ARTICLE pubs.acs.org/JPCC
Anomalous Elastic Behavior in hcp- and Sm-Type Dysprosium Oliver Tschauner,*,† Ognjen Grubor-Urosevic,† Przemyslaw Dera,‡ and Sean R. Mulcahy§,^ †
High Pressure Science and Engineering Center, Department of Physics, University of Nevada, Las Vegas, Las Vegas, Nevada 891544002, United States ‡ Center of Advanced Radiation Sources, Argonne National Laboratory, University of Chicago, Argonne, Illinois 60439, United States § Department of Geoscience, University of Nevada, Las Vegas, Las Vegas, Nevada 89154-4002, United States ^ Department of Geoscience, University of California Berkeley, Berkeley, CA, United States ABSTRACT: The compression behavior of elemental dysprosium in the hcp- and the Sm-type phases has been examined under hydrostatic pressure. Sm-type Dy has been found about 1% denser than the hcp phase. This increase in density is due to c-axis contraction in Sm-type Dy, whereas the a-axis even expands compared with the hcp-phase. Both the hcp- and the Sm-type phases show an inversion in the pressure derivative of the c/a ratio. For hcp-Dy this inversion is very sharp with minimal c/a at 2.5 GPa. At the same pressure, the compression behavior of hcp-Dy changes abruptly from dominantly c-axis compression to almost isotropic compression with slightly softer S11. The bulk modulus increases at this point by a factor of ∼2. Both hcp- and Sm-type Dy exhibit a crossover from highly anisotropic compression mostly along the c-axis to almost isotropic compression. We discuss these anomalies with respect to a possible Lifshitz transition and structural soft modes.
I. INTRODUCTION The principal features of the phase diagram of lanthanides were worked out many years ago.1 4 The sequence of structures occurring with increasing compression of a lanthanide metal is also found at ambient conditions as a sequence of structures occurring along with decreasing atomic number. Thus, at a given pressure, the lanthanide contraction induces stabilization of structures that occur for lighter lanthanides at lower (or hypothetically negative) pressure; the phases found in light lanthanides at ambient pressure occur in heavier lanthanides at elevated pressure with transformation pressure increasing with atomic number. Duthie and Pettifor,5 Johansson and Rosengren,2 and subsequently Skriver6 successfully explained these systematics as result of partial occupancy of 5d-like bands hybridized with the valence 6s-like bands. This regular sequence of lanthanide phase transformations suffers some exceptions4 because of magnetism (Eu, Yb) and contributions from f electrons to the valence state (Ce). The gradual change in chemical potential with pressure accounts for many of the interesting properties of lanthanides. In particular, it is worthwhile examining the detailed response of elastic properties to the changes in electronic density of state at Fermi level and to eventual topological changes of the Fermi surface. Here, we revisit the transition from the hcp- to Sm-type structure in one of the heavier lanthanides: dysprosium. Earlier compression studies on Dy mostly focused on the high-pressure phases,7,8 and the compression behavior of the hcp- and the Smtype phases is presently constrained only by a few data points. We show the occurrence of elastic anomalies in both phases in the absence of observable structural changes. In hcp-type dysprosium, these elastic anomalies are accompanied by sharp changes in the pressure derivative of the c/a ratio. r 2011 American Chemical Society
II. EXPERIMENTAL SECTION Dy specimens of 10 150 μm edge length and hexagonal prismatic or rhombohedral crystal habit were selected from aliquots of Dy metal crystallized in tantalum crucibles. The sample material was analyzed with a Jeol JXA-8900 Superprobe electron probe microanalyzer (EPMA) using 20 keV accelerating voltage and 10 nA beam current and found to have a purity of >97% (Dy 99.73%, Ce 0.08%, Nd 0.06%, Eu 0.06%, Sc 0.02%, all other impurities
