The structural phase transitions of americium under pressure were re-investigated. While 18 GPa was the highest pressure attained in previous work, the pressure range was extended up to 52 GPa in the present study. Important differences with previous studies were observed in three respects:
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phase Am III cannot be indexed in the monoclinic structure assigned to it previously. The resulting volumes are too large and would correspond to a widening of the lattice by an increasing-pressure phase transition. This contradiction is lifted when Am III is indexed as a trigonal distortion of the cubic close-packed Am II.
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5f delocalisation is suggested to occur only at 23 GPa, instead of the 10-12 GPa assumed on the basis of previous theoretical and experimental study. This view is supported by three facts: the absence of a volume discontinuity at the transition from Am II (ccp) to Am III (trigonal) at 13.5 GPa; a volume decrease of approximately 6% at the transition from Am III to Am IV (orthorhombic) at 23 GPa; the crystallographic similarity between Am II and Am III and the fact that the structure assigned to the latter phase also occurs in yttrium and lanthanum under pressure which both have no f electrons.
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The three phase transitions occurred at higher pressures than previously reported. The largest difference is found for the Am III to Am IV transition which was observed at 23 GPa in the present work, while earlier work placed it at 15 GPa.
These results contribute to the systematics of the high pressure behaviour of the f metals, in particular if they are discussed in the context of recent work on other heavy actinides, on cerium, and on other light lanthanides.