K. Hradil

On the one-dimensional 8 Å periodic superstructure in decagonal phases

Abstract Diffuse layers which are observed in different decagonal quasicrystals perpendicular to the unique ‘periodic’ axis are explained by one-dimensional (1D) columns of icosahedral A1 clusters along this axis. From results of X-ray and purely elastic neutron diffraction experiments we give a qualitative explanation by basically off-layer atomic displacements which are grossly uncorrelated in different columnar clusters. A reversible temperature behaviour of the 1D superstructure which vanishes at high temperatures can be understood in the framework of this interpretation. Diffuse intensity modulations within the diffuse layers are discussed in terms of atomic displacements and chemical ordering within the icosahedral clusters which are the basic structural units of the columns. Computer simulations of different columnar models are carried out, their Fourier transforms are compared with the observed patterns.

Periodic superorder and disorder of decagonal Al-Ni-Co and Al-Co-Cu(-Si) phases

Commensurate satellite reflections in diffraction patterns of decagonal Al-Ni-Co and Al-Cu-Co phases indicate a periodic superorder on the quasiperiodic substructure. From the intensity characteristics of these satellites a box-like modulation function is derived which may be visualized in different ways including approximant phases. An interpretation by fine-scale periodic lamella microdomains matches closely high-resolution electron microscopy observations and characteristic structural features of the average structure.

Superordering in Ni-rich and Ni-poor decagonal Al-Co-Ni phases

Abstract Single-crystal X-ray diffraction was carried out on a decagonal Ni-rich phase in the Al-Co-Ni system with a nominal composition of Al72.5 Co11Ni16.5. Apart from sharp Bragg reflections, satellite scattering was observed in Al72.5 Co11Ni16.5 which is different from the satellite scattering observed in a Ni-poor phase with nominal composition Al72.5 Co16.5Ni11. The two types of superorder scattering are interpreted in an unified way by periodic order superimposed on the quasiperiodic averaged substructure. This periodic regimes have a limited correlation length (short-range ordering). While the modulation period stays almost constant over a wide range of compositions in these decagonal alloys, the modulation vector is rotated by π/10 if passing from Ni-rich (S satellites) to Ni-poor (C satellites) compositions. Arguments are given in favour of this interpretation which is also supported by computer simulations.

Transient ordering states in decagonal Al-Co-Ni and Al-Cu-Co-Si phases

Abstract X-ray diffraction patterns of decagonal Al70Co15Ni15, Al72Ni12Co16 and Al62Cu20Co15Si3 show, apart from ‘sharp’ reflections, prominent diffuse maxima and diffuse streaks. From an analysis of high-resolution synchrotron data, arguments are given in favour of a more uniform interpretations. of a more uniform interpreatation of all these phenomena in terms of linear phason strain and phason fluctuations. Thus structural modifications, including one-dimensional quasicrystalline domains and periodic domains (approximants) can be conveniently described as transient ordering states, depending on temperature and composition

High resolution X-ray and neutron diffraction of super- and disorder in decagonal Al–Co–Ni

Abstract Synchrotron measurements at DESY and ESRF of decagonal quasicrystals with the nominal composition Al 70 Co 15 Ni 15 and Al 72 Co 16 Ni 12 , show that Bragg reflections with high |H i ∗ | values are surrounded by up to four weak subsidiary peaks. The positions of these additional peaks depend solely on the position of the Bragg reflection in the reciprocal perpendicular subspace and can be explained by a linear phason strain model. This model sheds new light on the current superorder models [K. Hradil et al. Phil. Mag. A 79 (1999) 1963] and is a further step towards an understanding of the real structures of decagonal quasicrystals.

Neutron and X-ray investigation of disordered quasicrystals

Abstract Disorder in decagonal quasicrystals is studied by “contrasting methods”, i.e. by joint analysis of X-ray and neutron data and by use of the anomalous dispersion method, to learn about the relative influence of the transition metals. First experimental results with decagonal Al–Ni–Co samples of different origin are discussed qualitatively. A comparative study of Patterson maps from neutron and X-ray Bragg data of d-Al72.5Ni16.5Co11 gives direct evidence for particular sites occupied by Co.