  Crystal and magnetic structures of the high-pressure stabilized perovskite
phase of TlMnO3 have been studied by neutron powder diffraction. The crystal
structure involves two types of primary structural distortions:
a+b-b-octahedral tilting and antiferrodistortive type of orbital ordering,
whose common action reduces the symmetry down to triclinic P -1. The orbital
pattern and the way it is combined with the octahedral tilting are different
from the family of LnMnO3 (Ln = lanthanide or Y) manganites who share with
TlMnO3 the same tilting scheme. The experimentally determined magnetic
structure with the k = (1/2,0,1/2) propagation vector and P_S-1 symmetry
implies anisotropic exchange interactions with a ferromagnetic coupling within
the (1,0,-1) planes and an antiferromagnetic one between them (A type). The
spins in the primary magnetic mode were found to be confined close to the
(1,0,-1) plane, which underlines the predominant role of the single ion
anisotropy with the local easy axes of Mn3+ following the Jahn-Teller
distortions of the octahedra. In spite of the same octahedral tilting scheme in
the perovskite structures of both LnMnO3 and TlMnO3 manganites, a coupling of
the secondary ferromagnetic component to the primary A-type spin configuration
through antisymmetric exchange interaction is allowed in the former and
forbidden in the latter cases.
