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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:

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:

CaKFe4As4 (1144) is a unique stoichiometric iron-based superconductor which harbours high upper critical fields and large critical current densities. In this work, we describe a study to optimize the synthesis conditions of stoichiometric polycrystalline samples of CaKFe4As4 and asses their structural, magnetic and transport properties. The samples were prepared over a wide temperature range (900-1100°C) and the pure phase formation is centered around 955°C. Outside this temperature region, impurity phases of KFe2As2 and CaFe2As2 can also form. Magnetic susceptibility and resistivity measurements establish that the critical temperature reaches ~34 K for the optimum synthesis conditions and the critical current reaches 2 × 104 A-cm−2. The post-annealing process demonstrates the stability of the 1144 phase up to 500°C, however, under higher temperature annealing, phase degradation occurs. Our study indicates that the formation of phase-pure 1144 occurs over a much narrower window and its highly prone to multi-phase formation as compared with the 122 family. As a result, the superconducting properties are enhanced for the pure 1144 phase but they are likely to be affected by the inter and intra-granular behaviour originating from the microstructural nature of polycrystalline CaKFe4As4, similar to other iron-based superconductors. Based on our study, we construct the phase diagram for polycrystalline 1144 and compared it with that reported for 1144 single crystal.

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.

Abstract:

The synthesis and structure of two new transition metal oxide tellurides, Sr2MnO2Cu1.82(2)Te2 and Sr2CoO2Cu2Te2, are reported. Sr2CoO2Cu2Te2 with the purely divalent Co2+ ion in the oxide layers has magnetic ordering based on antiferromagnetic interactions between nearest neighbors and appears to be inert to attempted topotactic oxidation by partial removal of the Cu ions. In contrast, the Mn analogue with the more oxidizable transition metal ion has a 9(1)% Cu deficiency in the telluride layer when synthesized at high temperatures, corresponding to a Mn oxidation state of +2.18(2), and neutron powder diffraction revealed the presence of a sole highly asymmetric Warren-type magnetic peak, characteristic of magnetic ordering that is highly two-dimensional and not fully developed over a long range. Topotactic oxidation by the chemical deintercalation of further copper using a solution of I2 in acetonitrile offers control over the Mn oxidation state and, hence, the magnetic ordering: oxidation yielded Sr2MnO2Cu1.58(2)Te2 (Mn oxidation state of +2.42(2)) in which ferromagnetic interactions between Mn ions result from Mn2+/3+ mixed valence, resulting in a long-range-ordered A-type antiferromagnet with ferromagnetic MnO2 layers coupled antiferromagnetically.