An-Pang Tsai, professor at Tohoku University, Sendai, Japan, passed away on May 25 2019 at the age of 60. He was a pioneer and a leader in the field of quasicrystals and complex intermetallic phases. With him the community has lost one of the brightest scientist in this field. This symposium is dedicated to his memory and illustrates the many different fields he has been contributing in crystallography, metallurgy, material science and solid-state physics and chemistry.
In this presentation we will highlight the many important contributions of the research conducted by An-Pang Tsai going from fundamental and basic research to their applications and the important impact it had on the scientific community. We can quote: the discovery of most of the thermodynamically stable quasicrystals including the stable binary quasicrystals named ‘Tsai-type’ quasicrystals, the physics allowing to understand which systems is favorable for quasicrystal growth, their atomic structure and physical properties, lattice dynamics, magnetism, mechanical properties. In view of applications he has been one of the first to promote the use of quasicrystals for light alloy reinforcement and more recently he developed a completely new field with new ideas for the development of new catalytic materials.
An-Pang Tsai also had a decisive impact in training the younger generation in the different labs he has ben leading. He always was enthusiast to explore new fields, proposing ambitious targets, but with the long-term perspective, without pressure and with the entire necessary scientific environment. He has coordinated researches in the field of quasicrystals and material sciences in Japan and abroad. In particular he started to organize annual meetings on quasicrystals in Japan in 1996, which continued for more than 20 years andhas organized or initiated number of international conferences.
From very early on in his career, An-Pang Tsai built up an impressive large number of collaborations all across the world, in Japan, Europe, USA, Canada, China and Taiwan. His generosity in sharing his discoveries, his constant curiosity and inspiring ideas have irrigated the quasicrystal community, promoting collaboration rather than competition, leading to major results in almost all aspects of quasicrystal researches and from experiments to theory. He always was available and ready to share his knowledge with senior scientists as well as with young students.
We have lost a great colleague and a good friend, and we will honour his memory by continuing his interdisciplinary research work, keeping his enthusiast and collaborative approach.
[1] de Boissieu M and Ishimasa T (2019) Acta Cryst. B 75 763.

The Al-Mn icosahedral quasicrystal discovered by Shechtman was a metastable phase [1]. Attempts at its structure solution immediately began after the discovery was declared. After the subsequent discovery of stable icosahedral quasicrystals, the study of their physical properties became more active in addition to the elucidation of their structures, but these quasicrystals were all formed in ternary systems [2]. Because chemical disorder of inherent in ternary icosahedral quasicrystals, it was difficult to achieve any structure solution which is comparable to what is obtained for ordinaryl crystal. Discovery of a stable binary icosahedral quasicrystal in a Cd-Yb alloy opened a route to the structure solution to at least for this particular type of quasicrystal [3]. This quasicrystal, now known as Tsai-type icosahedral quasicrystal, forms the largest group among known quasicrsytals and their approximant crystals forming systems. Although I was not directly involved in the discovery of this quasicrystal, I was present at the scene as a member of Tsai's group and took the first X-ray transmission Laue photographs (Fig. 1), which were appeared in the paper announcing the discovery [3]. In the beginning, the quality of Cd-Yb quasicrystal was not very good, but soon good quality was obtained, and higher-dimensional crystal structure analysis by means of single crystal X-ray diffraction became possible [4]. Here is the story of how we arrived at the structural solution of this particular icosahedral quasicrystal.

We had collaborated exciting themes in materials science together with Professor An Pang Tsai for 17 years (since 2002). Prof. Tsai began the investigation of catalytic materials in term of metallurgy at NIMS [1].

There are three important topics in the collaborative research with Professor Tsai. Firstly, we succeeded that novel catalytic materials were prepared by the leaching method of Al-Cu-Fe quasicrystalline (QC) [2]. The Al₆₃Cu₂₅Fe₁₂ QC is a promising precursor for Cu catalysts, whose constituent elements, compositions and quasi-periodic structure are in favor of processing high performance catalysts. Brittleness resulting from quasi-periodic structure enables one to obtain powder form for processing catalysts. Relatively low dissolution rate of Al due to quasi-periodic structure upon leaching with NaOH solution, generated homogeneous nanocomposite consisting of Fe₃O₄ and Cu and hence gave rise to high activity and thermal stability for steam reforming of methanol. Secondary, Prof. Tsai proposed a concept for a psudo-element material such as “PdZn = Cu” [3]. A clear correlation between electronic structure and CO₂ selectivity for steam reforming of methanol (SRM) was obtained with PdZn, PtZn, NiZn, and PdCd intermetallics on the basis of experiments and calculations. PdZn and PdCd also exhibited valence electronic densities of states and catalytic properties similar to that of Cu. Thirdly, a new concept of active sites for bulk-type metallic materials was proposed by Prof. Tsai, i.e., nano twin boundary [4]. According to the DFT calculation, surface density of the active six-coordinated atoms in nano porous gold (NPG) was comparable with that of supported gold nanoparticle catalysts. In addition, the energy profiles of reaction pathways for CO oxidation indicated that the six-coordinated sites created by twinning significantly contributed to the catalytic activity of NPG. I will overview of these topics in my presentation.

Two years have passed since Professor A.P. Tsai passed away. Taking over Prof. A.P. Tsai’s spirits, now we are conducting research on novel metallic catalysis materials under the new system. We hope that those concepts of Prof. Tsai’s will lead to a principal for the development of metallic functional materials as well as metallic catalysts in the future.

[1] A.P. Tsai, M. Yoshimura, “Highly active quasicrystalline Al-Cu-Fe catalyst for steam reforming of methanol”, Applied Catalysis, A, 214 (2001) 237-241.; M. Yoshimura, A.P. Tsai, “Quasicrystal application on catalyst”, J. Alloys Compounds, 342 (2002) 451-454.

