Structural metastability and Fermi surface Topology of SrAl2Si2

TL;DR

This study investigates the pressure- and temperature-induced phase transformations of SrAl2Si2, combining theoretical calculations and experimental data to map the transformation diagram and analyze changes in electronic structure.

Contribution

It provides the first combined theoretical and empirical analysis of SrAl2Si2 phase transformations and Fermi surface topology changes under pressure and temperature.

Findings

Low critical pressure (~5 GPa) for  to eta transition.

High activation barrier prevents phase transition at low P and T.

Fermi surface topology changes correlate with atomic rearrangements.

Abstract

SrAl2Si2 crystallizes into either a semimetallic, CaAl2Si2-type, \alpha phase or a superconducting, BaZn2P2-type, \beta phase. We explore possible \alpha --Pc;Tc--> \beta 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 \alpha to \beta 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 \alpha phase is separated from metastable \beta 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 \beta 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 SrAl2Si2-dimorphs or total chemical substitution of Ca by Sr in isomorphous \alpha CaAl2Si2; empirically, manifestation of such topology modification is evident when comparing the evolution of (magneto-)transport properties of members of SrAl2Si2-dimorphs and \alpha isomorphs.