Synthesis and characterisation of layered oxypnictides and oxychalcogenides

This thesis reports the synthesis and characterisation of a series of layered oxyarsenides A2M3As2O2 (A=Sr, Ba and M = Cr, Mn) and layered oxychalcogenides Sr2MO3CuX (M = Cr, Mn, Fe and X= S, Se). These materials are synthesised with different choices of the transition metal cations and it has been demonstrated that the nature of the metals can strongly influence the magnetic properties of these layered solids.
Variable temperature powder neutron diffraction on Ba2CrO2Cr2As2 has revealed long range antiferromagnetic ordering of the Cr2+ ions in both oxide and arsenide layers below 248 K and 470 K and the magnetic moments in both layers are aligned along the c-axis, where the two independent Cr2+ sublattices order with two different propagation vectors k = (½ ½ 0) in the oxide layer and k = (1 1 1) in the arsenide layer. The system has similarities and differences with the Sr analogue.
The partial substitution of Cr2+ by Mn2+ in Sr2CrO2Cr2As2 and Ba2CrO2Cr2As2 has been shown to change their crystal structures and magnetic properties depending on the Mn:Cr ratio. Variable temperature neutron powder diffraction enabled the magnetic structures to be determined and the compounds with substitution all show antiferromagnetic ordering, analogous to those seen in their parent samples. The most characteristic feature of the magnetic properties in these compounds is the appearance of two similar magnetic structures, with the magnetic spins in adjacent arsenide (oxide) layers aligned either parallel or antiparallel. Similar to Sr2CrO2Cr2As2, a spin-reorientation of the moments is found in the arsenide layer of Sr2Cr0.93Mn0.07O2Mn0.92Cr1.08.
PND measurements have revealed long-range AFM ordering in the oxide layers of Sr2MnO3CuSe. In replacing Se2- by S2-, a new magnetic structure is found in Sr2MnO3CuS which is different in detail to that of the selenide analogue. The deintercalation of a small amount of Cu in Sr2MnO3CuS results in adoption of the same magnetic structure as found for the selenide analogue and leads to a much larger ordered moment (3.60(4) μB/Mn3+) than in stoichiometric Sr2MnO3CuS (2.83(6) μB/Mn3+), with a value approaching that of Sr2MnO3CuSe (3.76(2) μB/Mn3+). The different magnetic behaviour between Sr2MnO3CuSe and Sr2MnO3CuS suggests that the Mn3+ ions in sulfide are slightly reduced and this introduces some Mn2+ ions and leads to moment reduction due to frustration. The neighbouring moments in all cases are found to be antiferromagnetically coupled in a checkerboard fashion with their moments directed perpendicular to the ab-plane.
Investigations into Sr2FeO3CuSe have revealed a spin reorientation transition of Fe moments between 1.5 K and 230 K with the tilted Fe3+ spins in the oxide planes gradually reorienting to lie in the ab plane. Similarly, Sr2CrO3CuSe demonstrates antiferromagnetic ordering of Cr3+ in the oxide layers at 4 K with their moments aligned within the ab-plane.
Finally, the synthesis of a novel and challenging compound Sr2CrO3CrAs has been successful. Variable temperature powder neutron diffraction reveals long range antiferromagnetic ordering of the Cr2+ ions in the arsenide layer below 478 K, with their moments directed along the c-axis. In contrast to other Sr2CuGaO3S-type compounds, there is no evidence for long range magnetic ordering in the oxide layers of Sr2CrO3CrAs.

Advisor: Clarke, S