Preparation, crystal structure and thermal expansion of a novel layered borate, Ba2Bi3B25O44

► A novel borate Ba2Bi3B25O44 has been synthesized by cooling a non-stoichiometric melt. ► The crystal structure is based upon a novel borate layered anion (∼13 Å thick). ► Anisotropy of thermal expansion increases on heating. ► Anisotropy dictated by the orientation of triborate groups.