Matthias Conrad

Large, larger, largest ‒ a family of cluster-based tantalum copper aluminides with giant unit cells. I. Structure solution and refinement

This is the first of two parts, where we report the structure determination of a novel family of cluster-based intermetallic phases of unprecedented complexity: cF444-Al(63.6)Ta(36.4) (AT-19), a = 19.1663 (1) A, V = 7040 A3, cF(5928-x)-Al(56.6)Cu(3.9)Ta(39.5), x = 20 (ACT-45), a = 45.376 (1) A, V = 93,428 A(3) and cF(23,256-x)-Al(55.4)Cu(5.4)Ta(39.1), x = 122 (ACT-71), a = 71.490 (4) A, V = 365,372 A3. The space group is F43m in all three cases. These cluster-based structures are closely related to the class of Frank-Kasper phases. It is remarkable that all three structures show the same average structure that resembles the cubic Laves phase.

A Dodecagonal Quasicrystalline Chalcogenide

Diffractograms with twelvefold rotational symmetry (depicted on the right) were obtained from the first quasicrystalline chalcogenide Ta1.6 Te. This compound was prepared on a preparative scale by the reduction of TaTe2 with tantalum below 1870 K. This tantalum-rich telluride, which is the first stable dodecagonal phase, has enabled an in-depth investigation of this unusual state of ordering.

Large, larger, largest - a family of cluster-based tantalum copper aluminides with giant unit cells. II. The cluster structure

This is the second of two papers, where we discuss the cluster structures of a novel family of cluster-based intermetallic phases of unprecedented complexity: cF444-Al(63.6)Ta(36.4) (AT-19), a = 19.1663 (1) A, V = 7040 A3, cF(5928-x)-Al(56.6)Cu(3.9)Ta(39.5), x = 20 (ACT-45), a = 45.376 (1) A, V = 93,428 A3 and cF(23,256-x)-Al(55.4)Cu(5.4)Ta(39.1), x = 122 (ACT-71), a = 71.490 (4) A, V = 365,372 A3. The space group is F43m in all three cases. The structures can be described as packings of clusters such as fullerenes, dodecahedra, pentagonal bifrusta and Friauf polyhedra. A characteristic feature of the two larger structures are nets of hexagonal bipyramidal Ta clusters (h.b.p.). The extremely short distance of 2.536-2.562 A between their apical Ta atoms indicates unusually strong bonding. The large h.b.p. nets are sandwiched between slabs of Friauf polyhedra resembling the structure of the mu phase.

The mesoscopic structure of disordered dodecagonal tantalum telluride : a high-resolution transmission electron microscopy study

A disordered dodecagonal quasicrystalline tantalum-rich telluride was prepared at the upper thermal stability limit of the related quasicrystalline phase by application of conventional high-temperature synthesis techniques. The structure of the disordered telluride was studied on a mesoscopic scale by means of high-resolution transmission electron microscopy (HRTEM). A selective imaging technique was employed in order to produce HRTEM images which allow a square-triangle tiling underlying the structure on a length scale of about 2 nm to be recognized The tiling was analysed with regard to specific deviations from a perfectly ordered quasicrystalline dodecagonal telluride and various crystalline approximants. The application of a general construction principle derived from the structures of two well characterized approximants allows the reconstruction of a realistic structural model for the disordered dodecagonal phase and the calculation of its composition. The disordered phase is interpreted as an interg...

Ta97Te60: a crystalline approximant of a tantalum telluride quasicrystal with twelvefold rotational symmetry.

