Bernd Harbrecht

Al69Ta39 - a new variant of a face-centred cubic giant cell structure

Abstract The structure of Al 69 Ta 39 , cF432, a = 1915.3(10) pm, F 43 m , Z =4, 38 variables, was determined from 523 X-ray intensities of a single crystal with I > 3 σ ( I ) and refined to wR ( I ) = 0.050. Al 69 Ta 39 represents a new structure type composed of four compositionally and three topologically distinct metal atom clusters as present in α-Mn-, γ-brass- and Cd 45 Sm 11 -type structures. The compositions of the clusters range from Al 10 Ta 16 to Al 23 Ta. Deviations from the crystallographic composition Al 72 Ta 39 are due to 50% occupation of one Al site. Eleven out of 17 primary coordination polyhedra are of the Frank-Kasper type. The remaining polyhedra can also be considered as being exclusively terminated by triangulated faces. They are either of b.c.c. type or they combine features of both Frank-Kasper and b.c.c. coordinations.

Phase relations in the AlTa system: on the translational symmetries of Al3Ta2 and AlTa

The Alue5f8Ta system was reinvestigated preparatively in the range 0.15 < xTa < 0.85. The translational symmetries of two phases referred to as Al3Ta2 and AlTa were determined by means of X-ray powder diffractometry in combination with electron diffraction. The lattice parameters of Al3Ta2 (f.c.c., cF) and AlTa (primitive monoclinic, mP) are a = 1915.80(3) pm and a = 1486.5(3) pm, b = 986.3(2) pm, c = 986.2(2) pm and β = 99.98(2)° respectively. The phase relations are discussed and previously reported data are critically reviewed.

Synthesis of tantalum tellurides : the crystal structure of Ta2Te3

Abstract Ta2Te3 was prepared by reducing TaTe2 with tantalum at 1350 K in a sealed molybdenum crucible. Ta2Te3 disproportionates above 1420 K yielding the ditelluride and as yet unknown Ta6Te5. The stability limit for the substitutional sesquitellurides NbxTa2−xTe3 is reached at x ≈ 1 . The novel layered-type structure of Ta2Te3 (C 2/m, N=4; a=2049.5(3) pm, b=349.96(4) pm, c=1223.7(2) pm, β=143.74(1)0 (Guinier), N(I0)=762 with I0>2σ(I0), 32 variables, R=0.032) can be considered a stuffed variant of a molybdenite structure type. It consists of corrugated layers ∞ 2 ( Te-Ta 4 3 - Te ) which, according to the shortest interlayer contacts Teue5f8Te (372.9 pm), order via van der Waals interactions. Extended homonuclear bonding regions (297.1 pm ≤ dTaue5f8Ta≤ 309.7 pm) within the metal layers contribute to the stability of the metallic sesquitelluride.

Phase relations in the AlTa system: on the translational symmetries of a triclinic structure and a new hexagonal giant cell structure

Abstract The translational symmetries of two structurally complex phases of the Alue5f8Ta system were determined by means of X-ray and electron diffraction methods. The thermal stability ranges of the phases were delimited and the phase relations in the medium range of the Alue5f8Ta system were unravelled: Al u Ta ( u ≈ 1.5), triclinic, aP ∗ , a = 1074.8(3) pm, b = 1107.8(4) pm, c = 1043.3(4) pm, α = 90.45(2)°, β = 97.60(2)° and γ = 63.19(2)°, is stable below about 1500 K and decomposes in the solid state to yield entropically stabilized Al 69 Ta 39 (cF432) and Al v Ta ( ν ≈ 1.4), hexagonal, hP ∗ , a = 3214(2) pm and c = 1341(1) pm. Al v Ta itself transforms in a peritectoid reaction above about 1620 K into Al 69 Ta 39 and Al w Ta ( w ≈ 1.0) of monoclinic symmetry.

