Peculiarities of crystal structure of terbium (III) trifluoroacetate trihydrate

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.