Possible quadrupole order in tetragonal Ba2CdReO6 and chemical trend in the ground states of 5d1 double perovskites.

The synthesis and physical properties of the double perovskite (DP) compound Ba2CdReO6 with the 5d1 electronic configuration are reported. Three successive phases originating from a spin-orbit-entangled Jeff = 3/2 state, confirmed by the reduced effective magnetic moment of 0.72 μB, were observed upon cooling. X-ray diffraction measurements revealed a structural transition from a high-temperature cubic structure to a low-temperature tetragonal structure at Ts = 170 K, below which the Jeff = 3/2 state was preserved. Magnetization, heat capacity, and thermal expansion measurements showed two more electronic transitions to a possible quadrupole ordered state at Tq = 25 K, and an antiferromagnetic order of dipoles accompanied by a ferromagnetic moment of ~ 0.2 μB at Tm = 12 K. These properties were compared with those of the sister compounds Ba2BReO6 (B = Mg, Zn, and Ca) and the chemical trend is discussed in terms of the mean-field theory for spin-orbit-coupled 5d electrons [G. Chen et al., Phys. Rev. B 82, 174440 (2010)]. The DP compound Ba2BReO6 provides a unique opportunity for a systematic investigation on symmetry breaking in the presence of multipolar degrees of freedom.