Ambient Temperature Synthesis and Structural Characterisation of Six Transition Metal Acetylenedicarboxylate Coordination Polymers

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

A Short, Versatile Route Towards Benzothiadiazinyl Radicals

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

Optimisation of the Magnetocaloric Effect in Low Applied Magnetic Fields in LnOHCO3 Frameworks

This study probes the structure and magnetocaloric effect of the LnOHCO3 (Ln = Gd3+, Tb3+, Dy3+, Ho3+ and Er3+) frameworks. A combination of single crystal X-ray and neutron powder diffraction indicate 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 effect of TbOHCO3 and DyOHCO3 have a peak entropy change of 30.99 and 33.34 J Kg-1 K-1 for a 2-0 T field change respectively, which is higher than than the promising GdOHCO3 framework above 4 K in moderate magnetic fields. The magnetic entropy change of TbOHCO3 and DyOHCO3 above 4 K for sub-2T field changes also exceeds that 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

Strong Coupling Superconductivity in the Vicinity of the Structural Quantum Critical Point in (CaxSr1-x)3Rh4Sn13

The family of the superconducting quasiskutterudites (CaxSr1?x)3Rh4Sn13 features a structural quantum critical point at xc=0.9, around which a dome-shaped variation of the superconducting transition temperature Tc 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 xc, which we interpret as a result of phonon softening accompanying the structural instability. Furthermore, a remarkably large enhancement of 2?/kBTc and ?C/?Tc beyond the Bardeen-Cooper-Schrieffer values is found in the vicinity of the structural quantum critical point. The phase diagram of (CaxSr1?x)3Rh4Sn13 thus provides a model system to study the interplay between structural quantum criticality and strong electron-phonon coupling superconductivity

Low Dimensional and Frustrated Antiferromagnetic Interactions in Transition Metal Chloride Complexes with Simple Amine Ligands

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 relationship between the strength of the magnetic coupling observed in these materials and their likely exchange pathways are also discussed

Investigations of the Magnetocaloric and Thermal Expansion Properties of the Ln3(adipate)4.5(DMF)2 (Ln = Gd–Er) Framework Series

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 non-renewable 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 characterisation of a Ln3(adipate)4.5(DMF)2 series (where Ln = Gd-Er) of Metal-Organic Framework (MOF) materials incorporating a flexible adipate ligand and their associated magnetocaloric and thermal expansion properties. The magnetocaloric performance of the Gd3(adipate)4.5(DMF)2 material was found to exhibit the highest magnetic entropy changes of the series with a peak entropy change of 36.4 J kg-1 K-1 for a 5-0 T field change at a temperature of 2 K which is suited for ultra-low temperature cooling applications. Thermal expansion properties were also investigated within these materials demonstrating modest negative and large positive thermal expansion identified along the different crystallographic axes within the MOF structures over a 100-300 K temperature range that demonstrated the novel mechanical properties of these adipate framework structures

Anomalous evolution of the magnetocaloric effect in dilute triangular Ising antiferromagnets Tb1−xYx(HCO2)3Tb_{1-x}Y_{x}(HCO_{2})_{3}

We investigate the effects of diamagnetic doping in the solid-solution series $Tb_{1-x}Y_{x}(HCO_{2})_{3}$, in which the parent $Tb(HCO_{2})_{3}$ phase has previously been shown to host a combination of frustrated and quasi-1D physics, giving rise to a triangular Ising antiferromagnetic ground state that lacks long range 3D order. Heat capacity measurements show three key features: (i) a low temperature Schottky anomaly is observed, which is constant as a function of x; (ii) the transition temperature and associated entropy change are both surprisingly robust to diamagnetic doping; and (iii) an additional contribution at T < 0.4 K appears with increasing x. The origin of this unusual behaviour is rationalised in terms of the fragmentation of quasi-1D spin chains by the diamagnetic $Y^{3+}$ dopant. Magnetocaloric measurements show a nonlinear dependence on x. The mass-weighted magnetocaloric entropy decreases across the series from the promising values in $Tb(HCO_{2})_{3}$; however, the magnetocaloric entropy per magnetic $Tb^{3+}$ ion first decreases then increases with increasing x. Our results establish $Tb_{1-x}Y_{x}(HCO_{2})_{3}$ as a model system in which to explore the functional ramifications of dilution in a low-dimensional magnet.Comment: 11 pages and 5 figures excluding supplementary informatio

One-dimensional magnetic order in the metal–organic framework Tb(HCOO)3Tb(HCOO)_3

Variable-temperature neutron scattering measurements, reverse Monte Carlo analysis and direct Monte Carlo simulation are used to characterise magnetic order in the metal–organic framework (MOF) $Tb(HCOO)_3$ over the temperature range 100K to 1.6K = T$_N$. The magnetic transition at T$_N$ is shown to involve one-dimensional ferromagnetic ordering to a partially-ordered state related to the triangular Ising antiferromagnet and distinct from the canonical “partially-disordered antiferromagnet” model. 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. We suggest 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

Pressure-induced and Composition-induced Structural Quantum Phase Transition in the Cubic Superconductor (Sr/Ca)_3Ir_4Sn_{13}

We show that the quasi-skutterudite superconductor Sr_3Ir_4Sn_{13} undergoes a structural transition from a simple cubic parent structure, the I-phase, to a superlattice variant, the I'-phase, which has a lattice parameter twice that of the high temperature phase. We argue that the superlattice distortion is associated with a charge density wave transition of the conduction electron system and demonstrate that the superlattice transition temperature T* can be suppressed to zero by combining chemical and physical pressure. This enables the first comprehensive investigation of a superlattice quantum phase transition and its interplay with superconductivity in a cubic charge density wave system.Comment: 4 figures, 5 pages (excluding supplementary material). To be published in Phys. Rev. Let

Spectroscopic Identification of Disordered Molecular Cations in Defect Perovskite‐like ALnLn(HCO2_2)(C2_2O4_4)1.5_{1.5} (LnLn = Tb‐Er) Phases

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