  Inspired by the peculiarities of the effective geometry of crystalline
structures, we reconsider thick brane scenarios from a metric-affine
perspective. We show that for a rather general family of theories of gravity,
whose Lagrangian is an arbitrary function of the metric and the Ricci tensor,
the background and scalar field equations can be written in first-order form,
and tensorial perturbations have a non negative definite spectrum, which makes
them stable under linear perturbations regardless of the form of the gravity
Lagrangian. We find, in particular, that the tensorial zero modes are exactly
the same as predicted by Einstein's theory regardless of the scalar field and
gravitational Lagrangians.
