Substantial Cd–Cd Bonding in Ca₆PtCd₁₁: A Condensed Intermetallic Phase Built of Pentagonal Cd₇ and Rectangular Cd_4/2Pt Pyramids

Abstract

The novel intermetallic Ca₆PtCd₁₁ is orthorhombic, <i>Pnma</i>, <i>Z</i> = 4, with <i>a</i> = 18.799(2) Å, <i>b</i> = 5.986(1) Å, <i>c</i> = 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 Cd_4/2Pt 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 <i>E</i>_F, 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