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In the dense metal-organic framework Na[Mn(HCOO)$_3$], Mn$^{2+}$ ions ($S=\frac{5}{2}$) occupy the nodes of a `trillium' hyperkagome net. We show that this material exhibits a variety of behaviour characteristic of geometric frustration: the N\'eel transition is suppressed well below the characteristic magnetic interaction strength; short-range magnetic order persists far above the N\'eel temperature; and the magnetic susceptibility exhibits a pseudo-plateau at $\frac{1}{3}$-saturation magnetisation. We demonstrate that a simple nearest-neighbour Heisenberg antiferromagnet model accounts quantitatively for each observation, and hence Na[Mn(HCOO)$_3$] is the first experimental realisation of this model on the trillium net. We develop a mapping between this trillium model and that on the two-dimensional Shastry-Sutherland lattice, and demonstrate how both link geometric frustration within the classical spin liquid regime to a strong magnetocaloric response at low fields.

This work reports a new series of A\(Ln\)(HCO\(_2\))(C\(_2\)O\(_4\))\(_{1.5}\) (A = [(CH\(_3\))\(_2\)NH\(_2\)]\(^+\) and Ln\(^{3+}\) = Tb\(^{3+}\)-Er\(^{3+}\)) compounds made solvothermally. These \({Cmce}\) phases combine monovalent and divalent ligands, which enables a scarce combination of A\(^+\) and B\(^{3+}\) cations in a hybrid perovskite-like compound. The ratio of ligands leads to ordered anion vacancies, which alternate with oxalate linkers along the \(c\)-axis. The A-site cations are disordered and cannot be identified crystallographically, likely a result of the larger pores of these frameworks compared to the recently reported AEr(HCO\(_2\))\(_2\)(C\(_2\)O\(_4\)) phases. Neutron and infrared spectroscopy, supported by elemental composition, enables these cations to be identified as [(CH\(_3\))\(_2\)NH\(_2\)]\(^+\) molecules. Magnetic property measurements suggest these materials have weak antiferromagnetic interactions but remain paramagnetic to 1.8 K.

We present a study on the nuclear and magnetic structures of two iron-based garnets with magnetic cations isolated on tetrahedral sites. Ca\(_2\)YZr\(_2\)Fe\(_3\)O\(_{12}\) and Ca\(_2\)LaZr\(_2\)Fe\(_3\)O\(_{12}\) offer an interesting comparison for examining the effect of increasing cation size within the diamagnetic backbone of the garnet crystal structure, and how such changes affect the magnetic order. Despite both systems exhibiting well-pronounced magnetic transitions at low temperatures, we also find evidence for diffuse magnetic scattering due to a competition between the nearest-neighbor, next nearest-neighbor, and so on, within the tetrahedral sites. This competition results in a complex noncollinear magnetic structure on the tetrahedral sublattice creating a mixture of ferro- and antiferromagnetic interactions above the long-range ordering temperature near 20 K and suggests that the cubic site of the garnet plays a significant role in mediating the superexchange interactions between tetrahedral cations.

This study probes the structure and the magnetic properties of members of the Ln(HCO\(_2\))(C\(_2\)O\(_4\)) (Ln = Sm\(^{3+}\)–Er\(^{3+}\)) family of coordination frameworks. These frameworks adopt Pnma orthorhombic symmetry with one-dimensional chains arranged on a distorted triangular lattice. The magnetic properties of the Gd–Ho members of this series indicate they remain paramagnetic down to 2 K, with Dy(HCO\(_2\))(C\(_2\)O\(_4\)) magnetically ordering at 0.6 K. The magnetocaloric effect of Gd(HCO\(_2\))(C\(_2\)O\(_4\)) is amongst the highest found in frameworks with a peak entropy change of 55.97 J kg\(^{-1}\) K\(^{-1}\) (218.42 mJ cm\(^{-3}\) K\(^{-1}\)) for a 5–0 T field change at T\(_{max}\) = 2 K, making this material a very good candidate for ultra-low temperature magnetic cooling. In contrast with related magnetocaloric materials lanthanides with high magnetocrystalline anisotropy do not generally improve the magnetocaloric performance of this family at higher temperatures and lower fields. Neutron diffraction experiments suggest that Tb(HCO\(_2\))(C\(_2\)O\(_4\)) and Ho(HCO\(_2\))(C\(_2\)O\(_4\)) lack significant local magnetic correlations, highlighting the key role these play in optimising the magnetocaloric performance in low fields in related phases; this emphasises the importance of designing materials with specific magnetic interactions to optimise magnetocaloric performance.

Herein we report the synthesis and structures of [(CH\(_3\))\(_2\)NH\(_2\)]Er(HCO\(_2\))\(_2\)(C\(_2\)O\(_4\)) and [(NH\(_2\))\(_3\)C]Er(HCO\(_2\))\(_2\)(C\(_2\)O\(_4\)), in which the inclusion of divalent oxalate ligands allows for the exclusive incorporation of A\(^+\) and B\(^{3+}\) cations in an ABX\(_3\) hybrid perovskite structure for the first time. We rationalise the observed thermal expansion of these materials, including negative thermal expansion, and find evidence for weak antiferromagnetic coupling in [(CH\(_3\))\(_2\)NH\(_2\)]Er(HCO\(_2\))\(_2\)(C\(_2\)O\(_4\)).

