Inelastic neutron scattering study of the anisotropic $S=1$ spin chain [Ni(HF${}_2$)(3-Clpyridine)${}_4$]BF${}_4$

[Ni(HF${}_2$)(3-Clpyridine)${}_4$]BF${}_4$ (NBCT) is a one-dimensional, $S=1$ spin chain material that shows no magnetic neutron Bragg peaks down temperatures of 0.1 K. Previous work identified NBCT to be in the Haldane phase and near a quantum phase transition as a function of $D\mathrm{/}J$ to the large-$D$ quantum paramagnet phase (QPM), where $D$ is the axial single-ion anisotropy and $J$ is the intrachain superexchange. Herein, inelastic neutron scattering results are presented on partially deuterated, ${}^{11}$B enriched NBCT polycrystalline samples in zero magnetic field and down to temperatures of 0.3 K. Comparison to density matrix renormalization group calculations yields $D/J = 1.51$andasignificantrhombicsingle-ionanisotropy$E$($E/D \approx 0.03$,$E/J \approx 0.05$)\mathrm{.}These$D$,$J$,and$E$ values place NBCT in the large-$D$ QPM phase but precipitously near a quantum phase transition to a long-range ordered phase.