  Melting of orthorhombic boron silicide B6Si has been studied at pressures up
to 8 GPa using in situ electrical resistivity measurements and quenching. It
has been found that in the 2.6-7.7 GPa range B6Si melts congruently, and the
melting curve exhibits negative slope of -31(2) K/GPa that points to a higher
density of the melt as compared to the solid phase. At very high temperatures
B6Si melt appears to be unstable and undergoes disproportionation into silicon
and boron-rich silicides. The onset temperature of disproportionation strongly
depends on pressure, and the corresponding low-temperature boundary exhibits
negative slope of -92(3) K/GPa which is indicative of significant volume
decrease in the course of B6Si melt decomposition.