Synthesis and crystal structure of Zr3Te

Abstract Zr 3 Te was prepared by arc-melting a compressed mixture of ZrTe 2 and zirconium and annealing of the sample at 1650 K in a sealed tantalum ampoule. The structure was determined by means of X-ray powder diffractometry and confirmed by a single-crystal structure analysis: a = 1132.7(1) pm, c = 563.64(7) pm, I - 4, Z = 4, tI 32, 1638 reflections, 38 variables, R F = 0.022. Zr 3 Te adopts a Ni 3 P-type structure. The structure comprises tetrakaidecahedral Zr 9 Te clusters sharing two faces, five edges and four vertices with other units of the same type. The specific orientation of the clusters affords a structure in which nearly all atoms are tetrahedrally close-packed.

Condensed tetrakaidecahedral clusters. The crystal structure of NiTa8Se8

Abstract The ternary selenides MTa 8 Se 8 (M ue5fc Co, Ni) were prepared from compacted, pre-reacted mixtures of the elemental components. Coarse crystalline NiTa 8 Se 8 was obtained from reactions performed in sealed molybdenum crucibles at 1620 K for 2 days with the use of iodine as a transport agent. The selenides crystallize in space group type Pnma with four formula units per unit cell. The lattice parameters are a = 2241.3(1) pm, b = 344.22(3) pm, c = 1684.1(1) pm and a = 2247.4(2) pm, b = 342.72(3) pm, c = 1690.3(1) pm for NiTa 8 Se 8 and CoTa 8 Se 8 respectively. The structure of NiTa 8 Se 8 was determined from X-ray intensities of a single crystal and refined to R ( I ) = 0.040 for 1411 structure factors and 104 variables. The highly porous intermetallic partial structure contains columns of face-shared, Ni-centred NiTa 9 tetrakaidecahedra which are covered by selenium atoms exhibiting a one-sided coordination with three, four or five tantalum atoms as nearest neighbours. NiTa 8 Se 8 is a metallic conductor.

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 .

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.

τ-Al2.9Ta2.7V1.4, a new type of pentagonal antiprismatic columnar structure

Abstract The so-called τ-phase occurring in the ternary system Alue5f8Taue5f8V was prepared at 1273 K from bcc-Al 35–48 Ta 32–42 V 17–30 solid solutions which themselves were obtained by arc-melting compressed mixtures of the elements. The structure of τ-Al 2.9 Ta 2.7 V 1.4 was determined by means of X-ray powder diffractometry: a = 509.94(2) pm; b = 1198.69(5) pm; c = 732.25(3) pm; space group, Cmca ; Z = 4; Pearson symbol: oC28; wR P = 0.040. The structure is made up of four crystallographically distinct metal atoms. Two sites are statistically occupied. All atoms have Frank-Kasper type coordination with 12, 15 and 16 nearest neighbours. The relative frequency of the distinct configurations is 3:2:2. τ-Al 2.9 Ta 2.7 V 1.4 forms a new type of pentagonal antiprismatic columnar structure based on a tetrahedral close-packed arrangement of the atoms.

The crystal structure of Cr1.73Mo6S8

Single crystals of Cr1.73Mo6S8 with triclinic symmetry were grown from a melt of an appropriate mixture of Mo2S3, molybdenum and chromium. A single crystal structure determination was performed in space group P1 (Ci1, No. 2; R(F0) = 0.032) including 2870 reflections with No. 2; R(F0) = 0.032) including 2870 reflections with F02>2δ(F02). The lattice parameters are: a = 652.2(2) pm, b = 649.7(2) pm, c = 644.9(1) pm, a = 94.68(2)°, β = 90.70(2)°, γ = 97.91(2)δ (Guinier data). The structure is isopointal with Fe2Mo6S8. The chromium atoms are located in parallel channels [010] Fe2Mo6S8. The chromium atoms are located in parallel channels [010] which are surrounded by Mo6S8 clusters linked via six intercluster Mo-S contacts. The coordination about the chromium atoms and the Cr-S distances are in accordance with the valence state +2 for chromium. Despite the low rotational symmetry the refinements indicate some disorder of the chromium atoms which is not resolved by releasing the centre of symmetry. In addition, indications of a lowering of translational symmetry do not exist.