  Static, charged black holes in the presence of a negative cosmological
constant and with a planar horizon are found in four dimensions. The solutions
have scalar secondary hair. We claim that these constitute the planar version
of the Martinez-Troncoso-Zanelli black holes, only known up to now for a curved
event horizon in four dimensions. Their planar version is rendered possible due
to the presence of two, equal and homogeneously distributed, axionic charges
dressing the flat horizon. The solutions are presented in the conformal and
minimal frame and their basic properties and thermodynamics analysed.
Entertaining recent applications to holographic superconductors, we expose two
branches of solutions: the undressed axionic Reissner-Nordstrom-AdS black hole,
and the novel black hole carrying secondary hair. We show that there is a
critical temperature at which the (bald) axionic Reissner-Nordstrom-AdS black
hole undergoes a second order phase transition to the hairy black hole
spontaneously acquiring scalar hair.
