  The magnetic ground state of the quasi-one-dimensional spin-1
antiferromagnetic chain is sensitive to the relative sizes of the single-ion
anisotropy ($D$) and the intrachain ($J$) and interchain ($J'$) exchange
interactions. The ratios $D/J$ and $J'/J$ dictate the material's placement in
one or other of three competing phases: a Haldane gapped phase, a quantum
paramagnet and an XY-ordered state, with a quantum critical point at their
junction. We have identified [Ni(HF)$_2$(pyz)$_2]$SbF$_6$, where pyz =
pyrazine, as a candidate in which this behavior can be explored in detail.
Combining neutron scattering (elastic and inelastic) in applied magnetic fields
of up to 10~tesla and magnetization measurements in fields of up to 60~tesla
with numerical modeling of experimental observables, we are able to obtain
accurate values of all of the parameters of the Hamiltonian [$D = 13.3(1)$~K,
$J = 10.4(3)$~K and $J' = 1.4(2)$~K], despite the polycrystalline nature of the
sample. Density-functional theory calculations result in similar couplings ($J
= 9.2$~K, $J' = 1.8$~K) and predict that the majority of the total spin
population of resides on the Ni(II) ion, while the remaining spin density is
delocalized over both ligand types. The general procedures outlined in this
paper permit phase boundaries and quantum-critical points to be explored in
anisotropic systems for which single crystals are as yet unavailable.
