Kondo insulator description of spin state transition in $\mathrm{Fe}{\mathrm{Sb}}_2$

The thermal expansion and heat capacity of $\mathrm{Fe}{\mathrm{Sb}}_2$ at ambient pressure agrees with a picture of a temperature induced spin state transition within the Fe $t_{2g}$ multiplet. However, high pressure powder diffraction data show no sign of a structural phase transition up to 7 GPa. A bulk modulus $\mathrm{B}=84\left(3\right)$ GPa has been extracted and the temperature dependence of the Grüneisen parameter has been determined. We discuss here the relevance of a Kondo insulator description for this material.

Figure 1

(Color online) (a) Crystal structure of $\mathrm{Fe}{\mathrm{Sb}}_2$ showing Fe (open, brown crosses) surrounded by Sb (full blue) octahedra. The two short Fe-Sb bond distances are shown in black and white. The $d_{xy}$ orbital is oriented in between short Fe-Sb bond distances along the $z$-axis (the $c$-axis of the unit cell). (b) The Fe-Sb-Fe bond angle $\alpha$ associated with the edge sharing octahedra along the $c$-axis of the unit cell. Note that increased occupancy of $d_{xy}$ orbitals is reflected in the increase of $\alpha$.

Figure 2

Temperature dependence of (a) normalized unit cell parameters, (b) bond distance lengths, and (c) Fe-Sb-Fe angle $\alpha$ between edge sharing octahedra and bond distance ratio (long/short). Note the increase of angle $\alpha$ above ${\mathrm{T}}_{\text{crossover}}$ (see text) and the presence of two regions of the Fe-Sb normalized short bond distance length contraction around it.

Figure 3

Pressure dependence of (a) normalized unit cell parameters and (b) unit cell volume of $\mathrm{Fe}{\mathrm{Sb}}_2$.

Figure 4

(Color online) The effects of pressure and temperature on the unit cell volume. The 8 GPa of pressure reduce the unit cell volume by 1.4%, 4.6 times exceeding the ambient pressure compression for 6.5% by temperature decrease from 300 K to 20 K.

Figure 5

(Color online) Magnetic susceptibility of $\mathrm{Fe}{\mathrm{Sb}}_2$ (data taken from Ref. 14). Note much smaller low temperature Curie tail than in FeSi. Solid and dashed lines represent fits using the model density of states shown in the inset.

Figure 6

(a) Heat capacity $C_P$ of $\mathrm{Fe}{\mathrm{Sb}}_2$. Inset shows contribution to heat capacity from the model density of states used to model magnetic susceptibility (see text). (b) Grüneisen parameter $\Gamma$ of $\mathrm{Fe}{\mathrm{Sb}}_2$.

Figure 7

Thermal expansion $\alpha =(1∕V){(\partial V∕\partial P)}_P$ of the unit cell volume of $\mathrm{Fe}{\mathrm{Sb}}_2$. Weak temperature dependence of the unit cell volume introduces some noise in thermal expansion data below 100 K. Inset shows the thermal expansion coefficient of the individual axes $\alpha =1∕l{(\partial l∕\partial P)}_P(l=a,b,c)$.