This work reports the ambient temperature synthesis and structural characterisation of six new first row transition metal acetylenedicarboxylate coordination polymers. The Co and two Ni compounds adopt structures in which the octahedral metals are connected into 1D chains via the acetylenedicarboxylate ligand. In contrast the Mn and two Zn compounds adopt 3D metal-organic frameworks; while the Mn compound is non-porous the two Zn structures contain dimeric or trimeric clusters connected into frameworks that are potentially porous. These two anionic metal-organic frameworks are, however, charged balanced by cations siting in their pores which greatly reduces the ability to access their porosity.

This work describes a homometallic spin-urn:x-wiley:09476539:media:chem202200855:chem202200855-math-0001 tetrabromocuprate adopting a bilayer structure. Magnetic-susceptibility measurements show a broad maximum centred near 70 K, with fits to this data using a Heisenberg model consistent with strong antiferromagnetic coupling between neighbouring copper atoms in different layers of the bilayer. There are further weak intralayer ferromagnetic interactions between copper cations in neighbouring dimers. First-principles calculations are consistent with this, but suggest there is only significant magnetic coupling within one direction of a layer; this would suggest the presence of a spin ladder within the bilayer with antiferromagnetic rung and weaker ferromagnetic rail couplings.

The development of sustainable and efficient cryogenic cooling materials is currently the subject of extensive research, with the aim of relieving the dependence of current low-temperature cooling methods on expensive and nonrenewable liquid helium. One potential method to achieve this is the use of materials demonstrating the magnetocaloric effect, where the cycling of an applied magnetic field leads to a net cooling effect due to changes in magnetic entropy upon application and removal of an external magnetic field. This study details the synthesis and characterization of a Ln

A family of substituted 1,2,4-benzothiadiazine 1-chlorides have been prepared by treatment of N-arylamidines in neat thionyl chloride at reflux. The S(iv) 1-chlorides are readily reduced under mild conditions to persistent 1,2,4-benzothiadiazinyl radicals which have been characterised by EPR spectroscopy and cyclic voltammetry. Crystallographic studies on isolated radicals indicate that the radicals dimerise via pancake bonding in the solid-state, resulting in spin-pairing and net diamagnetism., A family of 1,2,4-benzothiadiazinyl radicals are accessible from 1,2,4-benzothiadiazine 1-chlorides which can be prepared in a single step by treatment of N-arylamidines in neat thionyl chloride at reflux.

Probing the magnetic structure of magnetocaloric materials in applied fields, can reveal detailed insight into the mechanism of magnetic refrigeration thereby linking the magnetic states that form under applied magnetic fields to changes in magnetic entropy. This study probes the long range magnetic order in \(Tb(DCO_2)_3\) and \(TbODCO_3\) as a function of temperature and applied magnetic field, through neutron diffraction measurements. A triangular Ising antiferromagnetic phase is formed, in small applied magnetic fields in \(Tb(DCO_2)_3\), a spin flip transition occurs to a simple ferromagnetic structure in higher applied fields. \(TbODCO_3\) undergoes a phase transition, in applied fields, into two magnetic phases; namely a buckled chain ferromagnetic phase along the b-axis and a canted antiferromagnetic phase, with a ferromagnetic component along c-axis. Both of the states observed in \(TbODCO_3\) are consistent with Ising-like anisotropy previously reported in \(TbODCO_3\) and the coexistance of these is likely the result of applying a magnetic field to a powdered sample.

This study reports the facile synthesis, crystal structures and magnetic properties of five new Mn, Co and Cu complexes with chloride and simple amine ligands. The four hydrazinium complexes are discrete in nature while the O-methylhydroxylamine phase contains edge-sharing chains bridged by chloride ligands. Investigation of the magnetic properties of these materials reveals that two of these materials, Co(NH3NH2)2(H2O)2Cl4 and Cu(NH2OCH3)2Cl2, exhibit interesting antiferromagnetic properties arising from their low dimensional structures. Co(NH3NH2)2(H2O)2Cl4 appears to exhibit significant 2D magnetic frustration while the magnetic susceptibilities of Cu(NH2OCH3)2Cl2 are well fitted by a one-dimensional chain model. The relationships between the strength of the magnetic coupling observed in these materials and their likely exchange pathways are also discussed.

Variable-temperature and variable-pressure single-crystal diffraction studies are carried out on a mixed-valence perovskite dimethylammonium (DMA) iron formate compound, with the formula [(DMA3)(H2O)][FeII3FeIII(HCOO)12], in order to investigate potential electric ordering of the DMA cation from its fourfold type of dynamic disorder at ambient conditions. Mossbauer spectroscopy is additionally carried out at ambient conditions to confirm the presence and ratio of Fe2+ and Fe3+ cations. Below 200 K, a dynamic to static disorder of the DMA cation is observed, while the crystal symmetry and iron formate framework structure remain the same. Upon application of pressure, however, a phase transition occurs that lowers the symmetry above 3.3 GPa. This work highlights the further chemical modifications that are possible within the dimethylammonium metal formates family, i.e. doping upon the A-site with neutral molecules, leading to additional opportunities to tune their physical properties.

