Crystal Structure of Niobium Aluminium Cobalt, Nb (Al0. 62Co0. 38) 2

Z. Kristallogr. NCS 225 (2010) 621-622 / DOI 10.1524/ncrs.2010.0271 621 © by Oldenbourg Wissenschaftsverlag, München

Crystal structure of niobium aluminium cobalt, Nb(Al0.62Co0.38)2

Alexander Kerkau* and Guido Kreiner

Max-Planck-Institut für Chemische Physik fester Stoffe, Nöthnitzer Str. 40, 01187 Dresden, Germany

Received August 11, 2010, accepted and available on-line September 29, 2010; CSD no. 710054

the sample is single phase material. The composition found by four EDXS measurements (20 × 20 mm, 25 kV, standardless ZAF correction) corresponds to Nb0.99Al1.25Co0.76.Elemental analysis (nominal Nb, 33.3 at. %; Al, 41.3 at. %; Co, 25.3 at. %) found by ICP-OES: Nb, 33.1(2) at. %; Al, 41.1(2) at. %; Co, 25.8(2) at. %.

Discussion The title compound is ahexagonal Laves phase of C14 structure type [2]. The Nb atoms occupy the 4f site of the crystal structure. Al and Co atoms randomly occupy the M1 (6h)and M2 (2a)sites, thereby forming Kagomé (M1) and triangular (M2) layers alternately stacked parallel to the ab-plane. The Matoms form M4 tetrahedra which are joined alternately point-to-point (M2 sites) and base-to-base (M1 sites) forming infinite chains along [001]. These chains are linked together by vertex sharing in the ab- plane, forming large truncated M12 tetrahedra. All four hexagonal faces of these polyhedra are capped by Nb atoms. The Nb atoms occupying the center of these polyhedra have the coordination number 16 (Nb4M12). The atoms M1 and M2 are coordinated by two different distorted icosahedra (CN12, Nb6M6). The crystal structure is significantly distorted compared with an idealized crystal structure based on ahard sphere model. The distortion is manifested by the deviation of c/a =1.6164 from the ideal ratio (1.633) and by that of the fractional atomic coordinates x(M1) and z(Nb) from the idealized parameters 1/6 and 9/16, respectively. The deviation in x(M1) causes adistortion of the Kagomé layers Abstract in such away that M13 triangles which are capped by the M2 at- Al1.23Co0.77Nb, hexagonal, P63/mmc (no. 194), oms are expanding (d(M1—M1) =2.5430(4) Å), while the un- a =4.9935(1) Å, c =8.0713(2) Å, V =174.3 Å3, Z =4, capped triangles are contracting (d(M1—M1) =2.4505(3) Å). 2 Rgt(F) =0.019, wRref(F ) =0.048, T =293 K. The Nb net is slightly distorted with d(Nb—Nb) =3.0618(2) Å (3×)and 3.0048(3) Å (1×)). The interatomic distances d(Nb—M) Source of material range from 2.9015(2) Å to 2.9287(2) Å.Inthe title compound Asingle crystal of irregular shape and metallic luster was ob- (Nb(Al1–xCox)2 (x =0.38(1)), quenched from T=1423 K) Al at- tained from asample of nominal composition NbAl1.24Co0.76. oms occupy preferentially the Kagomé layer positions while Co The sample was prepared by arc-melting amixture of the ele- atoms occupy preferentially the triangular layer positions. The ments (Al, 99.99 %, Chempur; Co, 99.995 %, GfE-MIR; Nb, site occupation factor is 0.475(5) for Co at the 2a site compared 99.9 %Starck) on awater cooled copper hearth in Ar atmosphere. with 0.38 for arandom occupation. For the following heat treatment at 1423 Kfor 30 days the sample was weld-sealed in aniobium ampoule, which was in turn jack- Table 1. Data collection and handling. eted in an evacuated and sealed fused silica tube. Finally, the sample was quenched in cold water. Crystal: metallic light grey, irregular shape, size 0.031 × 0.037 × 0.082 mm Wavelength: Ag Ka radiation (0.56088 Å) Experimental details −1 The unit cell parameters were determined with the software μ:251.45 cm Diffractometer, scan mode: Rigaku R-Axis Spider, w WinXPow [1] from least-squares refinements of the 2q values of 2θmax:84.6° 26 reflections (Co Ka1 radiation, l =1.788965 Å)inthe range 20° N(hkl)measured, N(hkl)unique:3367, 521 <2q<100° using LaB6 powder (NIST SRM660a, a =4.1569(1) Criterion for Iobs, N(hkl)gt: Iobs >2s(Iobs),453 Å)asaninternal standard. Metallographic examinations were N(param)refined:12 Programs: WinXPow [1], Jana 2006 [3], carried out on aflat polished sample surface using optical and DIAMOND [4] scanning electron microscopy. The investigations confirm that ______*Correspondence author (e-mail: [email protected]) 622 Nb(Al0.62Co0.38)2

Table 2. Atomic coordinates and displacement parameters (in Å2).

Atom Site xyzU11 U22 U33 U12 U13 U23

* 1 M(1) 6h 0.16975(3) 2x 4 0.0071(1) 0.0040(1) 0.0068(1) ½U22 00 1 2 Nb(1) 4f 3 3 0.56386(2) 0.00470(5) U11 0.00520(6) ½U11 00 * M(2) 2a 0000.0076(2) U11 0.0045(2) ½U11 00

*Occupations M1 =Al0.645(5)Co0.355;M2=Al0.525(5)Co0.475.

Acknowledgments. The authors would like to thank Dr. H. Borrmann for the single crystal data collection, P. Scheppan and Dr. U. Burkhardt for metallographic examinations and EDXS measurements as well as S. Hückmann for collecting the powder diffraction data.

References

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