Synthesis, characterization, and low temperature electronic and magnetic properties of iron antimonide (FeSb2) single crystals

The narrow band gap semiconductor iron antimonide, FeSb2, has been grown as bulk single crystals, and investigated for its electronic and magnetic properties at low temperatures. The electronic behavior of FeSb 2 is associated with strong electron correlations, similar to the characteristic behavior observed in FeSi, another strongly correlated d-electron correlated semiconductor. Recent studies found an enhancement of the Seebeck coefficient and thermopower, two key parameters in thermoelectric performance, at temperatures below 77K, making FeSb2 potentially useful in cryogenic Peltier-cooling applications below liquid nitrogen temperatures. Crystals with sizes of 1.5-5.0mm are grown using three different synthesis approaches: chemical vapor-phase transport (CVT), using either chlorine (Cl 2(g)) or iodine (I2(g)) as transporting agents, and molten flux growth using excess antimony (Sb). Single crystal and powder x-ray diffraction experiments on select samples provided for structural phase identification and confirmed the absence of secondary impurity phases. FeSb2 crystallizes in the FeS2-marcasite structure type, featuring FeSb6 edge-sharing octahedra forming chains along the c-axis and corner-sharing in the a-b plane. The crystal surfaces were characterized using SEM-EDS and AFM measurements, revealing the characteristic morphological features resulting from CVT growth, as well as helping to identify surface contamination. Magnetic susceptibility measurements show weak temperature induced paramagnetism above 50K , with the diamagnetic-to-paramagnetic crossover above 100K. Temperature-dependent electronic transport properties, ρ(T), showed semiconducting behavior and the formation of a resistivity plateau between 10K-40K, corresponding to a secondary transport gap, ϵg, of about 3.9-7.2meV, similar in magnitude and in accordance to previous studies on FeSb2. Hybridization between iron (Fe)3d and antimony (Sb) 5p and 5s valence states appears to be responsible for complex electronic behavior at low temperatures, with the formation of a small secondary gap. Earlier studies have shown a significant enhancement of the Seebeck coefficient (thermopower) at the onset of this gap (10K-12K). Thermopower measurements in our sample also show peak values around this temperature, but the maximum values are several orders of magnitude lower than previous reports, while magnetic and electronic properties are in good agreement.