Directing Cluster Nuclearity in Semiconducting Lead Thioborates Built from Adamantane and Superadamantane Boron–Sulfur Clusters

Abstract Metal thioborates remain an underdeveloped class of chalcogenides because their synthesis is highly sensitive to vessel reactivity, amorphization, and the transport of volatile boron–sulfur species, all of which frustrate phase-selective access to well-defined cluster-based solids. Here we report controlled access to two tetragonal, noncentrosymmetric chiral lead thioborates, Pb2B2S5 (P41212) and Pb2B4S8 (I41), enabled by tuning stoichiometry, tube geometry, headspace, and cooling protocols to limit volatile transport and promote phase selectivity. Single-crystal X-ray diffraction reveals a cluster-based structural progression from discrete adamantane-like [B4S10]8– units in Pb2B2S5 to larger superadamantane-like [B10S20]10– clusters in Pb2B4S8, accompanied by greater Pb coordination complexity and site-occupancy disorder in the larger-cluster framework. Both compounds are indirect-gap semiconductors with optical gaps near 2.0 eV, but electronic-structure calculations indicate greater valence band dispersion in Pb2B2S5 than in Pb2B4S8, consistent with stronger intercluster electronic coupling in the smaller-cluster framework. Thermal analysis reveals pronounced supercooling, incongruent melting, and reversible decomposition-reformation equilibria, highlighting the narrow reaction window required to access these phases. Whereas Pb2B2S5 largely retains its bulk crystallinity under ambient conditions with only gradual surface degradation, Pb2B4S8 undergoes stepwise conversion to Pb2B2S5 and then PbS. These results show that controlling volatile boron–sulfur chemistry provides a route to phase-selective cluster assembly, noncentrosymmetric thioborate semiconductors, and functional hard-radiation-responsive materials. Leveraging its favorable band gap and Pb-rich framework for X-ray attenuation, dense polycrystalline Pb2B2S5 functions as an X-ray photoconductor with a reproducible, bias-dependent photoresponse under laboratory irradiation.