The ordered rocksalt-type oxide Na_3Cu_2SbO_6 was synthesized and its magnetic properties were investigated. The broad peak in the temperature-dependent magnetic susceptibility data near 92 K is indicative of the dominant low-dimensional short-range antiferromagnetic (AF) behavior. The data are very well fitted with the AF-AF alternating linear chain model with J_1/k=-79 K and J_2/J_1=0.39. The high-temperature data (T > 300 K) exhibit Curie-Weiss behavior with a Weiss temperature of -55(2) K. These results are very similar to those reported for the isostructural oxide Na_2Cu_2TeO_6 J. Xu et al., Inorg. Chem. 44, 5042 (2005). Recently, it was shown Y. Miura et al., J. Phys. Soc. Jpn. 75, 847071 (2006) that an AF-ferromagnetic (F) linear chain model gives an equally good fit to the low-temperature data for both compounds and that further analysis of magnetic heat capacity data supports the AF-F model. We reinvestigate this proposal by computing the intersite hopping integrals using both the tight-binding spin dimer analysis and the N~(th)-order muffin-tin-orbital downfolding procedure for both compounds. The calculations support the AF-AF model for the anti-monide. Further, the Weiss temperatures derived from the high-temperature experimental data, T > 300 K, are also consistent with the J values derived from the AF-AF model but not with those obtained from the AF-F alternative.
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