scandium(III), Yttrium(III), and Lanthanoid(III) Trifluoromethanesulfonates

Trivalent lanthanide-like metal ions coordinate nine water oxygen atoms, which form a tricapped trigonal prism in a large number of crystalline hydrates. Water deficiency, randomly distributed over the capping positions, was found for the smallest metal ions in the isomorphous nonahydrated trifluoromethanesulfonates, [M(H2O)n](CF3SO3)3, in which M = Sc(III), Lu(III), Yb(III), Tm(III) or Er(III). The hydration number n increases (n = 8.0(1), 8.4(1), 8.7(1), 8.8(1) and 8.96(5), respectively) with increasing ionic size. Deuterium (2H) solid-state NMR spectroscopy revealed fast positional exchange between the coordinated capping and prism water molecules; this exchange started at temperatures higher than about 280 K for lutetium(III) and below 268 K for scandium(III). Similar positional exchange for the fully nonahydrated yttrium(III) and lanthanum(III) compounds started at higher temperatures, over about 330 and 360 K, respectively. An exchange mechanism is proposed that can exchange equatorial and capping water molecules within the restrictions of the crystal lattice, even for fully hydrated lanthanoid(III) ions. Phase transitions occurred for all the water-deficient compounds at approximately 185 K. The hydrated scandium(III) trifluoromethanesulfonate transforms reversibly (DeltaH degrees = -0.80(1) kJ mol(-1) on cooling) to a trigonal unit cell that is almost nine times larger, with the scandium ion surrounded by seven fully occupied and two partly occupied oxygen atom positions in a distorted capped trigonal prism. The hydrogen bonding to the trifluoromethanesulfonate anions stabilises the trigonal prism of water ligands, even for the crowded hydration sphere of the smallest metal ions in the series. Implications for the Lewis acid catalytic activity of the hydrated scandium(III) and lanthanoid(III) trifluoromethanesulfonates for organic syntheses performed in aqueous media are discussed.

Superstructure of EU4CD25: a quasicrystal approximant

The quasicrystal approximant Eu(4)Cd(25), formerly designated EuCd(6), was synthesized and characterized by means of single-crystal X-ray diffraction. Superstructure reflections corresponding to an F-centered cubic unit cell with a doubled cell-parameter relative to the I-centered cubic sub-cell could be observed in the diffraction data. This gives the largest cubic unit cell reported to date among binary alloys. The superstructure exhibits structural features that are found in all the RE-Cd(6) approximants as well as a further ordering between vacant/occupied Cd(8) cubes and oriented Cd(4) tetrahedra, which causes the superstructure. A reversible high-temperature phase-transition was observed at 782 K by means of DSC.

The structure of alpha-Zn4Sb3: ordering of the phonon-glass thermoelectric material beta-Zn4Sb3

beta-Zn4Sb3 is an outstanding thermoelectric material mainly due to its extraordinarily low thermal conductivity, which is similar to that of glasses. Recently it was proposed that interstitial Zn atoms are responsible for this peculiar behavior. Here we report on the crystal and electronic stucture of the low-temperature polymorph alpha-Zn4Sb3. During the reversible phase transition the intricate disorder in beta-Zn4Sb3 disappears, and all Zn atoms localize completely. The electronic structure of alpha-Zn4Sb3 corresponds to that of a narrow-gap semiconductor.

Phase Stability in the Systems AeAl2-xMgx (Ae = Ca, Sr, Ba): Electron Concentration and Size Controlled Variations on the Laves Phase Structural Theme

The systems AeAl(2-x)Mgx (Ae = Ca, Sr, Ba) display electron concentration induced Laves phase structural changes. However, the complete sequence MgCu2 --> MgNi2 --> MgZn2 with increasing x (decreasing electron count) is only observed for Ae = Ca. Compounds SrAl(2-x)Mgx (0 < x < or = 2) and BaAl(2-x)Mgx (x = 0.85 and 2.0) were synthesized and structurally characterized by X-ray diffraction experiments. For the Sr system the structural sequence CeCu2 --> MgNi2 --> MgZn2 occurs with increasing Mg content x. Thus, larger Sr does not allow the realization of the MgCu2 structure at low x. For Ae = Ba a binary compound BaAl2 does not exist, but more Ba-rich Ba7Al13 forms. The reinvestigation of the crystal structure of Ba7Al13 by selected area and convergent beam electron diffraction in a transmission electron microscope revealed a superstructure, which subsequently could be refined from single X-ray diffraction data. The formula unit of the superstructure is Ba21Al40 (space group P31m, Z = 1, a = 10.568(1) angstroms, c = 17.205(6) angstroms). In Ba21Al40 a size match problem between Ba and Al present in Ba7Al13 is resolved. The structure of Ba7Al13 (Ba21Al40) can be considered as a Ba excess variant of the hexagonal MgNi2 Laves phase type structure. An incommensurately modulated variant of the MgNi2 structure is obtained for phases BaAl(2-x)Mgx with x = 0.8-1. At even higher Mg concentrations a structural change to the proper MgZn2 type structure takes place.