This study probes the structure and magnetocaloric effect of the LnOHCO3 (Ln = Gd3+, Tb3+, Dy3+, Ho3+, and Er3+) frameworks. The combination of single crystal X-ray and neutron powder diffraction indicates that these materials solely adopt the P212121 structure under these synthetic conditions and magnetic susceptibility measurements indicate they remain paramagnetic down to 2 K. We show that the magnetocaloric effects of TbOHCO3 and DyOHCO3 have peak entropy changes of 30.99 and 33.34 J kg–1 K–1 for a 2–0 T field change, respectively, which are higher than that of the promising GdOHCO3 framework above 4 K in moderate magnetic fields. The magnetic entropy changes of TbOHCO3 and DyOHCO3 above 4 K for smaller than 2 T field changes also exceed those of Gd3Ga5O12 and Dy3Ga5O12, making them suitable magnetic cooling materials for use at liquid helium temperatures using the low applied magnetic fields accessible using permanent magnets, advantageous for efficient practical cooling devices.

Materials with magnetic interactions between their metal centres play a tremendous role in modern technologies and can exhibit unique physical phenomena. In recent years, magnetic metal-organic frameworks and coordination polymers have attracted significant attention because their unique structural flexibility enables them to exhibit multifunctional magnetic properties or unique magnetic states not found in the conventional magnetic materials, such as metal oxides. Techniques that enable the magnetic interactions in these materials to be probed at the atomic scale, long established to be key for developing other magnetic materials, are not well established for studying metal-organic frameworks and coordination polymers. This review focuses on studies where metal-organic frameworks and coordination polymers have been examined using such microscopic probes, with a particular focus on neutron scattering and density-functional theory, the most-well established experimental and computational techniques for understanding magnetic materials in detail. This paper builds on a brief introduction to these techniques to describe how such probes have been applied to a variety of magnetic materials starting with select historical examples before discussing multifunctional, low dimensional and frustrated magnets. This review highlights the information that can be obtained from such microscopic studies, including the strengths and limitations of these techniques. The article then concludes with a brief perspective on the future of this area.

Probing the magnetic interactions in functional magnetic materials can reveal detailed insight into how to optimise the properties they possess while providing key understanding of the exotic phenomena they may host. This study probes the short and long range magnetic order in the \(LnODCO_{3}\) (where Ln = Tb, Dy, Ho, and Er) framework magnetocalorics using variable-temperature neutron scattering measurements. Reverse Monte Carlo analysis of neutron scattering data shows that \(TbODCO_{3}\), \(DyODCO_{3}\) and \(HoODCO_{3}\) develop short range Ising-like magnetic order between 1.5 and 20 K, consistent with dominant ferromagnetic correlations within chains along the b-axis. Through magnetic susceptibility measurements we identify that long range magnetic order develops in \(TbODCO_{3}\) and \(HoODCO_{3}\) at ∼1.2 and ∼0.9 K, respectively. Neutron diffraction measurements were conducted on \(HoODCO_{3}\) revealing incommensurate magnetic order develops between 1.2 and 0.9 K, before a commensurate magnetic phases emerges at 0.8 K with long-range ferromagnetic order in the chains. The results suggest Ising-like ferromagnetic chains associated with frustration are responsible for the improved magnetocaloric properties, of some members in this family, at higher temperatures and low applied fields.

Magnetic materials with strong local interactions but lacking long-range order have long been a curiosity of physicists. Probing their magnetic interactions is crucial for understanding the unique properties they can exhibit. Metal-organic frameworks have recently gathered more attention as they can produce more exotic structures, allowing for controlled design of magnetic properties not found in conventional metal-oxide materials. Historically, magnetic diffuse scattering in such materials has been overlooked but has attracted greater attention recently, with advances in techniques. In this study, we investigate the magnetic structure of metal-organic formate frameworks, using heat capacity, magnetic susceptibility and neutron diffraction. In Tb(DCO 2 ) 3 , we observe emergent magnetic order at temperatures below 1.2 K, consisting of two k -vectors. Ho(DCO 2 ) 3 shows diffuse scattering above 1.6 K, consistent with ferromagnetic chains packed in a frustrated antiferromagnetic triangular lattice, also observed in Tb(DCO 2 ) 3 above 1.2 K. The other lanthanides show no short- or long-range order down to 1.6 K. The results suggest an Ising-like one-dimensional magnetic order associated with frustration is responsible for the magnetocaloric properties, of some members in this family, improving at higher temperatures. This article is part of the theme issue ‘Mineralomimesis: natural and synthetic frameworks in science and technology’.

