(218e) A Comparative Study of Two Narrow Gap Semiconductors FeGa3 and FeSb2

Introduction

Fe-based semiconductors FeSb₂ and FeGa₃ have attracted great attention in recent years due to their interesting physical properties. Both of them have fairly small band gaps opened up by the hybridization of the transition metal d orbitals with the main group metal p orbitals. However, the intrinsic energy gap of FeGa₃ found above room temperature is one order of magnitude larger than intrinsic energy gap of FeSb₂.^1,2 In 2007, Bentien et al. reported a colossal Seebeck coefficient of -45000 µV/K at 10K in single crystal FeSb₂. ³ Recently, Seebeck coefficients of a similar order of magnitude (in the order of -16000 µV/K) was also found in FeGa₃ single crystalline samples.⁴ These two narrow-gap semiconductors therefore are potential candidates for low-temperature thermoelectric application. We are working on the optimization of the crystal growth of these compounds.

Results and discussion

FeSb₂ and FeGa₃were prepared by self-flux method from mixtures of the pure elements. The structures were refined from single-crystal X-ray data, showing FeSb₂ compound crystallizing in orthorhombic space group Pnnm (#58) with Z=2 and  lattice constants a=5.8328 Å , b=6.5376 Å and c=3.1973 Å, and FeGa₃ compound crystallizing in tetragonal space group P4₂/mnm (#136) with Z=4 and lattice constants a=6.2615 Å and c=6.5562 Å.

The resistivity of FeSb₂ decreases with increasing temperature which is clearly semiconducting, and a large decrease in resistivity of more than six orders of magnitude was observed in the temperature range from 0 to 150 K. The resistivity of FeGa₃decreases with increasing temperature which is also clearly semiconducting, and a large decrease in resistivity of more than seven orders of magnitude was observed in the small temperature range from 0 to 60 K.

The FeGa₃ is diamagnetic with susceptibility χ~-2*10^-5 emu/mol, weakly temperature dependent between 150-350 K, with some upturn below 150K which is the development of a Curie tail that originates from tiny amount of impurities in the system. Magnetic susceptibility of FeSb₂ increases with an increase in temperature from a low temperature of 25K and passes through a region of diamagnetic to paramagnetic crossover and becomes paramagnetic at high temperatures. The crossover temperatures are around 125K. Minimum susceptibility values were found at approximately 50K. At T<25K, the development of a Curie tail can be associated with saturation from impurity states dominating the magnetic contribution in the sample once all intrinsic states are “frozen” out.

In general, n-type behavior with electrons as majority carriers is observed over the whole temperature range for Seebeck measurements on FeGa₃, the onset of increasing of Seebeck coefficient consistent with the resistivity decresing. Seebeck measurements on FeSb₂crystals showed behavior consistent with the activated behavior also evidenced by resistivity data, with the increase in thermopower below 25K, and peaking around 10K. Interestingly, a crossover of negative value to positive value was observed at about 6K which might indicate an n-type to p-type transition.

Reference

1  C. Petrovic, J. W. Kim, S. L. Bud’ko, A. I. Goldman,11 P. Canfield, W. Choe, and G. J. Miller, Phys. Rev. B. 67, 155205 (2003).

2  Y. Hadano, S. Narazu, M. A. Avila, T. Onimaru, and T. Takabatake, J. Phys. Soc. Jpn. 78, 013702 (2009).

3  A. Bentien, S. Johnsen, G.K.H. Madsen, B.B. Iversen, and F. Steglich, Europhysics Letters. 8017008 (2007).

4.   M. Wagner-Reetz, D. Kasinathan, W. Schnelle, R. Cardoso-Gil, H. Rosner, Y. Grin,  Phys. Rev. B. 90, 195206 (2014).