[en] M_r=142.10, trigonal, P31c, a=5.129, c=8.639 A, V=196.82 A³, Z=2, D_x=2.397 g cm^-3, λ(neutron)=1.176 A, μ_c=0.74 cm^-1, T=200 K. Refinement for the ordered structure with 593 reflections with (sinθ)/λ ≤ 0.57 A^-1 gave R(F²)=0.055. The study has revealed merohedral twinning of 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 somewhat better R factor. (orig.)