This study examines the crystal structures of the AB(HCO2)3 (A = Li or Na and B = Mn or Co) metal-organic frameworks, which we find to adopt a chiral cubic P213 structure. This shows that the Li containing formates are isostructural with their Na analogues, extending the phase stability of this chiral architecture. The Mn containing compounds have a magnetic sublattice similar to ?-Mn, long of interest due to its highly frustrated antiferromagnetic coupling. In contrast the Co formates appear to have partially disordered alkali and transition metal cations, which prevents the formation of a clean ?-Mn-like magnetic sublattice. We have also re-examined the magnetic properties of NaMn(HCO2)3 finding it to be a simple paramagnet down to 2 K with only weak antiferromagnetic coupling.

Herein, we identify supramolecular self-associating amphiphiles (SSAs) as a novel class of antibacterials with activity towards methicillin-resistant Staphylococcus aureus. Structure–activity relationships have been identified in the solid, solution and gas phases. Finally, we show that when supplied in combination, SSAs exhibit increased antibacterial efficacy against these clinically relevant microbes.

This study probes the magnetic properties and interactions of the Ln(HCO2)3 (Ln = Tb3+–Er3+) frameworks. We show that the magnetocaloric effect of Tb(HCO2)3 is significantly higher above 4 K in moderate magnetic fields compared to the promising Gd(HCO2)3. While the peak performance of Tb(HCO2)3 is lower than Gd(HCO2)3, we also find that the Gd-rich members of the solid solution Gd1−xTbx(HCO2)3 blend the advantages of both end-members. Using neutron diffraction experiments, Tb(HCO2)3 is found to be antiferromagnetic below 1.7 K with ferromagnetic face-sharing chains and antiferromagnetic coupling between them. Analysis of magnetic diffuse scattering of the paramagnetic phase indicates that ferromagnetic coupling is retained, and it is likely that this plays a role in improving its magnetocaloric performance in low fields.

The deep red phosphor Sr4(PO4)2O:Eu2+, which has an excitation peak around 450 nm for blue LED applications, is reported. This behavior is unusual for most phosphate phosphors. The crystal structure of Sr4(PO4)2O:Eu2+ is found to be monoclinic P21 and isotypic with Ca4(PO4)2O:Eu2+, which also shows deep red emission. Sr4(PO4)2O:Eu2+ has a larger lattice volume than Ca4(PO4)2O:Eu2+, but their emission and excitation spectra at room temperature are very similar. The key factors are discussed for achieving a large redshift of the 5d levels of Eu2+ ion in order to emit red light. In particular, the importance of the anion polarizability and the distortions of the metal coordination polyhedra are discussed, including the effective coordination number. Importantly, Sr4(PO4)2O:Eu2+ lacks the yellow emission at 77 K, which is found in Ca4(PO4)2O:Eu2+. The differences in thermal quenching behavior for Eu2+ dopants in Sr4(PO4)2O:Eu2+ and Ca4(PO4)2O:Eu2+ are attributed to the degree of auto/photo-ionization due to differences in the band gaps of these compounds. The importance of the large band gap of the host lattice in avoiding non-radiative processes of energy relaxation was confirmed.

The ordered hexagonal perovskite ${\mathrm{Ba}}_{2}{\mathrm{CuTeO}}_{6}$ hosts weakly coupled $S=\frac{1}{2}$ spin ladders produced by an orbital ordering of ${\mathrm{Cu}}^{2+}$. The magnetic susceptibility $\ensuremath{\chi}(T)$ of ${\mathrm{Ba}}_{2}{\mathrm{CuTeO}}_{6}$ is well described by that expected for isolated spin ladders with exchange coupling of $J\ensuremath{\approx}86\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ but shows a deviation from the expected thermally activated behavior at low temperatures below ${T}^{*}\ensuremath{\approx}25\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. An anomaly in $\ensuremath{\chi}(T)$, indicative of magnetic ordering, is observed at ${T}_{\mathrm{mag}}=16\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. No clear signature of long-range ordering, however, is captured so far in NMR $1/{T}_{1}$, specific heat or neutron diffraction measurements at and below ${T}_{\mathrm{mag}}$. The marginal magnetic transition, indicative of strong quantum fluctuations, is evidence that ${\mathrm{Ba}}_{2}{\mathrm{CuTeO}}_{6}$ is in very close proximity to a quantum critical point between magnetically ordered and spin-gapped phases controlled by interladder couplings.

In this study we report the synthesis, crystal structures and magnetic properties of three frameworks wherein Cu cations are bridged by formate linkers into one-dimensional motifs. One of these compounds, Cu2(HCO2)3(C3N2H4)4(NO3), contains a ladder motif but remains paramagnetic to 2 K. This is likely because of the longer superexchange pathway along its chains due to the orientation of the Jahn–Teller axis of its Cu cations. In contrast Cu(HCO2)(NO3)(NH3)2 and Cu(HCO2)(ClO4)(NH3)2 feature Cu(HCO2) chains in which the Jahn–Teller axis is oriented perpendicular to the chain direction; these exhibit antiferromagnetic order below 12 and 7 K, respectively. Their magnetic susceptibilities are well fitted by a one-dimensional chain model but further examination of their magnetic properties reveals significant inter-chain magnetic coupling and a lack of spin dynamics. This suggests that these transitions correspond to the emergence of long-range magnetic order, highlighting the importance of detailed studies of frameworks containing low dimensional motifs to gain a deeper understanding of their magnetic behaviour.

