  The isostructural series of $S=1$ quantum magnets
Cs$_{1-x}$Rb$_{x}$FeCl$_{3}$ is investigated, using both thermodynamic
measurements and inelastic neutron scattering experiments. It is found that
increasing Rb content the system evolves from the gapped state at $x=0$,
through a quantum phase transition at $x\sim 0.35$, and to the magnetically
ordered state at larger $x$. Inelastic neutron experiments for $x=0$, $x=0.3$,
and $x=1$ demonstrate that the magnetic anisotropy and spin interactions are
continuously tuned by the chemical composition. For the intermediate
concentration all magnetic excitations are substantial broadened suggesting
that disorder plays a key role in this species. For the two end-compounds,
excitations remain sharp.
