High-Pressure Study of the Anomalous Rare-Earth Ternaries ${\mathrm{Eu}}_{1.2}$${\mathrm{Mo}}_6$${\mathrm{S}}_8$ and ${\mathrm{Eu}}_{1.2}$${\mathrm{Mo}}_6$${\mathrm{Se}}_8$

The two anomalous rare-earth ternaries, ${\mathrm{Eu}}_{1.2}$ ${\mathrm{Mo}}_6$ ${\mathrm{S}}_8$ and ${\mathrm{Eu}}_{1.2}$ ${\mathrm{Mo}}_6$ ${\mathrm{Se}}_8$, are investigated both resistively and magnetically under hydrostatic pressure up to ∼18 kbar. Superconductivity with an onset temperature of ∼11 K was observed abruptly at pressure above ∼7 kbar in ${\mathrm{Eu}}_{1.2}$ ${\mathrm{Mo}}_6$ ${\mathrm{S}}_8$. However, no superconductivity was detected in ${\mathrm{Eu}}_{1.2}$ ${\mathrm{Mo}}_6$ ${\mathrm{Se}}_8$ down to 1.2 K and up to 18 kbar. Several possibilities are discussed and contrasted.