Co adipate, Co(C6H8O4), has been found to order near 10 K into a magnetic structure featuring sheets of tetrahedral Co cations coupled antiferromagnetically in two dimensions through carboxylate groups. The emergence of this order is accompanied by magnetoelastic coupling, which drives anisotropic negative thermal expansion along the a-axis below 50 K, the first time such behaviour has been observed in a metal-organic framework. The monoclinic angle, β, has also been found to decrease on cooling, passing through a metrically orthorhombic phase without a phase transition; this unusual behaviour has been rationalised in terms of the thermal expansion along the principal axes.

A combination of variable-temperature neutron scattering, reverse Monte Carlo analysis and direct Monte Carlo simulation is used to characterise the emergence of magnetic order in the metal--organic framework (MOF) Tb(HCOO)$_3$ over the temperature range 100 K to 1.6 K $=T_{\rm N}$. We show that the magnetic transition at $T_{\rm N}$ involves one-dimensional ferromagnetic ordering to a partially-ordered state related to the triangular Ising antiferromagnet. In this phase, the direction of magnetisation of ferromagnetic chains tends to alternate between neighbouring chains but this alternation is frustrated and is not itself ordered. In neutron scattering measurements this partial order gives rise to Bragg-like peaks, which cannot be interpreted using conventional magnetic crystallography without resort to unphysical spin models. The existence of low-dimensional magnetic order in Tb(HCOO)$_3$ is stabilised by the contrasting strength of inter- and intra-chain magnetic coupling, itself a consequence of the underlying MOF architecture. Our results demonstrate how MOFs may provide an attractive if as yet under-explored platform for the realisation and investigation of low-dimensional physics., Comment: 5 pages, 3 figures

The elastic and anelastic properties of three different samples of Fe(x)O have been determined in the frequency range 0.1-2 MHz by resonant ultrasound spectroscopy and in the range 0.1-50 Hz by dynamic mechanical analysis in order to characterize ferroelastic aspects of the magnetic ordering transition at T(N) ~ 195 K. No evidence was found of separate structural and magnetic transitions but softening of the shear modulus was consistent with the involvement of bilinear coupling, λe(4)q, between a symmetry-breaking strain, e(4), and a structural order parameter, q. Unlike a purely ferroelastic transition, however, C(44) does not go to zero at the critical temperature, T*(c), due to the intervention of the magnetic ordering at a higher temperature. The overall pattern of behaviour is nevertheless consistent with what would be expected for a system with separate structural and magnetic instabilities, linear-quadratic coupling between the structural (q) and magnetic (m) driving order parameters, λqm(2), and T(N) > T*(c). Comparison with data from the literature appears to confirm the same pattern in MnO and NiO, with a smaller difference between T(N) and T*(c) in the former and a larger difference in the latter. Strong attenuation of acoustic resonances at high frequencies and a familiar pattern of attenuation at low frequencies suggest that twin walls in the rhombohedral phase have typical ferroelastic properties. Acoustic dissipation in the stability field of the cubic phase is tentatively attributed to anelastic relaxations of the defect ordered structure of non-stoichiometric wüstite or of the interface between local regions of wüstite and magnetite, with a rate controlling step determined by the diffusion of iron.

We report a new 2-D inorganic-organic framework material, MnDMS [Mn 2,2-dimethylsuccinate], featuring weakly bound hybrid layers in its bulk crystals that can be readily exfoliated into nanosheets via ultrasonication. The fully exfoliated hybrid nanosheets correspond to a unilamellar thickness of about 1 nm, while the partially exfoliated nanosheets (multilayer films) exhibit a typical thickness on the order of 10 nm. We used atomic force microscopy to characterize their surface topography and to map the variation of nanomechanical properties across the surface of the delaminated nanosheets. The morphology and crystallographic orientation of the exfoliated layers were further studied by transmission electron microscopy. Additionally, we investigated the elastic anisotropy underlying the bulk host material by means of single-crystal nanoindentation, from which the critical resolved shear stress (τ(crit)) needed for the micromechanical delamination of individual layers was determined to be relatively small (≲0.4 GPa).

New quenched-in fluorite-type materials with composition (BiO 1.5)0.94-x(LaO1.5)0.06(PbO) x, x = 0.02, 0.03, 0.04 and 0.05, were synthesised by solid state reaction. The new materials undergo a number of phase transformations during heating between room temperature and 750 °C, as indicated by differential thermal analysis. Variable temperature X-ray diffraction performed on the material (BiO1.5)0.92(LaO1.5) 0.06(PbO)0.02 revealed that the quenched-in fcc fluorite-type material first undergoes a transformation to a β-Bi 2O3-type tetragonal phase around 400 °C. In the range 450-700 °C, α-Bi2O3-type monoclinic, Bi 12PbO19-type bcc and β1/β 2-type rhombohedral phases, and what appeared to be a ε-type monoclinic phase, were observed, before a single-phase fluorite-type material was regained at 750 °C. © 2010 Elsevier Ltd © 2011 Elsevier Ltd. All rights reserved.

