Crystal Structure of Na2M2(BO3)2O (M=Al, Ga); Comparison with Other Layered Oxyborates and SiP2O7

A new oxyborate Na2Ga2(BO3)2O is evidenced. The mean crystal structure, determined by X-ray diffraction, is trigonal, space group P-31c (n°163), Z=2, with a=4.855(2) A, c=15.441(7) A, V=315.2(4) A3, R1=0.047, wR2=0.123. The three-dimensional network is built up from the stacking, along the c axis, of infinite [NaGa2B2O7]∞ sheets, separated by sodium layers. Na2Al2(BO3)2O is isostructural with Na2Ga2(BO3)2O. Ionic conductivity is expected.