Structural Phase Transitions in Yttrium under Pressure

The $4d$ transition metal yttrium was studied at pressures up to 34 GPa by energy dispersive x-ray diffraction. The crystal-structure sequence hc${\mathrm{p}}^{\to }$Sm-${\mathrm{type}}^{\to }$double-hcp was observed with increasing pressure. This sequence is well known for the regular trivalent lanthanide metals with decreasing $4f$-electron number and under high pressure. Its observation in yttrium is conclusive experimental evidence that these structural transitions result simply from changes in $d$-band occupancy ($s\to d$ transfer) without any significant participation of $f$ electrons.