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

Authors: 
Siegrist, T. - Presenter, Florida State University


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).