Abstract Ba 4Cu 2Al 3F 21 is orthorhombic : a = 5.299(1) Å, b = 7.318(1) Å, c = 41.529(7) Å, Z = 4. The crystal structure was solved in the space group P2 12 12 1 (n o19) from X-ray single crystal data using 5682 unique reflections (3698 with F O/σ(F) > 4). It consists in a succession along c of 8 layers of 2 different types of sheets. The first layer, formulated [Cu 2AlF 11] 4− in which copper-fluorine octahedra are linked by edges to form infinite distorted chains connected together by aluminium-fluorine octahedra, is followed by two [Al 2F 10] 4− layers of infinite cischains of aluminium-fluorine octahedra linked by two vertices and another [Cu 2AlF 11] 4− layer. These 4 layers are doubled along c by one of the screw-axes 2 1. The barium ions, 12-coordinated, are inserted between the sheets.