Alkaline-earth tri-mercurides A IIHg3 (A II=Ca, Sr, Ba): binary intermetallic compounds with a common and a new structure type

The alkaline-earth tri-mercurides AHg3 (A=Ca, Sr, Ba) were yielded from stoichiometric melts of the elements in pure phase (in the case of Sr with Sr11Hg54 as a by-product) and their structures were determined by means of single crystal X-ray data. As reported long ago from powder data, CaHg3 and SrHg3 crystallize in the Ni3Sn-type (P63/mmc, a=662.26(2)/689.39(3), c=501.64(2)/510.38(3) pm, Z=2, R1=0.0233/0.0306 for A=Ca/Sr). The structure consists of a hexagonal close packing of ordered layers AHg3 or a dense packing of anti-cuboctahedra [AHg12] (as cation coordination polyhedra, CCP) and [Hg6] octahedra fused via opposite faces to form columns along c. BaHg3 crystallizes in a unique structure type (P4/ncc, a=1193.04(3), c=958.02(5) pm, Z=12, R1=0.0461). It contains three crystallographically different Hg atoms, which form layers of distorted flat square pyramids. In contrast to the layers of the BaAl4-type, 1 5 ${1 \over 5}$ of the pyramids are missing. Due to the 45 degree rotation of adjacent layers, the connection between the layers is not a ‘apical-to-apical’ one like in BaAl4, but is established by ‘apical-to-basal’ bonds. Compared to the Ca and Sr compound, the CCPs of the two different Ba atoms, which are embedded between the pyramid layers, are increased to 12+4 and 14+2 (for Hg+Ba). For all title compounds and the Li phase LiHg3, which is isotypic to CaHg3, the electronic band structures were calculated within the framework of the FP-LAPW DFT method. Even though the compounds are metals and exhibit only very slight minima of the tDOS at the Fermi level, the electron transfer from the alkali/alkaline-earth element towards mercury is almost complete. Thus, Coulomb interactions and the optimized size and arrangement of the A CCPs, besides the flexible Hg–Hg bonding within the polyanion, determine the structure formation.

Cation substitution effects in the system Sr2-xBaxBi3 (0 < or = x < or = 1.3): structural distortions induced by chemical pressure

Crystals of the composition Sr(2-x)Ba(x)Bi(3) (0 < or = x < or = 1.3) have been synthesized from the elements and were characterized by single-crystal and powder X-ray diffraction methods. The compounds crystallize for x = 0, 0.45, 0.86, 1.08, 1.28 in the structure type of the parent compound Sr 2Bi 3 with space group Pnna (No. 52) and Z = 4. Substitution of Sr by Ba leads to a site preference for Ba. The anionic Bi substructures in the pseudoternary system simultaneously distort under remarkable elongation of one distinct Bi-Bi contact. Magnetic measurements for samples with x = 0, 0.45 and 1.08 reveal superconducting transitions at low temperatures. Linear muffin-tin orbital band structure calculations of Sr(2)Bi(3) show strong cation-anion interactions that greatly stabilize the structure. Besides showing characteristics of a typical metal, the band structure plot unveils the co-instantaneous occurrence of flat and steep bands around the Fermi level indicative for superconductivity.

Nonclassical Bonding in the Novel Structure of Ba2Bi3 and Unexpected Site Preference in the Coloring Variant Ba2BiSb2

The new phase Ba(2)Bi(3) crystallizes in the W(2)CoB(2) structure type. Its structure contains rigorously planar anionic layers of (4.6.4.6)(4.6(2))(2) nets with three- and four-bonded Bi, that are separated by Ba atoms. An unexpected site preference is observed in the coloring variant Ba(2)BiSb(2) with Sb occupying only the three-bonded sites. The nonclassical bonding in the anionic network can be rationalized from a reformulation of the Zintl concept as (Ba(2+))(2)[Bi(3)](3)(-)(e(-)). Bonding distances suggest that the extra electron fills Bi-Bi antibonding states. The densities of states obtained from TB-LMTO-ASA calculations show metallic character for both compounds.