KNb 4AsO 13 : Mr = 693,64 crystallizes in the orthorhombic system, space group: Cmcm, a = 10,477(4), b = 10,312(4), c = 10,006(4)Å, Z = 4. The crystal structure was refined from 760 independent reflections. The final R index and weighted Rw index are 0,026 and 0,030, respectively. The three-dimensional network is constituted of infinite chains (NbO 6NbO 6AsO 4) n parallel to the a axis and linked to each other by pairs of edge-sharing octahedra. Each pair links together four infinite chains. The potassium ions occupy interconnected cavities parallel to the c axis with an occupancy rate equal to 50%.