Observation of complete regular trivalent rare earth sequence in heavy lanthanide metal holmium under high pressure

Pressure-induced structural phase transformations in heavy rare earth holmium have been studied in a diamond anvil cell using energy dispersive X-ray diffraction at a synchrotron source to pressures of 62 GPa at ambient temperature. The complete sequence of hcp -> Sm-type -> dhcp -> fcc -> dfcc ( hR24) is observed for the first time with transitions at 10, 19, 54, and 58 GPa, respectively. The fcc phase is not observed as a pure phase. Each phase transition is correlated to predicted changes in d-band occupation at high pressure. As with many other rare earths, the regions of stability of the several phases are observed to overlap, and significant hysteresis is observed. The measured volume compression for Ho is V/V-0 = 0.526 at 62 GPa.