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Substantial Cd-Cd bonding in Ca6PtCd11: a condensed intermetallic phase built of pentagonal Cd7 and rectangular
Fakhili Gulo1, Saroj L Samal, John D Corbett
1Ames Laboratory, DOE and Department of Chemistry, Iowa State University, Ames, Iowa 50010, United States.
Abstract:
The novel intermetallic Ca6PtCd11 is orthorhombic, Pnma, Z = 4, with a = 18.799(2) Å, b = 5.986(1) Å, c = 15.585(3) Å. The heavily condensed network contains three types of parallel cadmium chains: apically strongly interbonded Cd7 pentagonal bipyramids, linear Cd arrays, and rectangular Cd4/2Pt pyramids. All of the atoms have 11-13 neighbors. Calculations by means of the linear muffin-tin orbitals method in the atomic spheres approximation indicate that some Cd-Cd interactions correspond to notably high Hamilton populations (1.07 eV per average bond) whereas the Ca-Ca covalent interactions (integrated crystal orbital Hamiltonian population) are particularly small (0.17 eV/bond). (Pt-Cd interactions are individually greater but much less in aggregate.) The Ca-Ca separations are small, appreciably less than the single bond metallic diameters, and unusually uniform (Δ = 0.14 Å). The Cd atoms make major contributions to the stability of the phase via substantial 5s and 5p bonding, which include back-donation of Cd 5s, 5p and Pt 5d into Ca 3d states in the principal bonding modes for Ca-Cd and Ca-Pt. Bonding Ca-Ca, Ca-Cd, and Cd-Cd states remain above EF, and some relative oxidation of Ca in this structure seems probable. Ca6PtCd11 joins a small group of other phases in which Cd clustering and Cd-Cd bonding are important.
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