The compounds, Li3MZn5(BH4)(15), M = Mg and Mn, represent the first trimetallic borohydrides and are also new cationic solid solutions. These materials were prepared by mechanochemical synthesis from LiBH4, MCl2 + or M(BH4)2, and ZnCl2,..The compounds are isostnictural,. and their crystal structure was characterized by. in situ synchrotron radiation powder X-ray and neutron diffraction and DFT calculations While diffraction provides an average view of the;structure as hexagonal (a = 15.371(3), c = 8.586(2),A, space group P6(3)/mcm for.s.Mg7eompoUnd,:at roam. temperature); the DFT optimization of locally ordered models, 'suggests a related ortho-hexagonal cell. Ordered models that maximize Mg Mg -separation have the lowest DFT energy, suggesting that the hexagonal structure seen by diffraction is a superposition of three "; such orthorhombic structures in three orientations along the hexagonal c-axis. No conclusion about the coherent length of the orthorhombic structure can be however done. The framework in Li(3)MZns(BH4)(5) is of a new type. It contains channels built from face-sharing (BH4)6 octahedra. While X-ray and neutron powder diffraction preferentially localize lithium in the center of the octahedra, thus resulting in two weakly interconnected frameworks of a new type, the DFT calculations clearly favor lithium inside the shared triangular faces, leading to two interpenetrated mco-nets (mco-c type) with the basic tile being built from three tfa tiles, which is the framework type of the related bimetallic LiZn2(13H(4))(5). The new borohydrides Li3MZn5(BH4)(5) are potentially interesting as solid-state electrolytes, if the lithium mobility within the octahedral channels is improved by disordering the site via heterovalent substitution. From a hydrogen storage point of view, their application seems to be limited as the compounds decompose to three known metal borohydrides.