High Pressure Phase Transformations in Heavy Rare Earth Metals and Connections to Actinide Crystal Structures
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1104-NN01-04
High Pressure Phase Transformations in Heavy Rare Earth Metals and Connections to Actinide Crystal Structures Yogesh K. Vohra1, Bagvanth Reddy Sangala1, Andrew K. Stemshorn1, and Kevin M. Hope2 1 Physics, University of Alabama at Birmingham (UAB), 310 Campbell Hall, 1300 University Boulevard, Birmingham, AL, 35294-1170 2 Biology, Chemistry, and Mathematics, University of Montevallo, Harman Hall, Station 6480, Montevallo, AL, 35115 ABSTRACT High-pressure studies have been performed on heavy rare earth metals Terbium (Tb) to 155 GPa and Holmium (Ho) to 134 GPa in a diamond anvil cell at room temperature. The Sm-type dhcp following crystal structure sequence was observed in both metals hcp monoclinic (C2/m) with increasing pressure. The last transformation to distorted fcc (hR-24) a low symmetry monoclinic phase is accompanied by a volume collapse of 5 % for Tb at 51 GPa and a volume collapse of 3 % for Ho at 103 GPa. This volume collapse under high pressure is reminiscent of f-shell delocalization in light rare earth metal Cerium (Ce), Praseodymium (Pr), and heavy actinide metals Americium (Am) and Curium (Cm). The orthorhombic Pnma phase that has been reported in Am and Cm after f-shell delocalization is not observed in heavy rare earth metals under high pressures.
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INTRODUCTION The appearance of low symmetry crystal structures with accompanying volume collapse under high pressures in f-elements is generally attributed to f-shell delocalization [1,2]. In light rare earth metals Cerium (Ce), Praseodymium (Pr), and Neodymium (Nd) low symmetry crystal structures like body centered tetragonal (bct), orthorhombic alpha-Uranium, and monoclinic C2/m crystal structures are observed under high pressures. Similarly, low symmetry crystal structures and volume collapse phase transformations have been reported in heavy actinide metals [3-6]. These low symmetry structures signal the participation of f-electrons in bonding. However, heavy rare earth metals have not been studied to extreme pressures to clearly establish the crystal structures trends and investigate any similarities with heavy actinide metals. In particular, it is of great interest to investigate the stability of an orthorhombic Pnma phase in heavy rare earth metals as this Pnma phase is well known to be a stable phase in heavy actinide metals under high pressures [3, 6]. EXPERIMENT The high-pressure angle-dispersive x-ray diffraction studies were carried out at the Advanced Photon Source (APS) beam-line ID-B. In both experiments on Tb and Ho, we employed a matched pair of beveled diamond anvils with a central flat of 100 m, with a bevel angle of 7 degrees, and a culet size of 300 m in diameter. The monochromatic x-ray beam utilized in the diffraction experiments had a wavelength = 0.3678 Å. Elemental Tb and Ho (99.9 % stated purity foil from Alfa Aesar) sealed under argon atmosphere were quickly loaded in a 50 m sample chamber along with a copper pressure marker [7]. The heavy rare earth
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metals are less reactive as compared to
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