  Single-crystal synchrotron x-ray diffraction measurements in strong magnetic
fields have been performed for magnetoelectric compounds GdMnO3 and TbMnO3. It
has been found that the P || a ferroelectric phase induced by the application
of a magnetic field at low temperatures is characterized by commensurate
lattice modulation along the orthorhombic b axis with q = 1/2 and q = 1/4. The
lattice modulation is ascribed to antiferromagnetic spin alignment with a
modulation vector of (0 1/4 1). The change of the spin structure is directly
correlated with the magnetic-field-induced electric phase transition, because
any commensurate spin modulation with (0 1/4 1) should break glide planes
normal to the a axis of the distorted perovskite with the Pbnm space group.
