Bridging the Salt-Inclusion and Open-Framework Structures: The Case of Acentric Ag4B4O7X2 (X = Br, I) Borate Halides

An acentric borate family, Ag4B4O7X2 (X = Br, I), has been prepared by slow cooling stoichiometric melts in evacuated silica ampules. Their crystal structure is comprised of two porous interpenetrating frameworks and demonstrates a further development of the “salt-inclusion” architecture toward a “covalent-inclusion” structure. The (Ag2X)+ sublattice shows strong anharmonic vibrations. Thermal expansion is strongly anisotropic because of the presence of condensed rigid kernite boron–oxygen chains aligned perpendicular to the c axes.