Synthesis and characterisation of iron-containing perovskites

Description

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.

Links & Downloads

1. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.757919

Tags

Chemistry ; Perovskite

Additional Fields

Identifer	oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:757919
Date	January 2018
Creators	Tang, Yawei
Contributors	Battle, Peter
Publisher	University of Oxford
Source Sets	Ethos UK
Detected Language	English
Type	Electronic Thesis or Dissertation
Source	http://ora.ox.ac.uk/objects/uuid:993996ed-92ee-4ddf-963d-d98eafc99580