Abstract
We have identified Y
2BaNiO
5 as a new Haldane state chain compound with a magnetic excitation gap Δ
min/k
B of 100±5K and Δ
min/|J|≈0.3. Single crystal results of χ(T→0) reveal a splitting of the excited magnetic state. The S=1 chains have been severed in a controlled way by the substitution of Zn or Mg for Ni, and the resulting modifications of χ(T) are presented.