Structural Metastability and Fermi Surface Topology of SrAl₂Si₂

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.