Fate of multipolar physics in 5d² double perovskites

In a cubic environment, the ground state of spin-orbit coupled 5d(2) ions is a non-Kramers E-g doublet, which hosts quadrupole and octupole moments. A series of 5d(2) osmium double perovskites Ba2MOsO6 (M = Mg, Ca, Zn, Cd) have recently been proposed to exhibit multipolar orders. We investigate the structural properties of these materials using ab initio calculations and find that the cubic structure is unstable for the Cd compound while the Mg, Ca, and Zn materials retain Fm (3) over barm symmetry. We show that Ba2CdOsO6 favors a rhombohedral R (3) over bar structure characterized by a(-)a(-)a(-) octahedral tiltings as indicated by unstable T-1g phonon modes. Trigonal distortions split the excited T-2g triplet into an E-g' doublet and an A(g) singlet, which may cross energy levels with the E-g doublet and suppress the multipolar physics. We find a window where E-g remains the lowest energy state under trigonal distortion, enabling the emergence of multipole phases in noncubic crystal environments.