Experimental and theoretical thermodynamic studies in Ba₂MgReO₆-the ground state in the context of Jahn-Teller effect

We address the degeneracy of the ground state multiplet on the 5d¹Re⁶⁺ion in double perovskite Ba₂MgReO₆using a combination of specific heat measurements and density functional calculations. For Ba₂MgReO₆, two different ground state multiplets have previously been proposed-a quartet (with degeneracyN= 4) (Hirai and Hiroi 2019J. Phys. Soc. Japan88064712) and a doublet (N= 2) (Marjerrisonet al2016Inorg. Chem.5510701). Here we employ two independent methods for the estimation of phonon contribution in heat capacity data to obtain the magnetic entropySmag, which reflects the degeneracy of the ground state multipletNthroughSmag=RlnN. In both cases, we obtain that in the temperature range covering 2 to 120 K the released entropy is better described bySmag=Rln2. The detailed nature of the ground state multiplet in Ba₂MgReO₆remains an open question.