We successfully synthesized the zinc-verdazyl complex [Zn(hfac)2]-(o-Py-V) [hfac = 1,1,1,5,5,5-hexafluoroacetylacetonate; o-Py-V = 3-(2-pyridyl)-l,5-diphenylverdazyl], which is an ideal model compound with an S = j ferromagnetic-antiferromagnetic alternating Heisenberg chain (F-AF AHC). Ab initio molecularorbital (MO) calculations indicate that two dominant interactions 7F and 7AF form the S = | F-AF AHC in this compound. The magnetic susceptibility and magnetic specific heat of the compound exhibit thermally activated behavior below approximately 1 K. Furthermore, its magnetization curve is observed up to the saturation field and directly indicates a zero-field excitation gap of 0.5 T. These experimental results provide evidence for the existence of a Haldane gap. We successfully explain the results in terms of the S = \ F-AF AHC through quantum Monte Carlo calculations with |7AF/7 F| = 0.22. The ab initio MO calculations also indicate a weak AF interchain interaction 7' and that the coupled F-AF AHCs form a honeycomb lattice. The 7' dependence of the Haldane gap is calculated, and the actual value of 7' is determined to be less than 0.01|7F|.

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