Ba₄Ga₂B₈O₁₈Cl₂·NaCl:  A New Polar Structure with NaCl Inclusion

The title compound, Ba₄Ga₂B₈O₁₈Cl₂·NaCl, is found to crystallize in a polar space group P4₂nm with cell dimensions of a = 12.1134(2) Å and c = 6.8456(1) Å. The basic building blocks of the structure are the B₄O₉ groups, which are interconnected by the GaO₄ tetrahedron to form a three-dimensional net with Ba²⁺ ion-, Cl^- ion-, and NaCl molecule-filled tunnels. This net structure is closely related to that of mineral hilgardite, with which many variant compounds have been found. Both a powder second-harmonic-generation test and calculations suggest that it possesses an optical nonlinearity comparable to that of potassium dihydrogen phosphate.