The phase composition of two series of mixed Nb5+-Sb5+ double perovskites formed between the pairs Ba2EuNbO6-Ba2PrSbO6 and Ba2NdSbO6-Ba2NdNbO6 have been studied using synchrotron X-ray powder diffraction methods. In both series extensive phase segregation is observed demonstrating limited solubility of Sb5+ in these Nb5+ perovskites, irrespective of the precise structures of the double perovskite. Evidence for a monoclinic I2/m phase in the series formed between tetragonal I4/m Ba2EuNbO6 and rhombohedral R over(3, -) Ba2EuNbO6 is presented. It is postulated that this phase segregation is a consequence of competing bonding requirements of the Nb5+ and Sb5+ cations associated with their electronic configurations. © 2007 Elsevier Inc. All rights reserved.

The structures of eight members of the series Sr1−xCexMnO3 with 0.075⩽x⩽0.4 have been established using synchrotron X-ray powder diffraction. These exhibit the sequence of structures cubic Pm 3 ¯ m ⟶ x ≈ 0.08 tetragonal I 4 / mcm ⟶ x ≈ 0.35 orthorhombic Imma with increasing Ce content. Unusual peak broadening due to domain wall scattering is observed near the composition-induced first-order cubic–tetragonal transition. The temperature dependence of the structures for three examples x=0.1, 0.2 and 0.35 is described. For the x=0.1 and 0.35 samples evidence for a first-order loss of the orbital ordering associated with a cooperative Jahn–Teller distortion is presented. The onset of the orbitally disordered state is accompanied by a discontinuous reduction in cell volume.

The crystal structures adopted by BaPrO3 at and above ambient temperature have been examined using a combination of synchrotron X-ray and neutron diffraction. BaPrO3 has been established to undergo a series of phase transitions from P b n m orthorhombic → I b m m orthorhombic → R over(3, ̄) c rhombohedral → P m over(3, ̄) m cubic. BaPrO3 is the second A2+B4+O3 perovskite found to adopt rhombohedral symmetry in preference to the I4/mcm tetragonal structure. Analysis of the octahedral tilting through the rhombohedral to cubic phase transition indicates that this transformation is continuous and tricritical in nature. The tricritical behaviour of this transition is likely to be a result of the competition between tetragonal and rhombohedral structures to be the preferred phase, with the rhombohedral symmetry adopted by BaPrO3 being stabilised by the unusually large B-site cation. © 2008 Elsevier Ltd. All rights reserved.

The structure of the cation-ordered double perovskite Ba2HoTaO6 was examined using synchrotron X-ray powder diffraction at fine temperature intervals over the range of 90-300 K. Ba2HoTaO6 has a cubic structure in space group F m over(3, ̄) m at room temperature. A proper ferroelastic phase transition to I4/m tetragonal symmetry occurs near approximately 260 K. Analysis of the spontaneous tetragonal strain versus temperature indicated that the phase transition is second order in nature. © 2007 Elsevier Ltd. All rights reserved.

An anhydrous manganese succinate, Mn(C4H4O 4), has been synthesised hydrothermally and studied by single-crystal X-ray diffraction. It adopts a succinate pillared structure in which layers of corner-sharing MnO6 octahedra alternate with sheets that contain chains of edge-sharing octahedra. This unique 3D framework structure contains highly distorted MnO6 octahedra, which are made possible by the lack of ligand field stabilisation energy for the high-spin Mn2+ ion. Attempts to dope the structure with other divalent transition-metal ions were accordingly unsuccessful. Magnetic sus-ceptibility and heat capacity measurements indicate that Mn(C4H4O4) undergoes antiferromagnetic ordering below 12 K, with a second antiferromagnetic transition at approximately 6 K. These two antiferromagnetic phases undergo further transitions in applied fields, underlining the subtle magnetic behaviour that is possible in inorganic-organic frameworks of this structural complexity. © 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

The crystal structures of the perovskite manganites SrxCa1-x-yNdyMnO3 with y=0.1 or 0.2 have been investigated using synchrotron X-ray powder diffraction. At room temperature the structures change from Pm over(3, -) m ↔ I 4 / mcm ↔ Pbnm depending on the cation distribution, the different structures exhibiting different tilts of the MnO6 octahedra. High temperature diffraction measurements demonstrate the presence of, an apparently continuous, isosymmetric I4/mcm to I4/mcm phase transition associated with the removal of long range orbital ordering. Heating the manganites to still higher temperatures results in a continuous transition to the cubic Pm over(3, -) m structure. A feature of such transitions is the continuous evolution of the octahedral tilt angle through the I4/mcm to I4/mcm phase transition. The orthorhombic structures do not exhibit orbital ordering and although a first order transition to the tetragonal structure is observed in Sr0.4Ca0.5Nd0.1MnO3, this high temperature tetragonal structure does not exhibit orbital ordering. © 2009 Elsevier Inc. All rights reserved.

