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Abstract

Na6Co2O6 was synthesized via the azide/nitrate route by reaction
between NaN3, NaNO3 and Co3O4. Stoichiometric mixtures of the starting
materials were heated in a special regime up to 500degreesC and
annealed at this temperature for 50 It in silver crucibles. Single
crystals have been grown by subsequent annealing of the reaction
product at 500degreesC for 500 It in silver crucibles, which were
sealed in glass ampoules under dried Ar. According to the X-ray
analysis of the crystal structure (P (1) over bar, Z = 1, a =
5.7345(3), b = 5.8903(3), c = 6.3503(3) Angstrom, alpha = 64.538(2),
beta = 89.279(2), gamma = 85.233(2)degrees, 1006 independent
reflections, R-1 = 8.34% (all data)), cobalt is tetrahedrally
coordinated by oxygen. Each two CoO4 tetrahedra are linked through a
common edge forming Co2O66- 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 lambda-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 Na6Co2O6,
the magnetic properties of Na5CoO4 have been measured. This compound
features isolated CoO4 tetrahedra and shows a Curie-Weiss behavior (mu
= 5.14 mu(B), Theta = -20 K) down to 15 K. An antiferromagmetic
ordering is observed in this compound below 10 K. (C) 2004 Published by
Elsevier Inc.