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 5d1 Re6+ ion in double perovskite Ba2MgReO6 using a combination of specific heat measurements and density functional calculations. For Ba2MgReO6, two different ground state multiplets have previously been proposed - a quartet (with degeneracy N=4) [1] and a doublet (N=2) [2]. Here we employ two independent methods for the estimation of phonon contribution in heat capacity data to obtain the magnetic entropy Smag, which reflects the degeneracy of the ground state multiplet N through Smag = RlnN. In both cases, we obtain that in the temperature range covering 2 to 120 K the released entropy is better described by Smag = Rln2. The detailed nature of the ground state multiplet in Ba2MgReO6 remains an open question.