On the question of commensurability – The Nowotny chimney-ladder structures revisited

A composite approach was used for the structure solution and refinement of two new Nowotny chimney-ladder phases, (Mo/Rh)11Ge18and (Mo/Rh)13Ge22, found during investigations in the Mo/Rh/Ge system. The descriptions of the phases with conventional models (in I¯42d(No. 122) with a=5.885(2) Å, c=52.62(2) Å for (Mo/Rh)11Ge18and a=5.922(1) Å, c=62.88(4) Å for (Mo/Rh)13Ge22) led to high numbers of unobserved reflections, high parameter/reflection ratios and an unstable refinement in the case of (Mo/Rh)13Ge22. Microscopy studies showed that the phases were in fact incommensurately modulated. Therefore, a composite model consisting of a rather stable diamond-like transition-metal substructure and a helical main-group-metal arrangement was developed. For the refinement of both phases, (Mo/Rh)11Ge18as well as (Mo/Rh)13Ge22, the same model – same space group pair (W: P4/nnc:q¯1q1 P: I41/amd: 1¯1ss), same atom positions and modulation waves – was used. The refinements converged rapidly, showing that the presented structure model is very general, and may well be applicable to any Nowotny chimney-ladder phase.

Zn(5)Sb(4)In(2-delta) - a ternary derivative of thermoelectric zinc antimonides

Zn(5)Sb(4)In(2-delta) (delta = 0.15(3)) was synthesized in the form of millimeter-sized crystals from reaction mixtures containing excess zinc. The ternary intermetallic compound is temperature polymorphic, and at room temperature it crystallizes with a new structure type in the orthorhombic space group Pbcn, where a = 7.1619(2), b = 17.1562(4), c = 8.6887(4) A, V = 1067.6(1) A(3), and Z = 4. The structure features 3(2)434 nets of Sb atoms that are stacked in antiposition to yield layers of square antiprisms sharing edges plus intervening tetracapped tetrahedra (tetreadersterns). The majority of Zn atoms occupy peripheral tetrahedra of such tetraedersterns, and attain at the same time the peculiar five-coordination by one like atom and four Sb atoms typical for the structures of binary zinc antimonides. The In and remaining Zn atoms are distributed in the tetragonal channels formed by the square antiprisms and display some disorder. At temperatures below 200 K Zn(5)Sb(4)In(2-delta) undergoes a phase transition into a more ordered structure with monoclinic symmetry (P2(1)/c) without any change of the unit cell. The thermoelectric properties of Zn(5)Sb(4)In(2-delta) were measured between 10 and 350 K. Exceptionally low thermal conductivity values (1 W/mK range) were obtained in the whole temperature range. Resistivity and thermopower values are characteristic of a heavily doped or degenerate semiconductor (2.5 mOmega cm and 160 muV/K, respectively, at room temperature) and show a discontinuity around 220 K. The thermoelectric figure of merit of Zn(5)Sb(4)In(2-delta) is higher than that of Zn(4)Sb(3) in the investigated temperature range.

Vacancy-ordering effects in AlB2-type ErGe2 - x(0.4 < x < or = 0.5)

In the Er-Ge system, the compostion range ErGe(2) to Er(2)Ge(3) has been investigated. Eight samples were produced by arc melting of the elements, and analyzed using X-ray powder diffraction. Nine crystal structures were found to be present in the samples. The structures are described as a homologous series and presented within the superspace formalism using the superspace group X2/m(alpha0gamma)0s, X representing the centring vector ((1/2), (1/2), 0, (1/2)). In this description the modulation vector q = (alphaa* + gammac*) is shown to be a direct measure of the Ge content as ErGe(2 - alpha) (alpha falls in the range 1\over 3 to (1/2)). The large composition range is achieved by extended vacancy ordering in the planar 6(3) net of Ge with subsequent relaxation.

Single-crystal X-ray study of Ba2Cu2Te4O11Br2 and its incommensurately modulated superstructure companion

Compounds containing lone-pair elements such as Te(IV) are very interesting from the structural point of view, as the lone-pair nonbonding regions create low-dimensional geometrical arrangements. We have synthesized two new compounds with these features-Ba(2)Cu(2)Te(4)O(11)Br(2) (I) and Ba(2)Cu(2)Te(4)O(11-delta)(OH)(2delta)Br(2) (II, delta approximately equal to 0.57)-as members of the AE-M-Te-O-X (AE=alkaline-earth metal, M=transition metal, X=halide) family of compounds by solid-state reactions. Preliminary single-crystal X-ray analysis indicated that compound I crystallizes in the orthorhombic system, but attempts at refinement proved unsatisfactory. Closer inspection of the reciprocal lattice revealed systematic, non-crystallographic absences that indicate twinning. The structure is in fact triclinic, space group C_1 (equivalent to P_1), with unit cell parameters (at 120 K) of a=10.9027(9), b=15.0864(7), c=9.379(2) A, beta=106.8947 degrees . It is layered and built from [TeO(3)E] tetrahedra, [TeO(3+1)E] trigonal bipyramids (where E is the lone pair of Te(IV)), [CuO(4)] squares and irregular [BaO(10)Br] polyhedra. The crystal structure of II shows the same basic structure as I but contains additional oxygen, probably in the form of OH groups. The presence of satellites reveals that ordering on this O site creates an incommensurate modulation, primarily affecting Br and Te. The modulated structure of II was solved in the triclinic superspace group X$\bar 1$(alphabetagamma)0 with the vector q approximately equal to1/16 c*.

Occupation Waves the Way You Have Never Seen Them: the Orthorhombic Quasicrystal Approximants RE13Zn58+δ (RE = Ho, Er, Tm, and Lu)

A series of binary quasicrystal approximants RE13Zn58+delta (RE = Ho, Er, Tm, and Lu) have been prepared, and structural studies were performed by means of single-crystal X-ray diffraction. All four compounds crystallize in the orthorhombic system, but while the Ho-containing compound crystallizes in space group Pcmn, the rest of the compounds crystallize in Pc21n. This work is a continuation of the structural studies on RE13Zn approximately 58 (RE = Ce, Pr, Nd, Sm, Gd, Tb, and Dy), which all belong to the hexagonal system. The sequence of RE13Zn58+delta compounds exhibits a large variability in local ordering and composition, and therefore the crystal structures are generally rather more complex than those previously reported and exhibit a number of different ordering modes. These four compounds are structurally closely related to each other.