[2] For example T. Tanabe, S. Kameoka, M. Terauchi and A.P. Tsai, “Microstructure of leached Al-Cu-Fe quasicrystal with high catalytic performance for steam reforming of methanol”, Applied Catalysis, A, 384 (2010) 241-251.

[3] A.P. Tsai, S. Kameoka and Y. Ishii, “PdZn=Cu: Can an intermetallic compound replace an element ?”, J. Physical Soc. Jpn., 73 (2004) 3270-3273.

[4] M. Krajci, S. Kameoka and A.P. Tsai, “Twinning in fcc lattice creates low-coordinated catalytically active sites in porous gold”, J. Chem. Phys., 145 (2016) 084703.; ibid., 147 (2017) 044713.

The name of An-Pang Tsai is in first line connected with his pioneer work on quiasicrystalline and related crystalline materials, e.g. on the atomic structure of quasicrystals [1]. Less known are the studies of his group on chemical properties, in particular on catalytic materials. A mutual origin of the interest to this research field may be found in the search for possible application fields for quasicrystals and investigations on surface properties of quasicrystalline and approximant phases, i.e. oxidation behaviour [2] or etching reactions [3]. Logical continuation of these studies is the work of An Pang Tsai and his group on hydrogen absorption on intermetallic compounds [4,5] and high catalytic activity of amorphous intermetallic hydrides in hydrogenation of ethylene and CO₂ [6,7]. The subsequent studies were devoted to the influence of real structure of materials (Renee catalyst) or electronic factors on the catalytic activity [8,9]. Coming back to the possible applications of quasicrystals, the group of A. P. Tsai was working on activation of quasicrystalline surface and fabrication of a fine nanocomposite layer with high catalytic performance [10]. In the following years several new results were produced by A. P. Tsai and his co-workers on composite catalyst with mixed lamellar structures and dual catalytic functions, dominant factors of porous gold for CO oxidation, effects of Cu oxidation states on the catalysis of NO+CO and N₂O+CO reactions, preparation of dispersive Au nanoparticles on TiO₂ nanofibers from Al-Ti-Au intermetallic compound. The last product of the work of An Pang Tsai in the field of catalysis – although not finished by himself – was the special issue of Science and Technology of Advanced Materials giving an comprehensive overview of current research activities around the world [11].

[1] Takakura, H., Goméz, C. P., Yamamoto, A., de Boissieu, M., Tsai A. P. (2007). Nature Materials 6(1), 58. [2] Yamasaki, M., Tsai A. P. (2002). J. Alloys Compd. 342(1), 473. [3] Saito, K., Saito, Y., Sugawara, S., Shindo, R, Guo, J.-Q., Tsai, A. P. (2004) Phil. Mag. A, 84(10), 1011. [4] Endo, N., Kameoka, S., Tsai, A. P., Zou, L., Hirata, T., Nishimura, Ch. (2009). J. Alloys Compd. 485, 588. [5] Endo, N., Kameoka, S., Tsai, A. P., Zou, L., Hirata, T., Nishimura, Ch. (2010). J. Alloys Compd. 490, L24.

[6] Endo, N., Kameoka, S., Tsai, A. P., Hirata, T., Nishimura, Ch. (2011). Mat. Trans. 52, 1794.

[7] Endo, N., Ito, Sh., Tomishige, K., Kameoka, S., Tsai, A. P., Hirata, T., Nishimura, Ch. (2011). Catalysis Today, 164, 293.

[8] Nozawa, K., Endo, N., Kameoka, S., Tsai, A. P., Ishii, Y. (2011). J. Phys. Soc. Jpn. 80, 064801.

[9] Murao, R., Sugiyama, K., Kameoka, S., Tsai, A. P. (2012). Key Eng. Mat. 508, 304.

[10] Kameoka, S., Tanabe, T., Satoh, F., Terauchi, M., Tsai A. P. (2014). Sci. Techn. Adv. Mat. 15, 1878.

[11] Special issue IMc (2019). Sci. Techn. Adv. Mat. 20

We will present several interesting structures of thin films grown on Tsai-type quasicrystal, icosahedral (i)-Ag-In-Yb, studied by various experimental techniques including scanning tunnelling microscopy (STM). The results include three dimensional quasicrystalline films of single elements [1] and molecular films [2] (Figure 1).

The i-Ag-In-Yb quasicrystal is built by rhombic triacontahedral (RTH) clusters and its surface is formed at the bulk atomic planes that bisect the RTH clusters [3]. When Pb is deposited on the fivefold i-Ag-In-Yb surface, the Pb atoms adsorb at the sites that were originally occupied by the cluster atoms and thus produce quasicrystalline film in three-dimension [1]. This observation is evidenced in other systems as well, namely Pb on the threefold and twofold i-Ag-In-Yb surfaces [4, 5] and In, Sb and Bi on the fivefold i-Ag-In-Yb surface [6].

We also found that Pentacene molecules deposited on the fivefold i-Ag-In-Yb surface adsorb at tenfold-symmetric sites of Yb atoms around surface-bisected RTH clusters, yielding quasicrystalline order [2]. The selective adsorption of Pentacene on Yb sites is also observed on the threefold and twofold surfaces of the same sample.

The phenomena of adsorption on selective sites is also found on Al-based quasicrystals. C₆₀ molecules preferably adsorb on Fe or Mn when deposited on surfaces of i-Al-Pd-Mn and i-Al-Cu-Fe [2, 7], yielding quasicrystalline order of C₆₀. The compatibility between the characteristic lengths of the substrate and the size of adsorbates has led to the growth of unprecedented epitaxial structures

His ideas about AuSi