Abstract Single crystals of Ba6Fe0.90Nb9.10O30 and K4(Fe1.56Nb6.44)O21.4, were grown from high temperature solution at 1380°C and 1150°C, respectively, by the spontaneous nucleation method. Single-crystal X-ray diffraction confirmed that Ba6Fe0.90Nb9.10O30 belonged to the tetragonal tungsten-bronze (TTB) structure, with the space group P4/mbm (No. 127) and the lattice parameters a = 12.5895(3) and c = 3.98910(10) Å. In contrast, K4Fe1.56Nb6.44O21.4 did not crystallize in the TTB structure and had a orthorhombic lattice instead, of which the space group was Pnma (No. 62) and the lattice parameters were a = 6.5371(6), b = 3.8212(3), and c = 18.6967(16) Å. Crystal structure analysis showed that the centers of the anion polyhedra in both crystals had distorted from the sites of the cations. The estimation from the empirical equations showed that Ba6Fe0.90Nb9.10O30 was ferroelectric below room temperature, whereas the ferroelectric transition temperature for K4Fe1.56Nb6.44O21.4 was as high as 820°C. Preliminary magnetic characterization suggested that Ba6Fe0.90Nb9.10O30 was likely to be antiferromagnetic at low temperature, while K4Fe1.56Nb6.44O21.4 was probably paramagnetic down to 10K.