Pb[PtBi₆I₁₂] – Infinite Chains of Heavy Atom Clusters

Abstract

Reaction of Bi with Pb, Pt and BiI3 above 330 °C yielded shiny, black, air insensitive crystals of the subiodide Pb[PtBi6I12]. Pb[PtBi6I12] crystallizes in the rhombohedral space group R3 with lattice parameters a = 1585.39(5) pm and c = 1098.77(3) pm at room temperature. The compound consists of cuboctahedral [PtBi6I12]2– clusters and Pb2+ cations in octahedral coordination between trigonal faces of two cuboctahedra, which concatenate them into linear chains. Six weaker Bi···I inter‐cluster bridges per cluster connect the chains. The structure can be interpreted as a superstructure of the NaCl structure type, with iodide and platinum(–II) as anions and bismuth(II) and lead(II) as cations, leaving 18 out of 39 cation positions unoccupied. The arrangement of the cuboctahedral clusters follows the motif of a body‐centered cubic packing. Pb[PtBi6I12] is a semiconductor with a measured bandgap of 0.69 eV. Full‐relativistic DFT calculations indicate a topologically trivial bandgap of 0.58 eV.

Design of High‐Temperature Syntheses on the Example of the Heavy‐Atom Cluster Compound Sn[PtBi6I12]

“…From the +II oxidation state for tin follows the formal charge of 2‐ for the cuboctahedral cluster in accordance with previous findings in similar compounds and general bonding considerations [12,13,16] . Sn II [Pt −II Bi II 6 I −I 12 ] is an electron‐precise situation with all valence electrons localized in chemical bonds or lone pairs and the compound should thus be a bulk semiconductor like the isostructural Pb[PtBi 6 I 12 ] [18] …”

“…Starting from Sn, Pt, Bi and BiI 3 in the molar ratio 1 : 1 : 2 : 4 (i. e. stoichiometric with respect to Sn[PtBi 6 I 12 ]) and using a temperature program that was based on the syntheses of Bi 2 [PtBi 6 I 12 ] 3 [17] and Pb[PtBi 6 I 12 ], [18] agglomerates of small black cubic crystallites and microcrystalline powder were formed. The X‐ray diffraction pattern of the product indicated that a substance had crystallized that resembled the two compounds mentioned above, but had slightly smaller lattice parameters than the lead compound (Figure S1).…”

“…Comparing these two demonstrates the large effect that the SOC has, greatly reducing the band gap in a similar manner as observed in Pb[PtBi 6 I 12 ]. [18] Nonetheless, despite this large reduction in the band gap, the parity between the valence and the conduction bands is not inverted and therefore a topologically trivial band gap is present -the calculated Z 2 invariant is (0;000) -and is therefore too large to allow the generation of a topological insulator following the mechanism introduced by Bernevig, Hughes and Zhang. [13] The band structure calculations also show that the electronic excitation occurs between bands strongly dominated by atomic contributions from Bi and I, while Pt and Sn play only a marginal role here (Figure 7 and S5 to S8).…”

“…. [18] Cuboctahedral [PtBi 6 I 12 ] 2À clusters are concatenated into linear chains by octahedrally coordinated Sn 2 + cations (Figure 6). The primary organization into chains parallel to the c axis also rationalizes why the lattice parameter a of the tin compound is only 2.2 pm (0.1 %) smaller than that of the lead compound, but the lattice parameters c differ by 9.1 pm (0.8 %).…”

“…In the case of Pb[PtBi 6 I 12 ], an excess of lead had to be used in the synthesis to prevent partial occupation of the lead(II) position by bismuth(III) corresponding to the formula (Bi 2x Pb 1-3x )[PtBi 6 I 12 ] with 0 � x � 1/3. [18] Therefore, we repeated the synthesis with an excess of one-third tin. Unexpectedly, this did not lead to a complete occupation of the counter cation position with tin (x � 0.05; Table S1) nor to larger crystals.…”