Synthesis and characterisation of iron-containing perovskites
Keywords: Perovskite

In this research project, iron-containing perovskites with the general formulae A3Fe2B’O9, A2A’Fe2B’O9 and A2A’FeB’B”O9 have been synthesised using conventional solid-state reactions. A combination of experimental techniques has been applied to characterise the samples. The main aspects studied include their crystal symmetry, cation ordering pattern and magnetic behaviour.
We have shown for the first time that Sr3Fe2TeO9 can adopt a trigonal perovskite-like structure with the Fe3+ and Te6+ cations ordering in a 2:1 sequence. However, the trigonal structure is disrupted by both nanotwinning and regions where the cations order in a 1:1 sequence. These disruptions prevent full antiferromagnetic ordering throughout the sample and the unordered spins form a spin-glass phase that coexists below 80 K. The identification of this disorder has allowed us to account for inconsistencies in the existing literature.
Ba3Fe2TeO9 is a 6H perovskite in which the cation sites in the face-sharing octahedra and the vertex-sharing octahedra are occupied by different fractions of Fe3+. At a temperature close to 300 K the atomic moments begin to order in an antiferromagnetic manner, and the spins that are unable to take part in the long-range magnetic order form clusters that freeze at 18 K. In the series SrxBa3-xFe2TeO9 (x=1~2.5), as x decreases the crystal structure switches from purely pseudo-cubic to purely hexagonal via a biphasic region, illustrating the effect of the radius ratio rA/rB on crystal structure.
A2LaFe2SbO9 (A=Ba, Sr, Ca) and CaLa2Fe2SnO9 have cation disorder on their A sites. Ca2LaFe2SbO9 and CaLa2Fe2SnO9 both adopt a monoclinic P21/n structure and Sr2LaFe2SbO9 adopts a triclinic P -1 structure, all of which show different levels of B-cation ordering. Ba2LaFe2SbO9 has a Pbnm structure with disordered B sites. These perovskites are not paramagnetic at 300 K, and they all adopt a G-type magnetic structure, which leads to long-range ferrimagnetic or weak ferromagnetic behaviour. The three antimonycontaining compounds contain a minor spin-glass-like phase below 50 K, while CaLa2Fe2SnO9 has a relatively well-developed magnetic backbone at 300 K.
We found that the six-coordinate cations in the P21/n perovskite SrLa2FeCoSbO9 order in a previously unreported manner. The observed cation distribution, with diamagnetic Sb5+ and magnetic Co2+ each partially occupying only one of the six-coordinate sites, results in ferrimagnetism below the Curie temperature of 215 K. CaLa2FeCoSbO9 and ALa2FeNiSbO9 (A=Ba, Sr, Ca) were prepared as analogues of SrLa2FeCoSbO9, and they have similar crystal structure and high Curie temperatures. However, TEM revealed the different levels of inhomogeneity present in these four compounds. The inhomogeneity is least significant in CaLa2FeCoSbO9, and it is most significant in BaLa2FeNiSbO9 where both primitive phase and body-centred phases have been observed in a single crystallite. Consequently, doubt has been cast on the interpretation of the diffraction data for these inhomogeneous samples.
A2LaFe2NbO9 (A=Sr, Ca) and CaLa2Fe2TaO9 were prepared with d0 cations for comparison with the perovskites containing d10 B-cations. Ca2LaFe2NbO9 and CaLa2Fe2TaO9 adopt the P21/n structure, and Sr2LaFe2NbO9 adopts the P -1 structure. These three perovskites show less well-developed ordering pattern than their antimony analogues due to the smaller difference in size of B-cations. They are not simple paramagnets at 300 K, and they all adopt a G-type magnetic structure with long-range ferrimagnetism. However, the formation of magnetic backbone is significantly slowed down from that in the d10 compounds and the temperature for the paramagnetic spins to freeze is lowered below 20 K. We propose that in these materials a J3 interaction occurs via the Fe3+ – O – Nb5+/Ta5+ – O – Fe3+ pathway to compete with the dominant J1 interaction. The J3 interaction is more significant when d0 cations are present because hybridisation of the empty d orbitals and the anion p orbitals facilitates virtual electron transfer.