Synthesis, structure and magnetic properties of Na 6Co 2O 6

Abstract

Na 6Co 2O 6 was synthesized via the azide/nitrate route by reaction between NaN 3, NaNO 3 and Co 3O 4. Stoichiometric mixtures of the starting materials were heated in a special regime up to 500°C and annealed at this temperature for 50 h in silver crucibles. Single crystals have been grown by subsequent annealing of the reaction product at 500°C for 500 h in silver crucibles, which were sealed in glass ampoules under dried Ar. According to the X-ray analysis of the crystal structure ( P 1 ̄ , Z=1, a=5.7345(3), b=5.8903(3), c=6.3503(3) Å, α=64.538(2), β=89.279(2), γ=85.233(2)°, 1006 independent reflections, R 1=8.34% (all data)), cobalt is tetrahedrally coordinated by oxygen. Each two CoO 4 tetrahedra are linked through a common edge forming Co 2O 6 6– anions. Cobalt ions within the dimers, being in a high spin state ( S=2), are ferromagnetically coupled ( J=17 cm –1). An intercluster spin exchange ( zJ′=−4.8 cm –1) plays a significant role below 150 K and leads to an antiferromagnetically ordered state below 30 K. Heat capacity exhibits a λ-type anomaly at this temperature and yields a value of 19.5 J/mol K for the transition entropy, which is in good agreement with the theoretical value calculated for the ordering of the ferromagnetic-coupled dimers. In order to construct a model for the spin interactions in Na 6Co 2O 6, the magnetic properties of Na 5CoO 4 have been measured. This compound features isolated CoO 4 tetrahedra and shows a Curie–Weiss behavior ( μ=5.14 μ B, Θ=−20 K) down to 15 K. An antiferromagmetic ordering is observed in this compound below 10 K.