Study of the compressibility of FeSi, MnSi, and CoS$_2$ transition-metal compounds at high pressures

Silicides and sulfides of transition metals attract great attention of researchers because of a wide spectrum of interesting magnetic, electronic, and optical properties. The crystal structure of FeSi, MnSi, and CoSi silicides is $P2_13\mathrm{(B20)}$, whereas FeS$_2$, CoS$_2$, and MnS$_2$ sulfides have a structure of pyrite $P\mathrm{a}3$. Despite the great interest in these systems and the cubic symmetry of crystals, the structure and compressibility of these compounds at high pressures are still insufficiently studied. There is a significant spread (more than a factor of two!) in the bulk modulus and its pressure derivative for a single compound. Most studies were performed under nonhydrostatic conditions. In this work, the compressibility of FeSi and MnSi silicides (at pressures up to $35$ GPa) and CoS$_2$ sulfide (up to $22$ GPa) has been studied by the X-ray diffraction method in a diamond anvil cell with the use of helium as the softest pressure-transmitting medium. The values obtained for the bulk modulus and its derivative — $B = 178 \pm3$ GPa and $B_p = 5.6 \pm 0.5$ for FeSi, $B = 167 \pm 3$ GPa and $Bp' = 4.6 \pm 0.5$ for MnSi, and $B = 94 \pm 2$ GPa and $Bp' = 6.9 \pm 0.5$ for CoS$_2$ — can be considered as the most reliable and can be used to test numerous theoretical models. The results for the compressibility of FeSi are important for the verification of models of the Earth’s core.