Structural Metastability and Fermi Surface Topology of SrAl₂Si₂

Abstract

SrAl2Si2 crystallizes into either a semimetallic, CaAl2Si2-type, α phase or a superconducting, BaZn2P2-type, β phase. We explore possible transformations by employing pressure- and temperature-dependent free-energy calculations, vibrational spectral calculations, and room-temperature synchrotron powder X-ray diffraction (PXRD) measurements up to 14 GPa using a diamond anvil cell. Our theoretical and empirical analyses together with all reported baric and thermal events on both phases allow us to construct a preliminary P–T diagram of transformations. Our calculations show a relatively low critical pressure for the α-to-β transition (4.9 GPa at 0 K, 5.0 GPa at 300 K, and 5.3 GPa at 900 K); nevertheless, our nonequilibrium analysis indicates that the low-pressure low-temperature α phase is separated from a metastable β phase by a relatively high activation barrier. This analysis is supported by our PXRD data at ambient temperature and P ≤ 14 GPa, which shows an absence of the β phase even after a compression involving three times the critical pressure. Finally, we briefly consider the change in the Fermi surface topology when atomic rearrangement takes place via either transformations among SrAl2Si2 dimorphs or total chemical substitution of Ca by Sr in the isomorphous CaAl2Si2 α phase; empirically, the manifestation of such a topology modification is evident upon comparison of the evolution of the (magneto)­transport properties of members of SrAl2Si2 dimorphs and α isomorphs.