Structure and absolute optical chirality of thulium pyrogermanate crystals

Authors:

• K. Stadnicka,
• A.M. Glazer,
• Marceli Koralewski,
• B.M. Wanklyn

Abstract

The crystal structure and absolute optical chirality of Tm ₂Ge₂O₇ (M_r=595.04) have been determined by X-ray diffraction at room temperature. An optically laevorotatory crystal had space group P4₁2₁2(D₄⁴), with a=6.764(2), c=12.293(2) AA, V=562.4(4) AA³, Z=4, D _x=7.027 mg m^-3, Mo K alpha , lambda =0.71069 AA, mu =42.03 mm^-1, F(000)=1032. The final R-factors were R=0.0276 and wR=0.0369 for 1568 reflections. It is found that Tm³⁺ is unusual in being coordinated by seven oxygens. The refractive indices and optical rotatory dispersion have been measured. Using a point dipole polarisability theory it was possible to compute the optical rotatory power, and this was found to be in close agreement with the observed value. It is found that the main contribution to the optical rotation is from the O1, O2 and O3 oxygens, with O3 playing the major role.