Optical rotation in RbTiOAsO₄(point groupmm2)

Measurement of optical rotation in RbTiOAsO 4 (RTA) with the tilter method resulted in an optical rotation of ρ 12 = +17(3) o /mm at a wavelength of 670nm, when a (100) sample was tilted about [001]. A tilt about [010] showed no rotation, as expected from the directional dependence of optical rotation calculated from the tensor in point group mm 2. The absolute Miller-indices of the sam-les were found using X-ray anomalous scattering. The calculations with a dipole-dipole model show that the As 5+ -ions in RTA correlate with the main structural contribution of the optical rotation. However, there seems to be an even larger intrinsic contribution due to the Ti 4+ – ions as a result of the distorted octahedral co-ordination with oxygen.