  The spin-orbit entangled quantum states in 4d/5d compounds, e.g., the
Jeff=1/2 and Jeff=3/2 states, have attracted great interests for their unique
physical roles in unconventional superconductivity and topological states.
Here, the key role of tetragonal distortion is clarified, which determines the
ground states of 4d1/5d1 systems to be the Jeff=3/2 one (e.g. K2NbCl6) or S=1/2
one (e.g. Rb2NbCl6). By tuning the tetragonal distortion via epitaxial strain,
the occupation weights of dxy/dyz/dxz orbitals can be subtly modulated,
competing with the spin-orbit coupling. Consequently, quantum phase transitions
between S=1/2 state and Jeff=3/2 state, as well as between different Jeff=3/2
states, can be achieved, resulting in significant changes of local magnetic
moments. Our prediction points out a reliable route to engineer new
functionality of Jeff states in these quantum materials.
