Аннотация:New acentric bismuth-sodium apatite (Bi5Na5)[PO4]6Cl2 with doubled c-axis and space group P-6, is obtained first in mild hydrothermal conditions in excess of NaCl in solution. Structural determination was carried out on single crystal, and new apatite demonstrates wide heterovalence isomorphic substitution Bi3+-Na+ in large cation positions. This fact determines doubling of c-axis and new space group P-6 compared with other widely known apatites. Non-centrosymmetry of the structure agrees with a distinct effect of optical second harmonic generation, but the SHG intensity is small due to proximity to the parent centrosymmetric apatite structure as well as lack of phase matching for SHG and YAG:Nd laser emission. From the structural point of view, optical nonlinearity is determined by difference in isomorphic occupation of positions by large cations with a slight deviation from the centrosymmetry of the whole structure. Disordered structural variants of isomorphic apatites are predicted, group-subgroup relation is found for three apatite varianst: well known and two new.