Simon Cassidy

Abstract:

© 2020 Elsevier Inc. Solid solutions between the known compounds Ca2FeO3CuCh and Sr2FeO3CuCh (Ch ​= ​S, Se) in which there are two fairly similar sites (8 and 9 coordinate) for the alkaline earth cations are not attainable under standard high temperature solid state syntheses under thermodynamic control. Instead compounds with greater condensation of FeO5 square pyramids form as these afford one 8-coordinate site and one 12-coordinate site for the alkaline earths which is better suited to the size-mismatched cations in the compounds Sr3-xCaxFe2O5Cu2Ch2 (Ch ​= ​S, Se; x ​= ​1, 2). Sr2CaFe2O5Cu2S2, SrCa2Fe2O5Cu2S2, Sr2CaFe2O5Cu2Se2 and SrCa2Fe2O5Cu2Se2 all crystallise in the tetragonal space group I4/mmm with two formula units in the unit cell with the crystal structure first described for Sr3Fe2O5Cu2S2. Oxide slabs composed of vertex-sharing FeO5 square pyramids are separated by Cu2Ch2 anti-fluorite-type layers. The larger Sr2+ ions have a strong preference for the 12-coordinate site in the oxide slabs, while Ca2+ cations dominate the 8-coordinate sites separating the oxide and chalcogenide slabs. Powder neutron diffraction reveals that all the compounds display antiferromagnetic long range ordering of the Fe3+ moments with ordering temperatures well above room temperature and exceeding 526 ​K in the case of Ca2SrFe2O5Cu2Se2.

Abstract:

© 2020 Elsevier Inc. Polycrystalline samples of the monoclinic perovskites CaLa2Ni2-xCuxWO9 (x ​= ​0.25 and 0.5) have been prepared and characterised by neutron diffraction and magnetometry. The Ni2+, Cu2+ and W6+ cations are partially ordered over two crystallographically-distinct six-coordinate sites such that one is ~63% occupied by W6+ and both are occupied by Ni2+ and Cu2+ in a ratio of (2-x):x. The composition x ​= ​0.25 behaves as a spin glass below 35 ​K whereas x ​= ​0.5 is ferrimagnetic below 120 ​K with an ordered moment of 0.774(5) μB per magnetic cation. The atomic moments order in a G-type pattern and the imbalance in the number of magnetic cations on the two sites results in a net magnetisation.

Abstract:

Polycrystalline samples of the monoclinic perovskites CaLa2Ni2-xCuxWO9 (x = 0.25 and 0.5) have been prepared and characterised by neutron diffraction and magnetometry. The Ni2+, Cu2+ and W6+ cations are partially ordered over two crystallographically-distinct six-coordinate sites such that one is ∼63% occupied by W6+ and both are occupied by Ni2+ and Cu2+ in a ratio of (2-x):x. The composition x = 0.25 behaves as a spin glass below 35 K whereas x = 0.5 is ferrimagnetic below 120 K with an ordered moment of 0.774(5) μB per magnetic cation. The atomic moments order in a G-type pattern and the imbalance in the number of magnetic cations on the two sites results in a net magnetisation.

Abstract:

Mixed-valent transition metal compounds display complex structural, electronic and magnetic properties which can often be exquisitely tuned. Here the charge-ordered state of stoichiometric CaFe3O5 is probed using neutron powder diffraction, Monte Carlo simulation and symmetry analysis. Magnetic ordering is dominated by the formation of ferromagnetic Fe3+–Fe2+–Fe3+ trimers which are evident above the magnetic ordering transition. Between TN =289 K and 281 K an incommensurate magnetically ordered phase develops due to magnetic frustration, but a spin Jahn-Teller distortion lifts the frustration and enables the magnetic ordering to lock in to a charge-ordered commensurate state at lower temperatures. Stoichiometric CaFe3O5 exhibits single phase behaviour throughout and avoids the phase separation into two distinct crystallographic phases with different magnetic structures and Fe valence distributions reported recently, which likely occurs due to partial Fe2+ for Ca2+ substitution. This underlines the sensitivity of the magnetism and chemistry of these mixedvalent systems to composition.