Abstract. M r = 142.10, trigonal, P3 lc, a = 5.129, c = 8.639 A, V= 196.82 A 3, z = 2, D x = 2.397 g cm -3, 2(neutron) = 1.176 A, gc = 0.74 cm -~, T-200 K. Refinement for the ordered structure with 593 reflections with (sin0)/;t_ 0.57 A -1 gave R(F 2) = 0.055. The study has revealed merohedral twinning of *CENG DRF/Sph-MDN 85x, 38041 Grenoble CEDEX, France.0108-2701/88/040592-04503.00 the crystal about (110), with twin components in nearly equal proportion, and large thermal vibration for the oxygens. Displacing the oxygens into three disordered positions gave a somewhat better R factor.