Structural metastability and Fermi surface Topology of SrAl2Si2

Abstract

SrAl 2 Si 2 crystallizes into either a semimetallic, CaAl 2 Si 2 -type, α phase or a superconducting, BaZn 2 P 2 -type, β phase. We explore possible α Pc,Tc − −− → β transformations by employing pressure-and temperature-dependent free-energy calculations, vibrational spectra calculations, and room-temperature synchrotron X-ray powder diffraction (XRPD) measurements up to 14 GPa using diamond anvil cell. Our theoretical and empirical analyses together with all baric and thermal reported 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 metastable β phase by a relatively high activation barrier. This analysis is supported by our XRPD data at ambient temperature and P ≤ 14 GPa which shows an absence of β phase even after a compression involving three times the critical pressure. Finally, we briefly consider the change in Fermi surface topology when atomic rearrangement takes place via either transformations among SrAl 2 Si 2 -dimorphs or total chemical substitution of Ca by Sr in isomorphous α CaAl 2 Si 2 ; empirically, manifestation of such topology modification is evident when comparing the evolution of (magneto-)transport properties of members of SrAl 2 Si 2 -dimorphs and α isomorphs.