Evidence for the singlet-dimer ground state in an S = 1 antiferromag netic bond alternating chain

Susceptibility, ESR and magnetization measurements have been performed on si ngle crystals of an S=1 bond alternating chain compound: [Ni(333-tet)(\mu-NO _2)](ClO_4) (333-tet = N,N'-bis(3-aminopropyl)propane-1,3-diamine) and the c ompound doped with a small amount of Zn. We observed an anomalous angular de pendence in the Zn-doped sample. These behaviors are well explained by the m odel based on the VBS picture for the singlet-dimer phase. The picture impli es that the free spins of S=1 with a positive single-ion anisotropy are indu ced at the edges of the chains without forming the singlet-dimer.