Hydrothermally synthesized first acentric apatite (Bi₅Na₅)[PO₄]₆Cl₂ with wide Bi-Na isomorphism and doubled c-axis

New acentric bismuth-sodium apatite (Bi5Na(5))[PO4](6)Cl-2 with doubled c-axis and space group P6 is obtained first under mild hydrothermal conditions in excess of NaCl in solution. Structural solution was carried out on single crystal. The new apatite demonstrated wide heterovalence isomorphic substitution of Bi3+-Na+ in large cation positions, which allowed the final chemical formula to be established. This fact determines the doubling of the c-axis and the new space group P6 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 a structural viewpoint, optical nonlinearity is determined by the difference in the 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, and a group-subgroup relation is found for three apatite variants: one well known and two new.