Two biaxial crystals having mirror planes were experimentally investigated for optical activity. No rotation occurred in (C9H12O4)2, the only crystal of this type previously reported to be optically active. The previous report had been based on the peculiar pattern the crystal production in a polarizing microscope. In NaNO2, a ferroelectric crystal of class mm2, the plane of polarization was rotated 15.7°+0.5°/mm as 6328-Å light propagated down either axis. Down one axis the rotation was clockwise, down the other counterclockwise. This indicates that the structure of NaNO2 consists of a right- and left-handed helix with their axes oriented along the optical axes. The angles between the helices are bisected by mirror planes.
© 1970 Optical Society of AmericaFull Article | PDF Article