Biomedical subjects

Walter Steurer

Publications and source records attributed to Walter Steurer.

3 recordsLinked to original sources

PubMed2004

Structural phase transitions from and to the quasicrystalline state.

Phase transitions from and to the quasicrystalline state show a typical signature due to some structural peculiarities. Both quasicrystals and most of their translationally periodic transformation products, the approximants, consist of the same basic structural units ('clusters'). This is the cause of low-energy interfaces between the newly forming phase and the parent phase. It is also the origin of the rather high stability of the frequently resulting orientationally twinned nanodomain structures. Owing to topological incompatibilities between quasiperiodic and periodic structures, purely displacive phase transitions are impossible. Diffusion of a significant fraction of atoms, at least on the scale of the cluster diameters, always has to take place. Locally similar icosahedral structural ordering between parent phase and nucleating phase is also responsible for the frequently occurring formation of icosahedral quasicrystals from undercooled liquid alloys or during devitrification of metallic glasses. The different types of experimentally observed phase transitions are discussed, from amorphous to quasicrystalline, quasicrystalline to ordered/disordered quasicrystalline and quasicrystalline to crystalline as a function of temperature, pressure, irradiation and high-energy ball milling, respectively.

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PubMed2002

General periodic average structures of decagonal quasicrystals.

The concept of periodic average structure is mutated from the theory of incommensurately modulated structures. For quasicrystals, this concept (up to now explored only in few cases) is becoming increasingly useful to understand their properties and to interpret some important structural features. The peculiar property of quasicrystals is that they admit not one but many (infinite) possible different average structures. Few of them, however, will be meaningful. Here are given a simple method (based on reciprocal space) for generating all the possible periodic average structures of decagonal quasicrystals and some new ideas about their meaning. By this method, the most significant average structures can be recognized from the diffraction pattern.

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PubMed2001

Structure solution of the basic decagonal Al-Co-Ni phase by the atomic surfaces modelling method.

The atomic surfaces modelling technique has been used to solve the structure of the basic Ni-rich Al-Co-Ni decagonal phase. Formula Al70.6Co6.7Ni22.7, space group P10, five-dimensional unit-cell parameters: d1 = d4 = 4.752 (3) A, d2 = d3 = 3.360 (2) A, d5 = 8.1710 (2) A; alpha12 = alpha34 = 69.295 degrees, alpha13 = alpha24 = 45 degrees, alpha14 = 41.410 degrees, alpha23 = alphai5 = 90 degrees (i = 1-4), V = 291.2 (7) A5; D(x) = 3.887 Mg x m(-3). Refinement based on /F/; 2767 unique reflections (/F/ > 0), 749 parameters, R = 0.17, wR = 0.06. Describing the structure of quasicrystals embedded in n-dimensional superspace in principle takes advantage of n-dimensional periodicity to select the minimal set of degrees of freedom for the structure. The method of modelling of the atomic surfaces yielded the first fully detailed structure solution of this phase. Comparison with numerous former, less accurate models confirms several features already derived, but adds a new essential insight of the structure and its complexity. The atoms fill the space forming recurrent structure motifs, which we will (generically) refer to as clusters. However, no unique cluster exists, although differences are small. Each cluster shows a high degree of structural disorder. This gives rise to a large configurational entropy, as much as expected in a phase which is stable at high temperature. On the other side, the cluster spatial arrangement is perfectly quasiperiodic. These considerations, corroborated by analysis of the structural relationship with neighbouring periodic phases, strongly suggest the existence of a non-local, long-range interaction term in the total energy which may be essential to the stability.

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