S S symmetry Article Icosahedral Polyhedra from D6 Lattice and Danzer’s ABCK Tiling 1 1, 2, Abeer Al-Siyabi , Nazife Ozdes Koca * and Mehmet Koca y 1 Department of Physics, College of Science, Sultan Qaboos University, P.O. Box 36, Al-Khoud, Muscat 123, Oman; [email protected] 2 Department of Physics, Cukurova University, Adana 1380, Turkey; [email protected] * Correspondence: [email protected] Retired. y Received: 10 November 2020; Accepted: 26 November 2020; Published: 30 November 2020 Abstract: It is well known that the point group of the root lattice D6 admits the icosahedral group as a maximal subgroup. The generators of the icosahedral group H3, its roots, and weights are determined in terms of those of D6. Platonic and Archimedean solids possessing icosahedral symmetry have been obtained by projections of the sets of lattice vectors of D6 determined by a pair of integers (m1, m2) in most cases, either both even or both odd. Vertices of the Danzer’s ABCK tetrahedra are determined as the fundamental weights of H3, and it is shown that the inflation of the tiles can be obtained as projections of the lattice vectors characterized by the pair of integers, which are linear combinations of the integers (m1, m2) with coefficients from the Fibonacci sequence. Tiling procedure both for the ABCK tetrahedral and the <ABCK> octahedral tilings in 3D space with icosahedral symmetry H3, and those related transformations in 6D space with D6 symmetry are specified by determining the rotations and translations in 3D and the corresponding group elements in D6. The tetrahedron K constitutes the fundamental region of the icosahedral group and generates the rhombic triacontahedron upon the group action. Properties of “K-polyhedron”, “B-polyhedron”, and “C-polyhedron” generated by the icosahedral group have been discussed. Keywords: lattices; Coxeter–Weyl groups; icosahedral group; projections of polytopes; polyhedra; aperiodic tilings; quasicrystals 1. Introduction Quasicrystallography as an emerging science attracts the interests of many scientists varying from the fields of material science, chemistry, and physics. For a review, see, for instance, the references [1–3]. It is mathematically intriguing, as it requires the aperiodic tiling of the space by some prototiles. There have been several approaches to describe the aperiodicity of the quasicrystallographic space such as the set theoretic techniques, cut-and-project scheme of the higher dimensional lattices, and the intuitive approaches such as the Penrose-like tilings of the space. For a review of these techniques, we propose the reference [4]. There have been two major approaches for the aperiodic tiling of the 3D space with local icosahedral symmetry. One of them is the Socolar–Steinhardt tiles [5] consisting of acute rhombohedron with golden rhombic faces, Bilinski rhombic dodecahedron, rhombic icosahedron, and rhombic triacontahedron. The latter three are constructed with two Ammann tiles of acute and obtuse rhombohedra. Later, it was proved that [6,7] they can be constructed by the Danzer’s ABCK tetrahedral tiles [8,9], and recently, Hann–Socolar–Steinhardt [10] proposed a model of tiling scheme with decorated Ammann tiles. A detailed account of the Danzer ABCK tetrahedral tilings can be found in [11] and in page 231 of the reference [4] where the substitution matrix, its eigenvalues, and the corresponding Symmetry 2020, 12, 1983; doi:10.3390/sym12121983 www.mdpi.com/journal/symmetry Symmetry 2020, 12, 1983 2 of 15 eigenvectors are studied. The right and left eigenvectors of the substitution matrix corresponding to the Perron–Frobenius eigenvalue are well known and will not be repeated here. Ammann rhombohedral and Danzer ABCK tetrahedral tilings are intimately related with the projection of six-dimensional cubic lattice and the root and weight lattices of D6, the point symmetry group of which is of order 256!. See for a review the paper “Modelling of quasicrystals” by Kramer [12] and references therein. Similar work has also been carried out in the reference [13]. Kramer and Andrle [14] have investigated Danzer tiles from the wavelet point of view and their relations with the lattice D6. In what follows, we point out that the icosahedral symmetry requires a subset of the root lattice D6 characterized by a pair of integers (m1, m2) with m1 + m2 = even, which are the coefficients of the orthogonal set of vectors li, (i = 1, 2, ::: , 6). Our approach is different than the cut and project scheme of lattice D6, as will be seen in the sequel. The paper consists of two major parts; the first part deals with the determination of Platonic and Archimedean icosahedral polyhedra by projection of the fundamental weights of the root lattice D6 into 3D space, and the second part employs the technique to determine the images of the Danzer tiles in