  Based on the first-principles calculations within local density approximation
and model analysis, we propose that the iso-structural compounds Cu2SnS3 and
Cu2SnSe3 are both the simplest nodal line semimetals with only one nodal line
in their crystal momentum space when spin-orbit coupling (SOC) is ignored. The
including of SOC drives Cu2SnS3 into a Weyl semimetal (WSM) state with only two
pairs of Weyl nodes, the minimum number required for WSM with time reversal
symmetry. In contrast, SOC leads Cu2SnSe3 to strong topological insulator (TI)
state. This difference can be well understood as there is a topological phase
transition (TPT). In it, the Weyl nodes are driven by tunable SOC and
annihilate in a mirror plane, resulting in a TI. This TPT, together with the
evolution of Weyl nodes, the changing of mirror Chern numbers of mirror plane
and the Z2 indices protected by time-reversal symmetry has been demonstrated by
the calculation of Cu2SnS3-xSex within virtual crystal approximation and an
effective k $\cdot$ p model analysis. Though our first-principles calculations
have overestimated the topological states in both compounds, we believe that
the theoretical demonstration of controlling TPT and evolution of Weyl nodes
will stimulate further efforts in exploring them.
