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 Sr_xBa_3-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 r_A/r_B 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.

Phase formation studies in the quaternary section Ca_2-<I>y</I>Sr<I>_y</I>Fe_2-<I>x</I>B'<I>_x</I>O<I>_γ</I> : B' = Nb, Ta, 0 ≤ <I>y</I> ≤ 2.0 and 0 ≤ <I>x</I> ≤ 2.0 were carried out. Results are summarised below for B' = Nb; for B' = Ta they were broadly similar. Four solid solution phases were found: i) a cubic solid solution, with a variable compositional extent <I>x</I>, which increased with substitution of Ca by Sr, from 0.45 ≤ <I>x</I> ≤ 0.65 for <I>y</I> = 0, to 0 ≤ <I>x</I> ≤ 1.2 for <I>y</I> = 2.0. ii) A limited orthorhombic solid solution along the Ca₂Fe₂O₅-Ca₂B'₂O₇ join with 0.8 ≤ <I>x</I> ≤ 1.0. iii) A complete range of orthorhombic, brownmillerite solid solution for <I>x</I> = 0 and iv) an orthorhomic, perovskite-related solid solution, isostructural with Ca₂Nb₂O₇, for <I>x</I> = 2.0. A combination of differential thermal analysis, thermogravimetry, Mössbauer spectroscopy, high temperature powder X-ray diffraction, Rietveld refinement, a.c. impedance spectroscopy and magnetic susceptometry has been used to characterise these new phases. Solid solution (i) has variable oxygen content with an oxygen-deficient perovskite structure. As synthesised, for <I>x</I> = 0.6 it contains a mixture of Fe⁴⁺ and Fe³⁺ in the ratio 13/87. The oxygen content could be modified by heat treatment under various atmospheres over the range 5.5. ≤ <I>γ</I> ≤ 5.92 which corresponded to the ratios: 93/7 to Fe³⁺/Fe²⁺ and 42/58 of Fe⁴⁺/Fe³⁺. At higher oxygen contents, <I>γ</I> > 5.7, transformation to an orthorhombic structure was observed. The electrical resistivity was dominated by grain boundary effects. Conduction is electronic and is attributed to hopping between iron in mixed oxidation states. The resistivity increased dramatically on reduction and became <I>n</I>-type, presumably due to a small fraction of Fe²⁺ present.

The investigation in this thesis focuses on the synthesis of cation-ordered perovskite phases by introducing anion vacancies into the structure. Complex cation-ordered phases Ba2YMO5 and Ba3YM2O7.5 (M = Fe, Co) have been synthesized using ceramic or citrate gel methods under flowing argon. Close inspection reveals that the structures are constructed from Y2M2O102 basic units which consist of two YO6 octahedra and two MO4 tetrahedra in a rock-salt type arrangement. In the structure of Ba2YMO5 (M = Fe, Co), the neighbouring Y2M2O102 units are connected with an equivalent one in the yz-plane with YO6 octahedra sharing an apex. In the structure of Ba3YM2O7.5 (M = Fe, Co), the basic units are connected to each other by the M2O7 dimers via a chain of Y – O – M – O – M – O – Y bonds. Complex cation ordering can be achieved by carefully controlling the anion vacancies and selecting the cations with different ionic radii. The anion vacancies present in Ba2YMO5 (M = Fe, Co) (space group P21/n) allow the intercalation of anions like O2- and F- into the lattice. The fluorination of Ba2YCoO5 leads to the formation of a new orthorhombic phase Ba2YCoO5F0.42 (space group Pbnm) in which the inserted fluoride ions are distributed in a disordered manner. In contrast, the topochemical oxidation of Ba2YFeO5 leads to the formation of a new orthorhombic phase Ba2YFeO5.5 (space group Pb21m), in which Fe4+ centres are located in 4-coordinate tetrahedral sites and 5-coordinate pyramidal sites, respectively. The polar structure of Ba2YFeO5.5 is confirmed by the observation of second-harmonic generation activity and pyroelectric behaviour. Ba2YFeO5.5 also exhibits a combination of ferromagnetic and antiferromagnetic behaviours at low temperature. LaCa2Fe2GaO8 adopts a six-layer structure consisting of an OOTLOOTR stacking sequence of layers of (Fe/Ga)O6 octahedra (O) and (Fe/Ga)O4 tetrahedra (T), related to that of the four-layer brownmillerite structure (space group Pbma). The chains of tetrahedra in the structure of LaCa2Fe2GaO8 exhibit a cooperative twisting distortion in which the twisting direction of the chains of tetrahedra alternates in adjacent tetrahedral layers. LaxSr2-xCoGaO5+δ (0.5 < x < 1) adopts brownmillerite structures which consist of octahedral and tetrahedral layers with mixed valence of Co2+/Co3+. The members with x = 0.5, 0.6 and 0.7 adopt structures with I2mb space group symmetry, in which all the tetrahedra twist in the same direction. The members with x = 0.8, 0.9 and 1.0 adopt structures with Imma space group symmetry, in which the chains of the tetrahedra twist in a disordered manner. A change in the Co3+ spin state from high spin (HS) to low spin (LS) is observed as the La/Sr ratio increases. The change of the Co3+ spin state can be rationalized on the basis of internal chemical pressure.