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

We address the degeneracy of the ground state multiplet on the 5Reion in double perovskite BaMgReOusing a combination of specific heat measurements and density functional calculations. For BaMgReO, two different ground state multiplets have previously been proposed-a quartet (with degeneracy= 4) (Hirai and Hiroi 2019064712) and a doublet (= 2) (Marjerrison201610701). 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 multipletthroughSmag=Rln. 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 BaMgReOremains an open question.