Multiferroic materials where ferromagnetism and ferroelectricity coexist are rare. We have succeeded in synthesizing new multiferroic materials Bi
2M MnO
6 (
M = Ni, Co) at high pressure of 6 GPa. The crystal structures of these compounds are a heavily distorted double-perovskite (space group
C2), with
M 2+ and Mn
4+ ions ordered in a rock-salt configuration. The monoclinic unit cell of Bi
2NiMnO
6 is
a = 9.4630(1),
b = 5.42553(8),
c = 9.5352(1) Å, and β= 107.840(1)°, and that of Bi
2CoMnO
6 is
a = 9.4886(2),
b = 5.4444(1),
c = 9.5678(2) Å, and β= 107.603(1)°. A polarized 6
s2 lone pair in Bi
3+ causes the noncentrosymmetric structural distortion, which leads to ferroelectric property. On the other hand, NaCl-type ordering of
M 2+ with
eg electrons and Mn
4+ without
eg electron produces ferromagnetic exchange paths, -
M 2+-O
2--Mn
4+-, according to the Kanamori-Goodenough rules. The ferroelectric and the ferromagnetic transition temperatures of Bi
2NiMnO
6 are 455 and 140 K, respectively. The ferroelectric transition temperature of Bi
2CoMnO
6 is above 600 K, while the ferromagnetic transition temperature is 95 K.
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