Pressure effects on an S = 1 Haldane compound Ni(C₅H ₁₄N₂)₂N₃(PF₆)

The pressure effects on an S = 1 Haldane compound Ni(C₅H ₁₄N₂)₂N₃(PF₆) (NDMAP) have been investigated through the magnetic susceptibility (χ) measurements using single crystalline samples and the powder x-ray structural analysis experiment under pressure up to 11.5 kbar. The crystal-axis dependence of χ has been analyzed with the results of quantum Monte Carlo simulation. The results suggest that both the intrachain interaction (J) and single-ion anisotropy (D) are enhanced by pressure: J = 31.5 K (P = 0 kbar) → 90.0 K (10.0 kbar) and D/J = 0.3 (P = 0 kbar) → 0.4 (9.5 kbar). As for J, there is the linear increase against pressure as dJ/dP = 2.0 K/kbar in the pressure region below P = 8.0 kbar, at around which dJ/dP rapidly changes to dJ/dP = 2.1 × 10¹ K/kbar. The ratio of D/J changes within 0.3-0.4, and this result suggests that D as well as J drastically change at around P = 8.0 kbar. Furthermore, at P = 9.5 kbar, the development of paramagnetic moment on the easy plane is detected, and it is assumed that the interchain interaction may be also enhanced under pressure. The structural analysis shows that the interchain shrinkage is larger than the intrachain one. However, a result concerning the rapid changes of J and D at around P = 8.0 kbar has been not observed, and we suppose that the changes of bond-angles such as -N-Ni-N- and -N-N-N- as well as interatomic shrinkage probably change under pressure. We assume that those bond-angles of NDMAP at around P = 9.0 kbar (J ≃ 70 K) may become close to those of Ni(C₅H₁₄N₂)₂N ₃(ClO₄) (NDMAZ), where the value of J is 71.0 K.