The crystal structure of a rational approximant of an unprecedented dodecagonal quasicrystal is reported. The atomic arrangement of the tantalum-rich telluride Ta(97)Te(60) has been determined from 30 458 symmetrically independent X-ray intensities of a crystal twinned by metric merohedry: a=2767.2(2), b=2767.2(2), c=2061.3(2) pm, space group P2(1)2(1)2(1), Pearson symbol oP628, 1415 variables, R(F)=0.059. A dodecagonal-shaped, vaulted Ta(151)Te(74) cluster with maximum symmetry 6mm can be seen as a characteristic motif of the structure. Each cluster measures about 2.5 nm across and consists of nineteen concentrically condensed hexagonal antiprismatic Ta(13) clusters which are capped with Te atoms. The Ta(151)Te(74) clusters can cover the plane distinctly in different ways, thereby forming a series of phases which are closely related both structurally and compositionally. In Ta(97)Te(60) the buckled clusters decorate the vertices of a square tiling at a 2 nm length scale to result in corrugated (infinity)(2)[Te(30)Ta(41)Ta(15)Ta(41)Te(30)] each about 1 nm thick. The lamellae are stacked along the c axis, corresponding to the direction of the pseudo-twelvefold axis of symmetry. Symmetry arguments are proposed that the twinning of the structure may be associated with a fine-tuning of weak interlayer Te-Te interactions which are reflected in a minimization of the deviation from the mean distance and a doubling of the stacking vector c.

Layered tantalum-rich chalcogenides

Abstract Selected ternary tantalum-rich chalcogenide systems were systematically explored by application of high temperature synthesis techniques. The layered metal-rich chalcogenides Ta 2 (Se,Te), Ta 2 (S,Se), Ta 9 (S,Se) 4 , Ta 5 (S,Se) 2 , and (M,Ta) 5 S 2 (M  Mo, W) were uncovered and characterized by means of X-ray diffraction. The phase relations in the tantalum-rich section of the TaSSe system were unravelled and the homogeneity ranges of five sulfide selenides were delimited. Electrical resistivities of polycrystalline single phase samples were measured within the range 15 to 320 K. The structures of the phases represent specific tetragonal order variants of the b.c.c.-structure of the elemental metals. Two-dimensional fragments of b.c.c.-tantalum are sandwiched by layers of chalcogen Q, thus forming Ta 4 O 2 or (M,Ta) 5 O 2 slabs as found in Ta 2 Se or (Nb,Ta) 5 S 2 . The cohesion of the slabs is, in accordance with the results of electronic band structure calculations (EH approximation) for Ta 2 Se, provided by van der Waals interactions. The new structure type of Ta 9 (S,Se) 4 results from a coherent chemical 1·1 intergrowth of Ta 4 O 2 and Ta 5 Q 2 . The structure was determined from X-ray intensities of a polycrystalline sample. The structural changes associated with the replacement of selenium by tellurium in Ta 2 Se-like Ta 2 Se 0.06 Te 0.34 were studied in detail on the basis of a precise single crystal structure analysis.

Synthesis and crystal structure of Hf2Te

Abstract Hf 2 Te was prepared by arc-melting appropriate compressed mixtures of HfTe 2 and hafnium. The structure of the new hafnium-rich telluride was determined by means of powder diffractometry, R B =0.0565. Hf 2 Te adopts a Nb 2 Se-type structure, monoclinic, space group C2/m, Z=8 , Pearson symbol mS24, a =1516.3(2) pm, b =370.26(4) pm, c =1006.4(1) pm, β=90.50(1)°. The structure comprises pairwise interpenetrating columns of trans-face-shared centred Hf 9 cuboids. The columns form puckered slabs which are interconnected by homo- and heteronuclear bonds. Tellurium atoms are one-sidedly coordinated by six hafnium atoms. The coordination configurations about the two distinct tellurium atoms represent Hf 6 Te hypho-fragments of a tri-capped trigonal prismatic Hf 9 Te closo-cluster. Characteristic features of four other M 2 Q structure types are elucidated. The valence electron concentration available for metal-metal bonding together with the cohesive energy of the respective metal are found to control the formation of the various M 2 Q metal partial structures.

Hexagonal approximants of a dodecagonal tantalum telluride — the crystal structure of Ta21Te13

Abstract The structure of a hexagonal approximant of the dodecagonal tantalum telluride dd-Ta 1.6 Te was studied by means of high resolution transmission electron microscopy and electron diffraction. A hierarchical building principle underlying the structures of two other approximants, Ta 97 Te 60 and Ta 181 Te 112 , was applied in order to deduce the crystal structure of the approximant with highest tellurium content, Ta 21 Te 13 .

