  The compounds A2Cu3O(SO4)3 (A=Na,K) are characterized by copper hexamers
which are weakly coupled along the b-axis to realize one-dimensional
antiferromagnetic chains below TN=3 K, whereas the interchain interactions
along the a- and c-axes are negligible. We investigated the energy-level
splittings of the copper hexamers by inelastic neutron scattering below and
above TN. The eight lowest-lying hexamer states could be unambiguously assigned
and parametrized in terms of a Heisenberg exchange Hamiltonian, providing
direct experimental evidence for an S=1 triplet ground-state associated with
the copper hexamers. Therefore, the compounds A2Cu3O(SO4)3 serve as novel
cluster-based spin-1 antiferromagnets to support Haldane's conjecture that a
gap appears in the excitation spectrum below TN, which was verified by
inelastic neutron scattering.
