CRYSTAL-STRUCTURE AND ABSOLUTE PIEZOELECTRIC D14 COEFFICIENT IN LEVOROTATORY BI12SIO20

Laevorotatory Bi12 SiO20 crystallizes in the cubic system with space group I23 and a = 10.10433±0.00005 Å at 298 K. The integrated intensities of 5488 reflections within a hemisphere of reciprocal space with (sinθ)/λ≤1.15 Å-1 were measured with monochromatic MoK α radiation. A total of 977 independent and significantly nonzero Fmeas were used to determine the crystal structure by the method of least squares. The final agreement factor R = 0.0316. The Si-O distance within the geometrically regular SiO4 tetrahedron is 1.647±0.005 Å. The Bi-O bond distances, ranging from 2.064±0.009 to 2.647±0.003 Å, with two additional contacts of 3.066 and 3.161 Å formed by the Bi inert 6s2 electron pair, are all within 0.025 Å of the corresponding distances in Bi 12GeO20. The absolute sense of the piezoelectric d 14 coefficient is such that tensile stress along [111] generates positive polarity on (111), the face toward which the oxygen apex of the SiO4 tetrahedra points, i.e. d14<0. The magnitude of d14 was determined in a static experiment to be 37.8±2. 0×10-12CN-1. © 1979 American Institute of Physics.