The Co–Ni distribution in decagonal Al_69.7(4)Co_10.0(4)Ni_20.3(4)

Abstract

Abstract. The Co--Ni distribution in d-Al 69.7(4) Co 10.0(4) Ni 20.3(4) was investigated based on X-ray and neutron diffraction data. The structure was modelled in higher dimensional space using the 'charge-flipping' and 'low-density elimination' methods and it was quantitatively refined in three-dimensional space employing a pseudo-approximant approach. In higher-dimensional description, the Co atoms are found at the centre of one of the two symmetry independent occupation domains, enclosed by regions mainly occupied by Ni. The other occupation domain is mostly occupied by Al. In physical space Co atoms are located in the centres of small Al pentagons and form pentagonal units, which are arranged in decagonal rings. On these sites Co is partly substituted by Ni, while all other transition metal sites are occupied by Ni and to a minor degree by Al. The fraction of Co found on transition metal sites decreases with decreasing Co--Co distances, whereby Co is replaced by Ni. IntroductionDecagonal Al--Co--Ni certainly belongs to the best-characterized quasicrystalline systems (for a review see Steurer, 2004). Despite its fundamental importance for the understanding of crystal-chemical and physical properties, however, only little is known about the chemical ordering of Co and Ni on transition metal (TM) sites. This may be explained by the fact that the most common structure determination techniques, i.e. X-ray diffraction and electron microscopy, provide almost no contrast between Co and Ni. Since only little experience regarding qualitative and quantitative reliability of CF/LDE methods is available, we cross-checked the results by employing a 3D approach where the quasicrystal structure is transformed into a virtual approximant by appropriate shearing of the nD lattice. The obtained pseudo-approximant was refined quantitatively using conventional crystallographic tools. Experimental and data reductionCrystals were grown by the Czochralski method by slowly pulling (0.15 mm/h) from an incongruent melt of initial composition Al 77 Co 6 Ni 17 at a temperature of about 1050 C. Use was made of [00001]-oriented native seeds that were prepared from a former crystal. The composition was analyzed from a sample taken from the neighbourhood of the crystals using a Cameca SX50 microprobe (15 kV, 10 nA). The experimental density of the sample was found to be 4190(2) kg/m 3 . The neutron diffraction experiment was done at the reactor FRM-II, Munich, Germany using the instrument RESI (Pedersen et al. 2006, Pedersen et al., 2007 at a wavelength of 1.042 A (Cu-422 monochromator, rotation images 0.5 , 1000 sec/image). This instrument is placed at the end of a neutron guide, providing very low background. It is equipped with a neutron sensitive image plate, providing a large coverage in reciprocal space, which is essential for quasicrystal work. The X-ray scattering experiment was done with an Oxford Diffraction diffractometer ('Onyx'-CCD detector, Mo--K a radiation, graphite monochromator). The diffraction pattern shows presence of ...

Structure of a crystalline approximant related to Al–Co–Ni decagonal quasicrystals studied by spherical aberration (Cs)-corrected scanning transmission electron microscopy and atomic-resolution energy dispersive X-ray spectroscopy

“…Also, the present chemical ordering, in which TM atoms forming the 0.76 nm pentagonal tiling and ones in MSs located at special positions in the 0.76 nm pentagonal tiling are different kinds of TM atoms, supports the justification of the new models. It should be noticed finally that Co atom positions determined from the present study are different from those proposed by X-ray and neutron diffraction in an Al 69.7 Co 10.0 Ni 20.3 DQCs [24]. In the result by the diffraction analysis, Co atoms are mainly located at vertices of pentagonal tiling with a bond length of 1.2 nm.…”

Structure of decagonal Al–Ni–Rh

“…A similar coordination has been reported for Co in o-Co 4 Al 13 (Grin et al, 1994). The similarity of Rh and Co is also recalled from EXAFS (Zaharko et al, 2001) and neutron diffraction studies (Weber et al, 2008) made on Ni-rich d-Al-Ni-Co for which both the techniques confirm Co pentagonal coordination with Al. The other two types of clusters commonly met in decagonal quasicrystals (Steurer, 2004) are identified around partly occupied sites at the center (Fig.…”

“…In contrast to d-Al-Ni-Co, where the close values for X-ray atomic scattering factors [f Ni (0) = 28, f Co (0) = 27] make those elements difficult to distinguish by X-ray diffraction, the distribution of the TM atoms can be easily determined in the case of d-Al-Ni-Rh [f Rh (0) = 45]. While the Ni/Co distribution in d-Al-Ni-Co had to be studied by other techniques such as neutron scattering (Weber et al, 2008) and polarized EXAFS (Zaharko et al, 2001) requiring large single crystals, in the case of d-Al-Ni-Rh this can be done on small crystals by X-ray diffraction.…”

Ab initiostructure solution by iterative phase-retrieval methods: performance tests on charge flipping and low-density elimination

“…Averaging of several successful CF or LDE runs can drastically improve the quality of structure solutions (Wu et al, 2004;Katrych et al, 2007;Weber et al, 2008) and may be done either in real space, by averaging the electron densities, or in reciprocal space, by averaging the resulting structure factors of n single runs with different random number generator seeds by applying…”

“…CF and LDE have proven particularly useful in the field of modulated structures and quasicrystals (QCs). In their higher-dimensional description, where no powerful standard structure solution methods were available before, CF and LDE work well and provide excellent structure solutions Palatinus, 2004;Katrych et al, 2007;Weber et al, 2008). For periodic structures it was shown that the CF algorithm was able to solve complex metallic structures with giant unit cells with more than 23 000 atoms (Weber et al, 2009), as well as several protein structures with diffraction data of atomic resolution (Dumas & van der Lee, 2008).…”