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.
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Vohra et al. (1981) studied this question.
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