Crystal Growth and Characterization of the Narrow-Band-Gap Semiconductors OsPn2 (Pn = P, As, Sb)

Using metal fluxes, crystals of the binary osmium dipnictides OsPn2 (Pn = P, As, Sb) have been grown for the first time. Single-crystal X-ray diffraction confirms that these compounds crystallize in the marcasite structure type with orthorhombic space group Pnnm. The structure is a three-dimensional framework of corner- and edge-sharing OsPn6 octahedra, as well as [Pn24–] anions. Raman spectroscopy shows the presence of P–P single bonds, consistent with the presence of [Pn2–4] anions and formally Os4+ cations. Optical-band-gap and high-temperature electrical resistivity measurements indicate that these materials are narrow-band-gap semiconductors. The experimentally determined Seebeck coefficients reveal that nominally undoped OsP2 and OsSb2 are n-type semiconductors, whereas OsAs2 is p-type. Electronic band structure using density functional theory calculations shows that these compounds are indirect narrow-band-gap semiconductors. The bonding p orbitals associated with the Pn2 dimer are below the Fermi energy, and the corresponding antibonding states are above, consistent with a Pn–Pn single bond. Thermopower calculations using Boltzmann transport theory and constant relaxation time approximation show that these materials are potentially good thermoelectrics, in agreement with experiment.

本研究首次通过金属助熔剂法，成功生长出二元锇基二pn化物（dipnictides）OsPn₂（Pn = P、As、Sb）的单晶。经单晶X射线衍射（single-crystal X-ray diffraction）分析证实，该系列化合物均以白铁矿（marcasite）结构型结晶，所属空间群为正交晶系Pnnm。其晶体结构由共享顶点与边的OsPn₆八面体构成三维骨架，并存在[Pn₂⁴⁻]阴离子。拉曼光谱（Raman spectroscopy）测试结果显示体系中存在P-P单键，这与[Pn₂⁴⁻]阴离子以及形式电荷为+4的锇阳离子（Os⁴⁺）的存在相吻合。光学带隙与高温电阻率测试结果表明，该系列材料均为窄带隙半导体。通过实验测得的塞贝克（Seebeck）系数结果显示，本征未掺杂的OsP₂与OsSb₂为n型半导体，而OsAs₂则为p型半导体。基于密度泛函理论（density functional theory, DFT）计算得到的电子能带结构表明，该系列化合物均为间接窄带隙半导体。与Pn₂二聚体相关的成键p轨道位于费米能级（Fermi energy）下方，对应的反键态则位于费米能级上方，这与Pn-Pn单键的成键特征相符。采用玻尔兹曼输运理论（Boltzmann transport theory）与恒定弛豫时间近似（constant relaxation time approximation）进行的热功率计算结果表明，该系列材料具备优异的热电潜力，与实验观测结果一致。