## Quasicrystals, explain those terms please!

What is:

-P-type icosahedral?

-Tsai-type quasicrystal?

-1/1 cubic approximant?

-six-dimensional lattice parameter a6D?

-why has (this is example) powder X-ray diffraction spectrum just 2 high peaks (at some angles), and why are they called 111112 and 011202 (underlined line should be above letters) and what does it mean?

Many thanks..
 Icosahedral means that the quasicrystal has the symmetry of an icosahedron, i.e. of a soccer ball. P-type probably means primitive (as opposed to I=body centered and F=face centered, not sufe if F-type QC exist) Tsai discovered a different type of QC that has 10-fold symmetry in one plane and periodic in the direction perpendicular to the plane (IIRC). These are called decagonal QC. Approximants are periodic crystals with similar local structure to a quasicrystal. The Bragg reflections of QC can be indexed in 6-dimensional reciprocal space. Hence you get 6 Miller indices rather than the usual 3 (HKL).

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