Comment on Microscopic magnetic modeling for the S = 1/2 alternating   chain compounds Na3Cu2SbO6 and Na2Cu2TeO6 (arXiv:1402.1091)
  In a recent paper by Schmitt et al. (arXiv:1402.1091), signs of two exchange interactions of one-dimensional alternating chains in distorted honeycomb systems of Na3Cu2SbO6 and Na2Cu2TeO6 are argued by theoretical calculations. Although the authors report that they have clarified that the interaction J1a is ferromagnetic between the spins within a structural dimer, and J1b is antiferromagnetic between these dimers, these results have been known for a long time by our two papers, which are cited by Schmitt et al. 
Masatoshi Sato and Yoshiaki Kobayashi
cond-mat.str-el