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.