  The GaV$_4$S$_{8-y}$Se$_y$ $(y = 0$ to $8)$ family of materials have been
synthesized in both polycrystalline and single crystal form, and their
structural and magnetic properties thoroughly investigated. Each of these
materials crystallizes in the $F\bar{4}3m$ space group at ambient temperature.
However, in contrast to the end members GaV$_4$S$_8$ and GaV$_4$Se$_8$, that
undergo a structural transition to the $R3m$ space group at 42 and 41 K
respectively, the solid solutions $(y = 1$ to $7)$ retain cubic symmetry down
to 1.5 K. In zero applied field the end members of the family order
ferromagnetically at 13 K (GaV$_4$S$_8$) and 18 K (GaV$_4$Se$_8$), while the
intermediate compounds exhibit a spin-glass-like ground state. We demonstrate
that the magnetic structure of GaV$_4$S$_8$ shows localization of spins on the
V cations, indicating that a charge ordering mechanism drives the structural
phase transition. We conclude that the observation of both structural and
ferromagnetic transitions in the end members of the series in zero field is a
prerequisite for the stabilization of a skyrmion phase, and discuss how the
absence of these transitions in the $y = 1$ to $7$ materials can be explained
by their structural properties.
