Structural distortion and chemical bonding in TlFeO3: Comparison with AFeO(3) (A = rare earth)

The crystal structure and the magnetic properties for thallium orthoferrite , TlFeO3, were characterized and compared with those of rare earth orthofer rites, AFeO(3) (A = rare earth). TlFeO3 has a GdFeO3-type perovskite struct ure (a = 5.3172(2) Angstrom, b = 5.4465(2) Angstrom, and c = 7.7927(3) Angs trom; space group, Pbnm). Its cell parameters and unit-cell volume consider ably deviate from a trend observed in the rare earth orthoferrites, which a re attributed to a specific coordination of Tl3+ ion. According to magnetiz ation measurements, TlFeO3 shows an antiferromagnetic behavior accompanied with weak ferromagnetism. The magnetic ordering temperature, T-N (560 K), f or TlFeO3 is lower than that for any other orthoferrite while the Fe-O-Fe s uperexchange angle (144.0 degrees) in TlFeO3 is comparable to that in ErFeO 3. The Mossbauer spectrum reveals a single Fell quadrupole doublet with an isomer shift delta = 0.18 +/- 0.02 mm/s and a quadrupole splitting value De lta = 0.47 +/- 0.02 mm/s at 623K (>T-N), whereas it shows a magnetic hyperf ine sextet with delta = 0.47 mm/s and a hyperfine field H-hf(Fe3+) = 535 kO e at 80 K (<T-N). The magnetism and the Mossbauer results underline a signi ficant competing effect of the Tl-O bond with the covalency of the Fe-O one . Despite the anomalous character of TlFeO3 in structure and magnetism, it is found that the TN values for all orthoferrites, including TlFeO3, are st rongly correlated with their crystallographic axial ratio, cl root 2a. Thro ugh this type of analysis, the electronic effects of A cation on structural distortion and magnetic behavior in perovskite are discussed., (C) 2001 Ac ademic Press.