CaAg – SrAg – BaAg: Zwischen CrB‐ und FeB‐Strukturtyp

Abstract

CaAg – SrAg – BaAg: Between CrB and FeB Structure Type Depending on both electronic (valence electron numbers) and geometric (atom size ratios) characteristics of the contributing elements, the 1:1 compounds AIIM of the heavier alkaline earth elements A and electron‐rich transition metals M form the well known CrB or FeB structure types. Both structure types exhibit M zig‐zag chains, which are stacked in different orientations against each other. In systematic studies of the pseudo‐binary sections CaxSr1−xAg and SrxBa1−xAg three new ternary phases with different stacking variants between the CrB (cubic stacking, c) and the FeB (hexagonal stacking, h2) structure type have been prepared and determined on the basis of single crystal x‐ray data. Starting from CaAg (CrB type, stacking c), up to 25 percent of Sr (i.e. in Ca0.75(2)Sr0.25(2)Ag: orthorhombic, space group Cmcm, a = 413.4(1), b = 1157.8(3), c = 467.4(2) pm, Z = 4, R1 = 0.0453) can be substituted without a change in the structure type. At a 1:1 ratio of Ca and Sr, a new structure type with stacking sequence (hc5)2 is formed (Ca0.48(3)Sr0.52(3)Ag: space group Pnma, a = 4800.1(1), b = 471.8(12), c = 624.11(9) pm, Z = 24, R1 = 0.0881). Further reduced cubic stacking ((hc2)2) is observed in the Sr rich region with Ca proportions between 39 and 25(2) % (space group Pnma, x = 0.39(1)/0.25(2): a = 2412.2(3)/2434.67(4), b = 472.75(5)/473.97(8), c = 626.17(7)/629.2(3) pm, Z = 12, R1 = 0.0430/0.0489) where the HT‐TbNi structure type is formed. The stability range of the SrAg structure type ((hc)2) starts at a Ca content of 5 % (Ca0.05Sr0.95Ag: space group Pnma, a = 1643.1(2), b = 476.79(7), c = 636.27(9) pm, Z = 8, R1 = 0.0318). From a new second modification of SrAg (space group Pnma, a = 4131.5(4), b = 478.79(6), c = 638.81(6) pm, Z = 20, R1 = 0.0394) up to a Ba proportion of 8 percent, the α‐CaCu structure type with sequence (hch2c)2 is formed. The stability range of the FeB structure with purely hexagonal stacking extends from Sr0.84(1)Ba0.16(1)Ag (a = 830.78(12), b = 479.35(8), c = 644.93(11) pm, Z = 4, R1 = 0.0413) to the border phase BaAg. In an accompanying computational study, the chemical bonding in the binary compounds CaAg, SrAg and BaAg is analyzed using FP‐LAPW band structure methods. Together with geometrical criteria, the results are used to compare and discuss the stability of the different stacking variants of the polar intermetallic 1:1 phases inbetween the CrB and the FeB structure type.

Anisotropic Reactivity of CaAg under Ethylene Epoxidation Conditions

“…The nature of the atomic interactions in an intermetallic compound yields valuable information on its cleavage, that is, atomic arrangements on the cleaved surfaces, being starting print for understanding of chemical behavior. The calculated electronic structure of CaAg (see SI, S8) is in agreement with findings reported previously. , The prominent feature are the highly localized, atomic-like Ag 4d states having a bandwidth of about 1.4 eV, much narrower than that in fcc Ag (S8, Figure S7). The QTAIM basins of Ag and Ca in CaAg (Figure A) reveal charge transfer of 1.2 electrons from Ca to Ag.…”

Influence of Element Substitution on the Cluster Arrangement in the Novel Structures Ca3Tl5, Sr3Tl5, and Sr3Sn5–xTlx (x = 1.8 and 2.2)

“…Contrary, in the Tm 3 Ga 5 type phases the cluster chains alternate with respect to the chain tilt angle along the a direction (Figure 1a) resulting in a herringbone pattern arrangement. This observation of different stacking variants is reminiscent of the different alignments of zigzag chains of atoms in CrB and FeB 38.…”

Analysis of Electronic Structures and Chemical Bonding of Metal‐rich Compounds. 2. Presence of Dimer (T–T)4– and Isolated T2– Anions in the Polar Intermetallic Cr5B3‐Type Compounds AE5T3 (AE = Ca, Sr; T = Au, Ag, Hg, Cd, Zn)

“…The d 10 s 2 p 1 case is found for compounds containing [Pt–Pt] 6– , [Au–Au] 4– , [Ag–Ag] 4– , and [Cu–Cu] 4– dimer anions 2, in which the transition‐metal elements behave as main group p‐elements forming a p σ –p σ σ‐bond. Anionic zigzag chains of silver atoms are found in the argentides SrAg and CaAg 11. The d 10 s 2 p 2 case is found for 18‐electron half‐Heusler compounds REML of rare‐earth elements RE (e.g., RE = Sc, Y; M = Ni, Pd, Au; L = As, Sn, Sb, Pb, Bi) 12, in which the M and L atoms form a diamond‐like lattice (i.e., the zinc blende lattice) 7.…”