  Magnetic structures of metastable perovskites ScCrO3, InCrO3 and TlCrO3,
stabilized under high-pressure and high-temperature conditions, have been
studied by neutron powder diffraction. Similar to the other orthochromites
LnCrO3 (Ln=lanthanide or Y), these materials crystallize into the orthorhombic
structure with Pnma10 symmetry. The spin configuration of the metastable
perovskites has been found to be C-type, contrasting with the G-type structure
usually observed in LnCrO3. First-principles calculations demonstrate that the
Ctype structure found in ScCrO3 and InCrO3 is attributed to a ferromagnetic
(FM) nearest-neighbor interaction, while in TlCrO3, this type of magnetic
ordering is stabilized by a strong next-nearest-neighbor antiferromagnetic
(AFM) exchange. The spins in the C-type magnetic structure line up along the
orthorhombic b-axis, yielding the Pnma magnetic symmetry. The dominant
mechanism controlling this spin direction has been concluded to be the single
ion anisotropy imposed by a uniaxial distortion of CrO6 octahedra.
