Pressure-induced structural phase transformations in a trivalent rare-earth metal praseodymium ͑Pr͒ were studied at room temperature in a diamond anvil cell to 103 GPa by energy dispersive x-ray diffraction using a synchrotron source. Our x-ray diffraction studies document the following crystal structure sequence: dhcp→fcc→distorted fcc(hR24 type)→monoclinic(C2/m)→␣-uranium with increasing pressure. We measure a 16.7% volume collapse at the transition to the ␣-uranium phase at 20 GPa. The high-pressure ␣-uranium phase in Pr was found to be stable to the highest pressure of 103 GPa, which corresponds to a volume compression V/V 0 ϭ0.407.
Ytterbium (Yb), a member of the rare earth group metals with a filled f shell and a divalent electronic character, has been studied under static high pressures in a diamond anvil cell to 202 GPa. We report a new transformation from a face-centered-cubic structure to a hexagonal phase with 3 atoms͞cell (space group P3 1 21 referred to as hP3) at 98 6 5 GPa. This hP3 phase is similar to that observed in trivalent neodymium and samarium at pressures near 40 GPa. Our high pressure study thus clearly demonstrates that Yb is no longer an "irregular" member of the rare earth metals, and divalent to trivalent electronic transition is completed at megabar pressures. [S0031-9007(99)08553-1]
Static high pressure diamond anvil cell experiments were performed on three polycrystalline Zr samples having varying interstitial impurity concentrations. Systematic increase in transition pressure with the increase in the amount of interstitial impurities is observed for the martensitic α →ω structural phase transition in Zr. Significant room temperature crystal grain growth is also observed for the two highest purity samples at the α →ω transition. In the case of the lowest purity sample interstitial impurities obstruct the α →ω transition, while possibly helping impede grain growth-even as the sample is heated to 1279 K.
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