  A comparative study of the crystalline and molecular structures of
trifluoroacetate trihydrates of Tb(III) prepared from Tb(OH)_3 and Tb_2(CO_3)_3
by reaction with trifluoroacetic acid is made. Elemental analysis of the two
reaction products in indicated them to be compounds of the same C_6H_6F_9O_9Tb.
According to the X-ray structural analysis the molecular structure of the first
product is described by the average statistical formula
Tb_2(CF_3COO)_5(CF_3COOH)(H_2O)_5(OH) (Tb-I) and the compound is a dimer with a
centre of inversion. In the second case the product is also a centrosymmetric
dimer but its formula is Tb_2(CF_3COO)_6(H_2O)_6 (Tb-II). The compounds only
difference is in the crystallographic positions of their hydrogen atoms. A
luminescent analysis using Tb^{3+} ^5D_4-> ^7F_0 transition indicated the
presence within the structure of Tb-I of the structure of Tb-II. Magnetic
measurements showed that Tb-II has an antiferromagnetic ordering at low
temperatures. In the case of Tb-I there also appears a ferromagnetic ordering.
On the basis of the available experimental results such a behaviour was
explained to be the result of some peculiar features of the hydrogen bonds in
the molecules.