Zr6STe2 – ein zirconiumreiches Sulfidtellurid mit einer Zr3Te-Teilstruktur vom Re3B-Typ

Zr6STe2 ist durch Reduktion eines Gemenges von ZrTe2 und ZrS2 mit Zirconium in gasdicht verschweisten Tantaltiegeln bei 1520 K zuganglich. Die raumlich gemittelte Kristallstruktur von Zr6STe2 lasst sich in der orthorhombischen Raumgruppe Cmcm beschreiben: a = 377.81(4), b = 1156.4(1), c = 887.96(8), Z = 2, Pearson-Symbol oC18, 320 Reflexe (I > 2σ(I)), 22 Variablen, Rw(I) = 0.088. Zr6STe2 bildet einen eigenen Strukturtyp, der sich struktursystematisch als teilweise aufgefullter Re3B-Typ einordnen lasst. Die Telluratome sind dreifach-uberdacht, verzerrt trigonal-prismatisch von neun Zirconiumatomen umgeben. Die Zr9Te-Tetrakaidekaeder sind uber gemeinsame trigonale Prismenflachen (langs [100]), Kanten (etwa parallel [001]) und Spitzen (langs [010]) zu einem dreidimensionalen Tetrakaidekaedernetzwerk [Zr9/3Te] verknupft, zu dessen Stabilitat homonukleare Zr–Zr-Bindungswechselwirkungen masgeblich beitragen. Zwischen den Tetrakaidekaedern befinden sich Zr6-Oktaeder, die uber Kanten (langs [100]) und Spitzen (langs [001]) zu Schichten verknupft sind. Infolge einer Verzerrung der Struktur weitet sich die Halfte der Oktaeder einer Schicht soweit auf, dass die Schwefelatome Platz darin finden. Den Beugungsbefunden nach erfolgt die Ausordnung der Leerstellen in benachbarten Oktaederschichten unabhangig voneinander. Zr6STe2 – a Zirconium-rich Sulfide Telluride with a Zr3Te Partial Structure of the Re3B Type Zr6STe2 is accessible through the reduction of a mixture of ZrTe2 and ZrS2 with zirconium in fused tantalum tubes at 1520 K. The spatially averaged crystal structure of Zr6STe2 is described in the space group Cmcm, a = 377.81(4), b = 1156.4(1), c = 887.96(8), Z = 2, Pearson symbol oC18, 320 reflexions (I > 2σ(I)), 22 variables, Rw(I) = 0.088. Zr6STe2 crystallizes in a unique structure type, which can be seen as a filled Re3B type structure. The tellurium atoms are surrounded by nine zirconium atoms situated at the vertices of a distorted, tricapped trigonal prism. The Zr9Te tetrakaidecahedra are connected by common triangular prism faces parallel [100], edges approximately along [001] and common vertices along [010], thus forming a three-dimensional tetrakaidecahedral network [Zr9/3Te], which is decisively stabilized by homonuclear Zr–Zr-interactions. The tetrakaidecahedra are arranged in such a way, that Zr6 octahedra occur. The octahedra are arranged into layers by sharing edges parallel [100] and vertices along [001]. As a result of a distortion of the structure, every second octahedron is expanded to such an extent as to be able to smoothly accommodate sulfur atoms. According to the modulation of the diffraction intensities, the vacancy ordering in adjacent layers of octahedra occurs independently of each other.

Periodic and aperiodic arrangements of dodecagonal (Ta, V)151Te74 clusters studied by transmission electron microscopy: The method’s merits and limitations

A series of metal-rich tellurides in the ternary system Ta/V/Te crystallizes in closely related structures representing different arrangements of a basic disc-like (Ta, V) 151 Te 74 cluster. The centres of these clusters define various periodic or aperiodic square-triangle tilings on a length scale of about 2 nm. The structure of the approximant Ta 83 V 14 Te 60 with its underlying square tiling has been studied under various imaging conditions by high-resolution transmission electron microscopy and complementary image simulations. At a large defocus e-180 nm, a (Ta, V) 151 Te 74 cluster is represented by a bright dot surrounded by 12 others forming a dodecagon. The application of this selective imaging technique allows us to determine the tilings, i.e., the cluster arrangements, of new approximants and of quasicrystalline phases almost unambiguously. More complex contrast features as observed in some regions of the sample and the streaks turning up in reciprocal space are interpreted as stacking faults along the pseudo-12-fold axis.