The structure of the defect pyrochlore CsTi0.5W1.5O6 has been investigated using electron, synchrotron X-ray and neutron diffraction methods. The material is cubic a = 10.2773 Å with displacive disorder of the Cs cations along the 〈1 1 1〉 direction. The local structure, revealed by the diffuse structure in the electron diffraction patterns shows there is correlated displacement of the heavy Cs atoms along the 〈1 1 0〉 directions. The thermal expansion of the material is also described. © 2008 Elsevier Ltd. All rights reserved.

The structures of eight members of the series of double perovskites of the type Ba2LnB′O6 (Ln=La3+–Sm3+ and Y3+ and B′=Nb5+ and Ta5+) were examined both above and below room temperature using synchrotron X-ray powder diffraction. The La3+ and Pr3+ containing compounds had an intermediate rhombohedral phase whereas the other tantalates and niobates studied have a tetragonal intermediate. This difference in symmetry appears to be a consequence of the larger size of the La3+ and Pr3+ cations compared to the other lanthanides. The temperature range over which the intermediate symmetry is stable is reduced in those compounds near the point where the preferred intermediate symmetry changes from tetragonal to rhombohedral. In such compounds the transition to the cubic phase involves higher order terms in the Landau expression. This suggests that in this region the stability of the two intermediate phases is similar.

The structures of 28 compounds in the two series Ba2LnSbO6 and Ba2LnNbO6 have been examined using synchrotron X-ray and in selected cases neutron powder diffraction at, below and above ambient temperature. The antimonate series is found to undergo a sequence of phase transitions from monoclinic to rhombohedral to cubic symmetry with both decreasing ionic radii of the lanthanides and increasing temperature. Compounds in the series Ba2LnNbO6, on the other hand, feature an intermediate tetragonal structure instead of the rhombohedral structure exhibited by the antimonates. This difference in symmetry is thought to be caused by π-bonding in the niobates that is absent in the antimonates. The bonding environments of the cations in these compounds have also been examined with overbonding of the lanthanide and niobium cations being caused by the unusually large B-site cations. © 2006 Elsevier Inc. All rights reserved.

The family of the superconducting quasiskutterudites (Ca(x)Sr(1-x))(3)Rh(4)Sn(13) features a structural quantum critical point at x(c)=0.9, around which a dome-shaped variation of the superconducting transition temperature T(c) is found. Using specific heat, we probe the normal and the superconducting states of the entire series straddling the quantum critical point. Our analysis indicates a significant lowering of the effective Debye temperature on approaching x(c), which we interpret as a result of phonon softening accompanying the structural instability. Furthermore, a remarkably large enhancement of 2Δ/k(B)T(c) and ΔC/γT(c) beyond the Bardeen-Cooper-Schrieffer values is found in the vicinity of the structural quantum critical point. The phase diagram of (Ca(x)Sr(1-x))(3)Rh(4)Sn(13) thus provides a model system to study the interplay between structural quantum criticality and strong electron-phonon coupling superconductivity.

This study probes the magnetic and ferroelectric ordering of the NH4M(HCO2)3 (M = Mn(2+), Fe(2+), Co(2+) and Ni(2+)) frameworks using neutron diffraction, improving the understanding of the origins of the properties of these fascinating multiferroics. This rare study of the magnetic structure of a family of metal-organic frameworks shows that all four compounds exhibit antiferromagnetic coupling between neighbouring cations bridged by formate ligands. The orientation of the spin, however, changes in a highly unusual way across the series with the spins aligned along the c-axis for the Fe(2+) and Ni(2+) frameworks but lying in the ab plane for the other members of the series. This work also sheds new light on the nature of the ferroelectric order-disorder transition in these materials; probing changes in the ammonium cation across the transition and also shows that the Ni(2+) framework does not undergo a transition to the polar P63 phase due to the smaller size of the Ni(2+) cation. Finally trends in their anisotropic negative thermal expansion, which potentially enhances their ferroic behaviour, are quantified.

The family of the superconducting quasiskutterudites (Ca(x)Sr(1-x))(3)Rh(4)Sn(13) features a structural quantum critical point at x(c)=0.9, around which a dome-shaped variation of the superconducting transition temperature T(c) is found. Using specific heat, we probe the normal and the superconducting states of the entire series straddling the quantum critical point. Our analysis indicates a significant lowering of the effective Debye temperature on approaching x(c), which we interpret as a result of phonon softening accompanying the structural instability. Furthermore, a remarkably large enhancement of 2Δ/k(B)T(c) and ΔC/γT(c) beyond the Bardeen-Cooper-Schrieffer values is found in the vicinity of the structural quantum critical point. The phase diagram of (Ca(x)Sr(1-x))(3)Rh(4)Sn(13) thus provides a model system to study the interplay between structural quantum criticality and strong electron-phonon coupling superconductivity.

