Metal-Rich Compounds of the d-Metals

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

“…For the cubic 2/1 CA Ca 13 Au 56.79(6) Al 21.20 , the most intense PXRD peaks fall in the region 24.81−30.67 nm −1 and lead to VEC values spanning 1.208 to 2.108 e/a, averaging 1.814 e/a, which corresponds to a composition Ca 13 Au 47.46 Al 30.54 (Table S7). These VEC values fall at the border between Hume−Rothery and polar intermetallic classifications 13 even though not all the compounds evaluated contain elements that may participate in polar-covalent interactions as typically found for polar intermetallics.…”

“…The equivalent isotropic thermal parameters associated with these sites are the largest of all the positions in the asymmetric unit by a factor of ∼2−3. For the prototypic Tsai-type cubic 2/1 CA (Yb/Ca) 13 Cd 64 , which shows somewhat longer distances of 1.734(6)−1.98(1) Å from the center to the 12 partially occupied Cd sites at ambient conditions, a refinement outcome of spatially disordered, distorted tetrahedra is one of the hallmarks of the Tsai-type CA. 8b In this case, to obtain tetrahedra from the 12 positions in the shell, the atoms would be unrealistically close to one another (≤2.00(4) Å) and therefore, although this shell may be geometrically seen as 4 spatially disordered tetrahedra, from XRD, the only chemical conclusion is that there are 4 Au/Al sites split among 12 positions.…”

“…Among crystalline intermetallic compounds, these e/a values fall near the upper end of Hume−Rothery phases, which occur for 1.00 ≤ e/a ≤ 2.00, and just below polar intermetallic compounds, which occur for 2.00 < e/a < 4.00. 13 Hume− Rothery phases are densely packed structures with atoms of similar sizes and electronegativities from among the late-and post-transition metals. One example is the class of γ-brasses (e/ a = ∼1.60 to ∼1.75), which are body-centered cubic packings of 26-atom clusters involving four fused icosahedra.…”

“…In many cases, "A" is a divalent or trivalent metal and "B" is Cd or a combination of a group 11 element with a group 13/14 element. For example, (Ca/ Yb) 13 Cd 76 8b (VEC = 2.00 e/a) and Sc 16 Cu 46 Al 38 i-QC (VEC = 2.08 e/a) 43 can be formulated, respectively, as (Ca/Yb)Cd 5.846 and Sc(Cu 0.548 Al 0.452 ) 5.25 . For a few examples of Bergman-type CAs with a monovalent active metal, the condensed formulation is AB 2.0−2.4 .…”

Uncovering new transition metal Zintl phases by cation substitution: the crystal chemistry of Ca3CuGe3 and Ca2+nMnxAg2−x+zGe2+n−z (n = 3, 4)

“…Polar intermetallic compounds (PICs) form a large class of extended solids, combining electropositive 'active' metals (s-or f-block metals mainly) with electronegative post-transition pblock elements and/or late transition (noble) metals. [1][2] Interest in polar intermetallic compounds were originally driven by their structural diversity and exotic bonding features. [2][3][4][5] In fact, they represent intermediates between semiconductors and typical metallic systems.…”