Anomalous Elastic Behavior in hcp- and Sm-Type Dysprosium

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 S 11 . 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.