The quasiskutterudite superconductor ${\mathrm{Sr}}_{3}{\mathrm{Rh}}_{4}{\mathrm{Sn}}_{13}$ features a pronounced anomaly in electrical resistivity at ${T}^{*}\ensuremath{\sim}138\text{ }\text{ }\mathrm{K}$. We show that the anomaly is caused by a second-order structural transition, which can be tuned to 0 K by applying physical pressure and chemical pressure via the substitution of Ca for Sr. A broad superconducting dome is centered around the structural quantum critical point. Detailed analysis of the tuning parameter dependence of ${T}^{*}$ as well as insights from lattice dynamics calculations strongly support the existence of a structural quantum critical point at ambient pressure when the fraction of Ca is 0.9 (i.e., ${x}_{c}=0.9$). This establishes the ${({\mathrm{Ca}}_{x}{\mathrm{Sr}}_{1\ensuremath{-}x})}_{3}{\mathrm{Rh}}_{4}{\mathrm{Sn}}_{13}$ series as an important system for exploring the physics of structural quantum criticality without the need of applying high pressures.

The technique of standard addition in combination with powder X-ray diffraction was used to identify and quantify the amount of Ln3+ segregating into secondary phases from Ln3+doped alkaline earth aluminates. Results indicate that Ln3+ ions are more soluble in CaAl2O4 than SrAl2O4 and BaAl2O4, with this being rationalized by the structural details of the A sites. These results indicate that the enhancement of the luminescence afterglow obtained by doping AAl2O4:Eu2+ samples with Ln3+ ions is a result of much lower doping levels than previously thought.

This work reports the synthesis and structures of six new AB(HCO2)3 (A = Rb+ or Cs+ and B = Mn2+, Co2+ or Ni2+) frameworks containing the largest monoatomic cations on the A-site. RbMn(HCO2)3 is found to adopt a distorted perovskite framework with a 412[middle dot]63 topology and a mixture of syn-anti and anti-anti ligands, while the remaining compounds adopt a chiral hexagonal structure with a 49[middle dot]66 topology. The structures of these frameworks clarify the effect of ionic size on the formation of the five known architectures adopted by the AB(HCO2)3 frameworks, which have attracted attention as a new class of potential multiferroics, and in particular the chiral hexagonal structure within this. This also highlights the role of molecular A-site cations in stabilising the 49[middle dot]66 topology for frameworks where such cations are too large or small to support this structure on the basis of size alone, possibly due to hydrogen bonding. The magnetic properties of the RbB(HCO2)3 and CsMn(HCO2)3 frameworks are also reported with the Rb+ compounds featuring weak ferromagnetic behaviour and the latter being purely antiferromagnetic. In conjunction with a comparison of the other isostructural AB(HCO2)3 frameworks we find that compounds adopting the 49[middle dot]66 topology have much higher magnetic ordering temperatures than those with the RbMn(HCO2)3 structure, highlighting the importance of understanding the structure-property relationships of the ternary formates.

Several double perovskite compounds of the type Ba 2 NdB′O 6− δ (B′=Sn 4+ , Nb 5+ and Sb 5+ ) were synthesised. Room temperature neutron diffraction patterns were obtained in order to determine the precise structure of these compounds. It was found that monoclinic Ba 2 NdSnO 5.5 and Ba 2 NdNbO 6 and rhombohedral Ba 2 NdSbO 6 all featured only out-of-phase tilting. The structures of each of these three compounds revealed consistent overbonding of the Nd 3+ cations most likely as a result of the significant size difference between the Nd 3+ ions and B′ cations.

A novel family of cerium-doped garnet oxide phosphors, (Ca 1-xSrx)3RE2Ge3O 12:Ce,Li (2%, 2%) (x=0-1, RE = Y, Lu), was prepared using a conventional high-temperature ceramic route and was found to exhibit green luminescence under blue excitation. These compounds were characterised by synchrotron X-ray diffraction and photoluminescence spectroscopy. The results from this study were compared with other known Ce-doped garnets, including yttrium aluminium garnet, leading to a deeper understanding of the relationship between crystal structure and emission wavelength. In particular, the relationship between Ce3+ emission and the distortion factor of the dodecahedral doping site (d88/d81) was examined. © 2013 Elsevier B.V. All rights reserved.

The local nuclear and magnetic structure of wustite, Fe1-xO, and the coupling between them, has been examined using reverse Monte Carlo refinements of variable-temperature neutron total scattering data. The results from this analysis suggest that the individual units in a tetrahedral defect cluster are connected along vectors into a Koch-Cohen-like arrangement, with the majority of octahedral vacancies concentrated near these defects. Bond valence calculations indicate a change in the charge distribution on the cations with the charge on the tetrahedral interstitials increasing on cooling. The magnetic structure is more complex than previously thought, corresponding to a non-collinear spin arrangement described by a superposition of a condensed spin wave on the established type-II antiferromagnetic ordering. This leads to an architecture with four groups of cations each with different spin directions. The cations within the interstitial clusters appear to be weakly ferromagnetically coupled and their spins are correlated to the spins of the octahedral cations closest to them. This work not only provides further insight into the local structure of wustite but also a better understanding of the coupling between defect structures and magnetic and charge-ordering in complex materials., 9 pages, 8 figures