Structural phase transitions in yttrium up to 183 GPa

Angle-dispersive x-ray powder diffraction experiments have been performed on yttrium metal up to 183 GPa. We find that the recently discovered $oF16$ structure observed in the high-$Z$ trivalent lanthanides is also adopted by yttrium above 106 GPa, pressures where it has a superconducting temperature of $\ensuremath{\sim}20$ K. We have also refined both tetragonal and rhombohedral structures against the diffraction data from the preceding ``distorted-fcc'' phase and we are unable to state categorically which of these is the true structure of this phase. Finally, analysis of yttrium's equation of state reveals a marked change in the compressibility upon adoption of the $oF16$ structure, after which the compression is that of a ``regular'' metal. Electronic structure calculations of $oF16$-Y confirm its stability over $oF8$ structure seen in Nd and Sm, and provide insight into the nature of the shift of orbital character from $s$ to $d$ under compression.