  Spin ordering in TbBaCo2O5.5 and its temperature transformation is studied.
Polymorphism due to the oxygen ordering with the average content close to 5.5
is investigated. One of ceramic samples (I), in addition to the main phase ap x
2ap x 2ap, Pmmm, (Z=2), contained about 25% of the phase ap x ap x 2ap, Pmmm,
(Z=1), with statistical distribution of oxygen over the apical sites, where ap
is parameter of perovskite cell. The other sample (II) contained only a main
phase with well defined octahedral and pyramidal sublattices. Treatment of
neutron diffraction patterns of the sample I itself gives a sophisticated spin
structure. Knowing the structure of sample II, one can chose only proper
magnetic lines, which give exactly the same results as for sample II. Above the
Neel temperature TN ~ 290K, there is a structural transition to the phase 2ap x
2ap x 2ap, Pmma. At TN, the spins order with the wave vector k19 = 0 (Phase 1).
At T1 ~ 255K, a magnetic transition takes place to the phase 2 with k22 = b3/2.
At T2 ~ 170K, the crystal structure changes to 2ap x 2ap x 4ap, Pcca (Z=4). The
wave vector of the spin structure becomes again k19 = 0 (Phase 3). The basis
functions of irreducible representations of the group Gk have been found. Using
results of this analysis, the magnetic structure in all phases is determined.
The spins are always parallel to the x axis, and the difference is in the
values and mutual orientation of the moments in the ordered non-equivalent
pyramidal or octahedral positions. Spontaneous moment at T = 260 K is due to
ferrimagnetic ordering of the moments in pyramidal sites.
(Dzyaloshinskii-Moriya canting is forbidden by symmetry.) The moment values
together with the ligand displacements are used to analyze the picture of
spin-state/orbital ordering in each phase.
