The crystal and magnetic structures of polycrystalline BiCoO 3 have been determined by the Rietveld method from neutron diffraction data measured at temperatures from 5 to 520 K. BiCoO 3 (space group P4mm; Z ) 1; a ) 3.72937(7) Å and c ) 4.72382(15) Å at room temperature; tetragonality c/a ) 1.267) is isotypic with BaTiO 3 and PbTiO 3 in the whole temperature range. BiCoO 3 is an insulator with a Ne ´el temperature of 470 K. A possible model for antiferromagnetic order is proposed with a propagation vector of k ) ( 1 / 2 , 1 / 2 , 0). In this model, magnetic moments of Co 3+ ions are parallel to the c direction and align antiferromagnetically in the ab plane. The antiferromagnetic ab layers stack ferromagnetically along the c axis, forming a C-type antiferromagnetic structure. Refined magnetic moments at 5 and 300 K are 3.24(2)µ B and 2.93(2)µ B , respectively. The structure refinements revealed no deviation from stoichiometry in BiCoO 3 . BiCoO 3 decomposed in air above 720 K to give Co 3 O 4 and sillenite-like Bi 25 -CoO 39 .

Structural properties of polycrystalline single-phased BiMnO3 samples prepared at 6 GPa and 1383 K have been studied by selected area electron diffraction (SAED), convergent beam electron diffraction (CBED), and the Rietveld method using neutron diffraction data measured at 300 and 550 K. The SAED and CBED data showed that BiMnO3 crystallizes in the centrosymmetric space group C2/c at 300 K. The crystallographic data are a = 9.5415(2) A, b = 5.61263(8) A, c = 9.8632(2) A, beta = 110.6584(12) degrees at 300 K and a = 9.5866(3) A, b = 5.59903(15) A, c = 9.7427(3) A, beta = 108.601(2) degrees at 550 K, Z = 8, space group C2/c. The analysis of Mn-O bond lengths suggested that the orbital order present in BiMnO3 at 300 K melts above TOO = 474 K. The phase transition at 474 K is of the first order and accompanied by a jump of magnetization and small changes of the effective magnetic moment and Weiss temperature, mueff = 4.69 microB and theta = 138.0 K at 300-450 K and mueff = 4.79 microB and theta = 132.6 K at 480-600 K.