The 4d transition metal yttrium was studied at pressures up to 34 GPa by energy dispersive x-ray diffraction. The crystal-structure sequence hcp→Sm-type→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→d transfer) without any significant participation of f electrons. © 1981 The American Physical Society.