  Impurity substitution is a general strategy to study the intrinsic properties
of a quantum material. However, when the target element has more than one
Wyckoff position in the lattice, it is a big challenge but with extreme
necessity to know the exact position and order of the occupancy of impurity
atoms. Via comprehensive experimental and theoretical investigations, we
establish herein the roadmap for Ni substitution in Fe3GaTe2, a van der Waals
ferromagnet with the Curie temperature TC even reaching ~ 380 K. The results
unambiguously reveal that in (Fe1-xNix)3GaTe2, Ni atoms initially form an van
der Waals interlayer gap Ni3 sites when x < 0.1, and then gradually occupy the
Fe2 sites. After replacing the Fe2 sites at x of ~ 0.75, they start to
substitute for the Fe1 sites and eventually realize a full occupation at x =
1.0. Accordingly, TC and saturation magnetic moments of (Fe1-xNix)3GaTe2 both
show nonlinear decrease, which is tightly tied to the Ni occupancy order as
well as the different roles of Ni3, Fe1 and Fe2 sites in the spin Hamiltonian.
The results not only yield fruitful insights into the essential roles of
different Fe sites in producing the above room temperature high TC, but also
set a paradigm for future impurity substitution study on other quantum
materials.
