High-pressure high-temperature behaviour of the lanthanide metals
Munro, Keith Alistair
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The high-pressure behaviour of the lanthanide series of metals has been the subject of study since the work of Percy Bridgman in the 1940s. Differences in said behaviour between the different lanthanide metals are attributed to the increasing occupation of the 4f electron shell as Z increases. Upon compression, or as Z decreases, the trivalent lanthanides (La to Lu, excluding Eu and Yb) undergo a common phase transformation sequence through various close packed structures: hcp → Sm-type (the structure adopted by samarium at ambient conditions) → dhcp → fcc → distorted fcc (d-fcc). Upon further compression, the lanthanide metals experience a first order transition to a "volume collapsed" phase. Many studies have focused on the low-Z members of the series, since the various phase transitions occur at much lower pressure where it is comparatively easy to collect high quality data. By contrast, the other members of the series have received comparability little attention, and there are even fewer reports of the structural behaviour of the lanthanide metals at high pressure and high temperature. This thesis contains the results of angle-dispersive x-ray powder diffraction experiments at high pressure and high temperature of the various members of the lanthanide metals. Ce has been the subject of many previous studies, but a systematic x-ray diffraction study of the fcc/d-fcc phase boundary has never been attempted. Furthermore, the location in P-T space of the high temperature fcc/bct/d-fcc triple point has only been inferred, due to the lack of data on the fcc/bct phase boundary at high temperature. The high-pressure high-temperature phase diagram of Ce is presented and discussed. La is unique amongst the lanthanide metals due to its empty 4f shell at ambient conditions. Despite this, La undergoes the common lanthanide transformation sequence up to the d-fcc phase, after which it undergoes a re-entrant transition back to the fcc phase at 60 GPa. The diffraction peaks of d-fcc La are shown in this thesis to undergo changes in intensity upon compression, indicating a transformation to the oI 16 structure found in Pr. La is one of the few elements whose behaviour has been unknown above 100 GPa, and results of La's structural behaviour upon compression to 280 GPa are presented and discussed. At 76 GPa, La begins a transition from the fcc phase to a new phase with the bct structure. Finally, the d-fcc→fcc re-entrant phase transition has been determined at various temperatures, and the d-fcc stability region has been mapped out. Finally, x-ray diffraction experiments were performed on Gd up to 100 GPa and ~700 K, to determine the structure of the d-fcc phase and the "volume collapsed" phase. While d-fcc Gd does not undergo pressure-induced changes similar to its low Z brethren, the d-fcc Gd remains stable up to 41 GPa at 700 K, putting a constraint on the d-fcc stability region. The data collected on Gd's "volume collapsed" phase cannot be fitted to the currently accepted mC4 structure. This has implications for our understanding of the lanthanide series as a whole, since most of of the heavier members, and some of the lighter lanthanides, are reported to adopt the mC4 structure.