D6. Inflation of the Danzer tiles are related to a redefinition of the pair of integers (m1, m2) by the Fibonacci sequence. Embeddings of basic tiles in the inflated ones require translations and rotations in 3D space, where the corresponding transformations in 6D space can be easily determined by the technique we have introduced. This technique, which restricts the lattice D6 to its subset, has not been discussed elsewhere. The paper is organized as follows. In Section2, we introduce the root lattice D6, its icosahedral subgroup, and decomposition of its weights in terms of the weights of the icosahedral group H3 leading to the Archimedean polyhedra projected from the D6 lattice. It turns out that the lattice vectors to be projected are determined by a pair of integers (m1 , m2). In Section3, we introduce the ABCK tetrahedral Danzer tiles in terms of the fundamental weights of the icosahedral group H3. Tiling by n 1+ p5 inflation with τ where τ = and n Z is studied in H3 space by prescribing the appropriate 2 2 rotation and translation operators. The corresponding group elements of D6 are determined noting that the pair of integers (m1 , m2) can be expressed as the linear combinations of similar integers with coefficients from Fibonacci sequence. Section4 is devoted for conclusive remarks. 2. Projection of D6 Lattice under the Icosahedral Group H3 and the Archimedean Polyhedra We will use the Coxeter diagrams of D6 and H3 to introduce the basic concepts of the root systems, weights, and the projection technique. A vector of the D6 lattice can be written as a linear combination of the simple roots with integer coefficients: X6 X6 X6 λ = niαi = mili, mi = even, ni, mi Z. (1) i=1 i=1 i=1 2 Here, αi are the simple roots of D6 defined in terms of the orthonormal set of vectors as α = l l + , i = 1, ::: , 5 and α = l + l , and the reflection generators of D act as i i − i 1 6 5 6 6 r : l l + and r : l l . The generators of H can be defined [13] as R = r r , R = r r and i i ! i 1 6 5 ! − 6 3 1 1 5 2 2 4 R3 = r3r6, where the Coxeter element, for example, can be taken as R = R1R2R3. The weights of D6 are given by ! = l , ! = l + l , ! = l + l + l , ! = l + l + l + l , 1 1 2 1 2 3 1 2 3 4 1 2 3 4 (2) = 1 (l + l + l + l + l l ), ! = 1 (l + l + l + l + l + l ). 2 1 2 3 4 5 − 6 6 2 1 2 3 4 5 6 The Voronoi cell of the root lattice D6 is the dual polytope of the root polytope of !2 [15] and determined as the union of the orbits of the weights !1, !5 and !6, which correspond to the holes of the root lattice [16]. If the roots and weights of H3 are defined in two complementary 3D spaces E and k E as βi(β´i) and vi(v´i), (i = 1, 2, 3), respectively, then they can be expressed in terms of the roots and ? weights of D6 as Symmetry 2020, 12, x FOR PEER REVIEW 3 of 16 = ( + ), = ( + ), Symmetry 2020, 12, 1983 √ √ 3 of 15 =β = 1((α++ τα)),, v = = 1 (!(+τω+), ), (3) 1√p2+τ 1 5 1 p2√+τ 1 5 1 1 β2 = (α2 + τα4), v2 = (!2 + τω4), (3) p2+τ p2+τ =β = 1((α++ τα)), v== 1 (!(+τω+). ). 3 √p2+τ 6 3 3 p√2+τ 6 3 1+ p5 ´ √ For the complementary 3D space, replace βi by βi, vi by v´i in (3), and τ = 2 by its algebraic For the complementary 3D space, replace by , by ́ in (3), and = by its 1 p5 1 conjugate σ = − = τ−√. Consequently, Cartan matrix of D6 (Gram matrix in lattice terminology) algebraic conjugate2 =− =−. Consequently, Cartan matrix of (Gram matrix in lattice and its inverse block diagonalize as terminology) and its 2inverse block diagonalize as 3 2 3 2 1 0 0 0 0 2 1 0 0 0 0 6 7 6 7 6 − 7 6 − 7 6 1 2 2 −1 1 0 0 0 0 0 0 07 6 21 −1 2 0τ 00 0 0 0 0 7 6 −⎡ − 7⎤ ⎡6 − − ⎤ 7 6 0−1 1 2 2 −11 0 0 0 0 07 6−10 2τ −2 0 0 0 0 0 0 7 C = 6 ⎢ 7⎥ ⎢6 ⎥ 7 6 ⎢ 0− −1 2− −1 0 07⎥ ⎢6 0 −− 2 0 0 0⎥ 7 =6 0 0 1 2 1 1 7 !→ 6 0 0 0 2 1 0 7 6 ⎢ 0 0− −1 2− −1− −17⎥ ⎢6 0 0 0 2 −1− 0⎥ 7 6 0 0 0 1 2 0 7 6 0 0 0 1 2 σ 7 6 ⎢ 0 0 0− −1 2 07⎥ ⎢6 0 0 0 −1− 2 −−⎥ 7 4 ⎣0 0 0 0 0 0 −11 0 0 2 25⎦ ⎣4 00 0 0 0 0 0 0 −σ 2 2⎦ 5 2 − 3 2 − 3 (4) 2 2 2 2 1 1 2 + τ 2τ2 τ3 0 0 0 (4) 6 7 6 7 6 2222117 26 + 2 2 2 3 0 0 0 7 6 ⎡2 4 4 4 2 2⎤ 7 ⎡6 2τ 4τ 2τ 0 0 0 ⎤ 7 6 2444227 ⎢6 2 4 2 0 0 0 ⎥ 7 16 ⎢2 4 6 6 3 3⎥ 7 1 6 τ3 2τ3 3τ2 0 0 0 7 C1==1 6 ⎢246633⎥ 7⟶ 1⎢6 2 3 0 0 0 ⎥ 7 − 2 6 2468447 2 6 2 3 7 26 ⎢2 4 6 8 4 4⎥ 7 !2 ⎢6 00 0 0 0 0 2 + 2 + σ 2 σ2 σ ⎥ 7 6 ⎢123432⎥ 7 6 7 6 1 2 3 4 3 2 7 ⎢6 00 0 0 0 0 2 2σ2 44σ2 22σ3⎥ 7 6 ⎣123423⎦ 7 ⎣6 0 0 0 2 3 ⎦ 7 4 1 2 3 4 2 3 5 4 0 0 0 σ3 2σ3 3σ2 5 FigureFigure 11 shows shows how how one can illustrateillustrate thethe decompositiondecomposition of of the the 6D 6D space space as as the the direct direct sum sum of of twotwo complementary complementary 3D 3D spaces spaces where thethe correspondingcorresponding nodes nodesαi ,(i,(=1,2,...,6)= 1, 2, ::: , 6), β,i , (i=, (1, 2, = 31,) 2, 3) representrepresent the the corresponding corresponding simple simple roots, andand β´i stands stands for forβi in thein complementarythe complementary space.