Previously unknown quasicrystal periodic approximant found in space.

We report the discovery of Al34Ni9Fe2, the first natural known periodic crystalline approximant to decagonite (Al71Ni24Fe5), a natural quasicrystal composed of a periodic stack of planes with quasiperiodic atomic order and ten-fold symmetry. The new mineral has been approved by the International Mineralogical Association (IMA 2018-038) and officially named proxidecagonite, which derives from its identity to periodic approximant of decagonite. Both decagonite and proxidecagonite were found in fragments from the Khatyrka meteorite. Proxidecagonite is the first natural quasicrystal approximant to be found in the Al-Ni-Fe system. Within this system, the decagonal quasicrystal phase has been reported to transform at ~940 °C to Al13(Fe,Ni)4, Al3(Fe,Ni)2 and the liquid phase, and between 800 and 850 °C to Al13(Fe,Ni)4, Al3(Fe,Ni) and Al3(Fe,Ni)2. The fact that proxidecagonite has not been observed in the laboratory before and formed in a meteorite exposed to high pressures and temperatures during impact-induced shocks suggests that it might be a thermodynamically stable compound at high pressure. The most prominent structural motifs are pseudo-pentagonal symmetry subunits, such as pentagonal bipyramids, that share edges and corners with trigonal bipyramids and which maximize shortest Ni-Al over Ni-Ni contacts.

AAuAl (A = Ca, Sc, and Ti): Peierls Distortion, Atomic Coloring, and Structural Competition.

Using density functional theory, the crystal structure variation of AAuAl (A = Ca, Sc, and Ti) from orthorhombic Co2Si-type to distorted hexagonal Fe2P-type and then Ni2In-type structures is shown to correlate with their electronic structures and valence electron counts, sizes of the active metals A, and site preferences for Au and Al atoms, which are arranged to maximize Au-Al nearest neighbor contacts. An evaluation of chemical pressure imposed by the varying A metals using total energy vs volume calculations indicates that larger unit cell volumes favor the orthorhombic structure, whereas smaller volumes favor the hexagonal structures. The electronic origin of the Mg2Ga-type crystal structure of ScAuAl, refined as a distorted Fe2P-type supercell doubled along the c-axis, indicates a Peierls-type distortion mechanism of the Au chains along the c-axis.

From Quasicrystals to Crystals with Interpenetrating Icosahedra in Ca-Au-Al: In Situ Variable-Temperature Transformation.

The irreversible transformation from an icosahedral quasicrystal (i-QC) CaAu4.39Al1.61 to its cubic 2/1 crystalline approximant (CA) Ca13Au56.31(3)Al21.69 (CaAu4.33(1)Al1.67, Pa3̅ (No. 205); Pearson symbol: cP728; a = 23.8934(4)), starting at ∼570 °C and complete by ∼650 °C, is discovered from in situ, high-energy, variable-temperature powder X-ray diffraction (PXRD), thereby providing direct experimental evidence for the relationship between QCs and their associated CAs. The new cubic phase crystallizes in a Tsai-type approximant structure under the broader classification of polar intermetallic compounds, in which atoms of different electronegativities, viz., electronegative Au + Al vs electropositive Ca, are arranged in concentric shells. From a structural chemical perspective, the outermost shell of this cubic approximant may be described as interpenetrating and edge-sharing icosahedra, a perspective that is obtained by splitting the traditional structural description of this shell as a 92-atom rhombic triacontahedron into an 80-vertex cage of primarily Au [Au59.86(2)Al17.14□3.00] and an icosahedral shell of only Al [Al10.5□1.5]. Following the proposal that the cubic 2/1 CA approximates the structure of the i-QC and on the basis of the observed transformation, an atomic site analysis of the 2/1 CA, which shows a preference to maximize the number of heteroatomic Au-Al nearest neighbor contacts over homoatomic Al-Al contacts, implies a similar outcome for the i-QC structure. Analysis of the most intense reflections in the diffraction pattern of the cubic 2/1 CA that changed during the phase transformation shows correlations with icosahedral symmetry, and the stability of this cubic phase is assessed using valence electron counts. According to electronic structure calculations, a cubic 1/1 CA, "Ca24Au88Al64" (CaAu3.67Al2.67) is proposed.

An Icosahedral Quasicrystal and Its 1/0 Crystalline Approximant in the Ca-Au-Al System.

A new icosahedral quasicrystalline phase, CaAu4.5-xAl1.5+x [0.11 ≤ x ≤ 0.40(6); CaAu4.4Al1.6, aQC = 5.383(4) Å, and Pm3̅ 5̅], and its lowest-order 1/0 cubic crystalline approximant phase, CaAu3+xAl1-x [0 ≤ x ≤ 0.31(1); a = 9.0766(5)-9.1261(8) Å, Pa3̅ (No. 205), and Pearson symbol cP40], have been discovered in the Ca-poor region of the Ca-Au-Al system. In the crystalline approximant, eight [Au3-xAl1+x] tetrahedra fill the unit cell, and each tetrahedron is surrounded by four Ca atoms, thus forming a three-dimensional network of {Ca4/4[Au3-xAl1+x]} tetrahedral stars. A computational study of Au and Al site preferences concurs with the experimental results, which indicate a preference for near-neighbor Au-Al interactions over Au-Au and Al-Al interactions. Analysis of the electronic density of states and the associated crystal orbital Hamilton population curves was used to rationalize the descriptions of CaAu4.5-xAl1.5+x [0.11 ≤ x ≤ 0.46(6)] and CaAu3+xAl1-x [0 ≤ x ≤ 0.31(1)] as polar intermetallic species, whereby Ca atoms engage in polar covalent bonding with the electronegative, electron-deficient [Au3-xAl1+x] tetrahedral clusters and the observed phase width of the crystalline approximant.

Chemical excision of tetrahedral FeSe(2) chains from the superconductor FeSe: synthesis, crystal structure, and magnetism of Fe(3)Se(4)(en)(2).

Fragments of the superconducting FeSe layer, FeSe2 tetrahedral chains, were stabilized in the crystal structure of a new mixed-valent compound Fe3Se4(en)2 (en = ethylenediamine) synthesized from elemental Fe and Se. The FeSe2 chains are separated from each other by means of Fe(en)2 linkers. Mössbauer spectroscopy and magnetometry reveal strong magnetic interactions within the FeSe2 chains which result in antiferromagnetic ordering below 170 K. According to DFT calculations, anisotropic transport and magnetic properties are expected for Fe3Se4(en)2. This compound offers a unique way to manipulate the properties of the Fe-Se infinite fragments by varying the topology and charge of the Fe-amino linkers.