The self-hosting structure of β-Ta (original) (raw)
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The self-hosting structure of beta-Ta
Acta Crystallographica Section B Structural Science
Using electrodeposition from a bath of molten fluorides, single crystals of tetragonal beta-tantalum have been obtained for the first time at normal pressure. The unit-cell parameters are a = 10.211 (3), c = 5.3064 (10) Angstrom, space group P (4) over bar2(1)m. The beta-Ta structure belongs to the sigma-type Frank-Kasper structures which are typical for binary intermetallic compounds and beta-U. In comparison to the sigma-type, additional intercalated Ta atoms (population factor similar to 0.01) have been detected between the atoms located in the channels of the structure. The shorter interatomic distances observed between the channel atoms in comparison with the atoms of the framework justify the 'self-hosting' characteristic. beta-Ta exhibits common features with the complex tetragonal structures of the high-pressure phases for the elements Rb, Ba, Sr, Bi and Sb.
Acta Crystallographica Section B Structural Science, 2000
A sample of Ta 2 O 5 , ditantalum pentaoxide, heat-treated in à toroid'-type high-pressure chamber at P = 8 GPa and T = 1470 K, was studied by X-ray powder diffraction and highresolution transmission electron microscopy (HRTEM). Two high-pressure modi®cations of Ta 2 O 5 , isostructural with B-Nb 2 O 5 and Z-Nb 2 O 5 , were identi®ed from the X-ray powder pattern. Both structures were re®ned by the Rietveld method from the X-ray diffraction data: B- 2 O 5 , a = 12.7853 (4), b = 4.8537 (1), c = 5.5276 (2) A Ê , = 104.264 (2) , V = 332.45 A Ê 3 , Z = 4, space group C2/c; Z-Ta 2 O 5 , a = 5.2252 (1), b = 4.6991 (1), c = 5.8534 (1) A Ê , = 108.200 (2) , V = 136.53 A Ê 3 , Z = 2, space group C2. The Z-Ta 2 O 5 modi®cation is new. The Ta atoms are six-coordinated in B-Ta 2 O 5 and seven-coordinated in Z-Ta 2 O 5 .
The apparent'five-fold'nature of large T 2 (AI 6 Li 3 Cu) crystals
Metallurgical and Materials Transactions A, 1988
Large T2 (A16Li3Cu) crystals which display apparent five-fold symmetry have been studied using several different electron microscopy techniques. Electron microprobe X-ray analysis was used to determine the phases present in two different alloys cast with bulk compositions close to stoichiometric T2. Convergent beam electron diffraction (CBED) of these crystals indicates that they do not display five-fold rotational symmetry, and TEM images and selected area diffraction from thin foil specimens indicate a microcrystalline or twinned structure is responsible for the apparent fivefold symmetry of T2. Microdiffraction patterns obtained from the thinnest regions of the crystals are consistent with a cubic structure having a lattice parameter of ~0.40 nm.
The formation of ordered ω-related phases in alloys of composition Ti4Al3Nb
Acta Metallurgica et Materialia, 1990
During cooling of an alloy of composition Ti4A13Nb from a B2 phase field above 1100°C, a metastable trigonal (P~ml) to-related phase, designated co", forms along with small amounts of D0j9 and LI 0 phases. The to" phase exhibits partial collapse of 111 planes and reordering relative to its B2 parent. An apparently equilibrium low temperature phase with the B82 structure was found after 26 days of annealing at 700°C. Both co" and B82 structures were verified by means of transmission electron microscopy and by single crystal X-ray diffraction. The latter permitted detailed analysis of the collapse parameters and site occupancies. The observed transformation path, B2(Pm~m)--*to"(P3ml)---,B82 (P6~/mmc), occurs in two steps. The first involves a subgroup transition during cooling that is primarily displacive with reordering consistent with the trigonal symmetry imposed by the to-collapse. The second involves a supergroup transition during prolonged annealing that is primarily replacive and constitutes a chemical disordering. The direct equilibrium transformation, B2--,B8 v without the formation of an intermediate trigonal phase, can only occur by a reconstructive transformation.
Materials
The formation and crystal structure of the binary Cu3As phase have been re-investigated. Some physical properties were then measured on both single crystal and polycrystalline bulk. Cu3As melts congruently at 835 °C. At room temperature (RT), this compound has been found to crystallize in the hexagonal Cu3P prototype (hP24, P63cm) with lattice parameters: a = 7.1393(1) Å and c = 7.3113(1) Å, rather than in the anti HoH3-type (hP24, P–3c1) as indicated in literature. A small compositional range of 74.0–75.5 at.% Cu (26.0–24.5 at.% As) was found for samples synthesized at 300 and 400 °C; a corresponding slight understoichiometry is found in one out of the four Cu atomic sites, leading to the final refined composition Cu2.882(1)As. The present results disprove a change in the crystal structure above RT actually reported in the phase diagram (from γ’ to γ on heating). Instead, below RT, at T = 243 K (−30 °C), a first-order structural transition to a trigonal low-temperature superstructu...
Convergent beam electron diffraction analysis of the T 1 (Al 2 CuLi) phase
Metall Mater Trans a, 1988
Point and space group analysis of large T~ (AlzCuLi) crystals was performed by convergent beam electron diffraction. The structure of Tj was determined to be hexagonal, possessing a 6/mmm point group and P6/mmm (No. 191) space group. The lattice constants were found to be a ~ 0.497 nm and c = 0.93 nm. This structure is in agreement with an existing model of T~, although discrepancies between the observed and calculated intensities of certain reflections were evident. Electron probe X-ray microanalysis of these T~ crystals indicates the composition is on the copper-rich side of stoichiometric A12CuLi. The slight deviation in the composition of Tl from stoichiometry and the presence of planar defects in the microstructure may account for the discrepancy in the intensity of certain reflections.