LaSeTe2—Temperature Dependent Structure Investigation and Electron Holography on a Charge-Density-Wave-Hosting Compound

Single crystals of LaSeTe(2) have been prepared by reaction of the elements in a LiCl/RbCl flux at 970 K for seven days. Satellite reflections observed in diffraction experiments indicate the presence of an incommensurate lattice distortion, which is of the charge-density-wave (CDW) type. The modulated structure has been solved from X-ray data at 173, 293, and 373 K. LaSeTe(2) crystallizes in the 3+1-dimensional orthorhombic superspace group Cmcm(00gamma)s00 (No. 63.2) with lattice parameters of a=4.295(1), b=25.371(4), c=4.306(1) A (173 K), a=4.297(1), b=25.408(4), c=4.309(1) A (293 K), and a=4.309(1), b=25.481(6), c=4.321(1) A (373 K). The modulation vector q=(0, 0, 0.288) does not change over the temperature interval. Electron holographic investigations confirm the existence of the modulation and help to visualize the charge-density wave.

Structure of δ1-CoZn7.8, an example of a phason pinning–unpinning transformation?

The novel structure of the δ1 phase in the cobalt–zinc system was determined by single-crystal X-ray diffraction. The structure features fused double icosahedra linked by edge- and face-sharing. The δ1-CoZn7.8 structure is incommensurately modulated along the columnar b direction. The extent of the linear pentagonal antiprismatic intergrowth is limited to a maximum of two overlapping icosahedra because of geometric demands and radii relations. Even this limited fusion of icosahedra leads to strain that is believed to be the origin of the structural modulation. The compound was synthesized using a centrifugation-aided filtration method which yielded single crystals grown on cobalt pieces in a zinc-rich melt. The (3 + 1)-dimensional superspace group is F2/m(0β0)s0 and the modulation wavevector was determined to q = 0.234b*. The partial amorphization of the sample when subjected to mechanical grinding is an indication of a metastable structure. The incommensurability of the structure may be seen as an ordered pinning of phasons.

(3 + 1)-dimensional structure refinement of the fresnoite framework-structure type compound Ba(2)TiGe(2)O(8)

The incommensurately modulated structure of the fresnoite framework-structure type compound Ba(2)TiGe(2)O(8) has been solved using a (3 + 1)-dimensional superspace approach. The structure is orthorhombic and adopts the superspace group Cmm2(0,beta,1/2)s00 with beta approximately 0.635 at room temperature. The refinement was based on neutron powder diffraction data obtained from a powdered single crystal grown by Czochralski pulling. The modulation parameters that were obtained support the idea that frozen-in rigid-unit modes cause the modulation. The modulation is mainly manifested by positional displacements of O atoms. Barium ions are either eightfold, ninefold or tenfold coordinated in the one-dimensional modulated structure. A significant improvement of the bond-valence sum for both barium positions is achieved by the introduction of the positional modulation. This finding strongly suggests that underbonded barium positions are critically involved in provoking the incommensurate modulation in Ba(2)TiGe(2)O(8).

Superlattice formation in the lithiated vanadium oxide phases Li(0.67)V(6)O(13) and LiV(6)O(13)

Two new lithiated phases of V(6)O(13) were formed by carefully tuning the temperature of electrochemical lithiation in a "coffee-bag" type Li-ion battery at 2.78 V versus Li/Li(+). These were studied by single-crystal X-ray diffraction. A phase with the composition Li(2/3)V(6)O(13) was obtained at 308 K with a unit cell three times the volume of the original V(6)O(13) cell. A single crystal discharged at ambient temperature was shown to be LiV(6)O(13) and twice the unit-cell volume of the original V(6)O(13) cell. On lithiation, the structures retain their basic V(6)O(13) structure of alternating single and double layers of VO(6) octahedra. The lithium ions occupy chemically equivalent sites, where they coordinate fivefold to O atoms, and associate with the single layers of VO(6) octahedra. The insertion of lithium causes a significant elongation of one of the V-O bonds in each structure, which expands from 1.65 to 1.89 A; this is due to the charge reduction of a specific V atom.