Experimental Studies of S=1 Heisenberg Planar Antiferromagnetic Chain Ni(C$\text{}_{11}$H$\text{}_{10}$N$\text{}_{2}$O)$\text{}_{2}$Ni(CN)$\text{}_{4}$

In this paper we discuss the results of experimental studies of specific heat and magnetic susceptibility of chain compound Ni(C$\text{}_{11}$H$\text{}_{10}$N$\text{}_{2}$O)$\text{}_{2}$Ni(CN)$\text{}_{4}$. We have observed a Schottky type anomaly at T=2.2 K in a temperature dependence of specific heat and the compound was identified as a S=1 Heisenberg antiferromagnetic chain with the subcritical exchange coupling D/k$\text{}_{B}$=5 K, D/|J|=5.2 and the strong in-plane anisotropy E/k$\text{}_{B}$=2.8 K. It has been found that the in-plane anisotropy affects the region of validity of a diluted exciton model [1] and a strong coupling model [2] for S=1 Heisenberg chains with the strong planar anisotropy. The analysis suggests that the in-plane anisotropy should be considered in any attempt to find a compound potentially located in the boundary of Haldane and "large-D" phases.