Crystal structure and absolute piezoelectric d14 coefficient in laevorotatory Bi12SiO20

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Abstract
Laevorotatory Bi12 SiO20 crystallizes in the cubic system with space group 123 and a=10.10433±0.00005 Å at 298 K. The integrated intensities of 5488 reflections within a hemisphere of reciprocal space with (sinϑ/γ?1.15 Å−L were measured with monochromatic MoKα 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 3.06 and 3.161 Å formed by the Bi inert 6s2 electron pair are all within 0.025 Å of the corresponding distances Bi12 GeO2. The absolute sense of the piezoelectric d14 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.,d1450. The magnitude of d14 was determined in a static experiment to be 37.8±2.0x10−12CN−1.