Structural and magnetic phase transitions of the ${\text{V}}_4$-cluster compound ${\text{GeV}}_4{\text{S}}_8$

The phase transitions of the strongly correlated tetrahedral ${\text{V}}_4$-cluster compound ${\text{GeV}}_4{\text{S}}_8$ have been studied by low-temperature powder x-ray diffraction, magnetic susceptibility, and specific heat measurements. The crystal structure is cubic at room temperature (space group $F\overline{4}3m$) and transforms to orthorhombic (space group $Imm2$) at $T_S=30\text{K}$. A Jahn–Teller distortion reduces the symmetry of the ${\text{V}}_4$-cluster from $\overline{4}3m$ to $mm2$. The second transition at 18 K is the onset of antiferromagnetic ordering without symmetry change but with a certain increase in the distortion. The latter reflects a strong magnetoelastic coupling at $T_N$. Specific heat anomalies at 30 and 18 K confirm the two phase transitions.

Figure 1

Cubic (440) reflection and splitting at lower temperatures. Solid lines mark the orthorhombic reflection positions at 25 K, dashed lines mark the assumed rhombohedral reflection positions.

Figure 2

Rietveld fit of ${\text{GeV}}_4{\text{S}}_8$ at 25 K. Lower tick marks are ${\text{GeV}}_4{\text{S}}_8$, upper tick marks the impurity phase ${\text{V}}_2{\text{O}}_3$

Figure 3

${\text{V}}_4$-cluster units of ${\text{GeV}}_4{\text{S}}_8$ at 300 $\left(\overline{4}3m\right)$, 25 ($mm2$, paramagnetic), and 4 K ($mm2$, magnetically ordered). Distances of the longest and shortest V-V bonds are in pm.

Figure 4

(Color online) Magnetic susceptibility $(△)$ and inverse magnetic susceptibility $(+)$ of ${\text{GeV}}_4{\text{S}}_8$ at 1 T. Insert: Hysteresis of the susceptibility measured at 0.1 T. Red (gray) line: increasing temperature; black line: decreasing temperature)

Figure 5

(Color online) Specific heat of ${\text{GeV}}_4{\text{S}}_8$. Red (gray) markers: increasing temperature; black markers: decreasing temperature.