Experimental

disorder in solvent or counterion; R factor = 0.029; wR factor = 0.076; data-to-parameter ratio = 11.1. A new non-centrosymmetric compound, trisodium tetra-niobium triarsenic nonadecaoxide, Na3Nb4As3O19, has been synthesized by a solid-state reaction at 1123 K. The structure consists of AsO4 tetrahedra and NbO6 octahedra sharing corners to form a three-dimensional framework containing two types of tunnels running along the c axis, in which the sodium ions are located. Na+ cations occupying statistically several sites, respectively, are surrounded by seven, six and four O atoms at distances ranging from 2.08 (1) to 2.88 (4) Å. The title structure is compared with those containing the same groups, viz. M2XO13 and M2X2O17 (M = transition metal, and X = As or P). Related literature For physical properties of this class of compound, see