KBi<sub>2</sub><i><sub>-</sub></i><i><sub>x</sub></i>Pb<i><sub>x</sub></i> (0 < <i>x</i> ≤ 1): A Zintl Phase Evolving from a Distortion of the Cubic Laves-Phase Structure
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The quasibinary system KBi2-xPbx has been investigated, both experimentally and theoretically. Phases with compositions 0 ≤ x ≤ 1.2 were synthesized and structurally characterized by X-ray diffraction experiments. For low values of x (0 ≤ x < 0.6), KBi2-xPbx adopts the cubic Laves-phase structure MgCu2 (space group Fd3̄m), which contains a rigid framework of corner-condensed symmetry-equivalent tetrahedra formed by randomly distributed Bi and Pb atoms. For compositions x ≥ 0.6, these tetrahedra become alternately elongated and contracted. The distortion of the framework lowers the space-group symmetry to F4̄3m (KBi1.2Pb0.8, F4̄3m, Z = 8, a = 9.572(1) Å). Magnetometer measurements show that KBi2 (x = 0) is metallic and goes through a superconducting transition below 3.5 K. First principles calculations reveal that the Fd3̄m → F4̄3m distortion is largest for KBiPb (x = 1.0), which at the same time turns into a semiconductor. Thus, F4̄3m KBiPb corresponds to a proper charge-balanced Zintl phase, K+[BiPb]-, with separated polyanionic tetrahedra, (Bi2Pb2)2-. However, it was not possible to prepare F4̄3m KBiPb. Syntheses attempting to increase the Pb content in KBi2-xPbx above x = 0.8 yielded additional, not yet characterized, ternary phases.
