RESUMEN
Two-dimensional materials have proven to be a prolific breeding ground of new and unstudied forms of magnetism and unusual metallic states, particularly when tuned between their insulating and metallic phases. Here we present work on a new metal-to-insulator transition system FePS_{3}. This compound is a two-dimensional van der Waals antiferromagnetic Mott insulator. We report the discovery of an insulator-metal transition in FePS_{3}, as evidenced by x-ray diffraction and electrical transport measurements, using high pressure as a tuning parameter. Two structural phase transitions are observed in the x-ray diffraction data as a function of pressure, and resistivity measurements show evidence of the onset of a metallic state at high pressures. We propose models for the two new structures that can successfully explain the x-ray diffraction patterns.
RESUMEN
Magnetoelastic coupling associated with the hexagonal-monoclinic transition in a natural sample of the mineral pyrrhotite, Fe7S8, has been analysed in terms of separate coupling of spontaneous strains with two discrete order parameters, q v for Fe/vacancy ordering and q m for magnetic ordering. Coupling of the two order parameters separately with strain gives rise to two terms for coupling between them, λ [Formula: see text] and λ [Formula: see text], and a pattern of evolution in which q v varies continuously and q m discontinuously through a single transition point. The transition is ferrimagnetic and ferroelastic but the relatively slow relaxation rate for Fe/vacancy ordering, in comparison with magnetic ordering, results in elastic and anelastic properties which are quite different from those observed in other ferroic or multiferroic materials with two instabilities. Instead of classical elastic softening, there is stiffening of the elastic constants which scales with [Formula: see text] and [Formula: see text]. Instead of the normal pattern of acoustic loss associated with the mobility and subsequent freezing for ferroelastic twin walls, the loss is consistently low throughout the temperature range 300 K-875 K.
RESUMEN
We present an overview of our recent work in tuning and controlling the structural, magnetic and electronic dimensionality of 2D van-der-Waals antiferromagnetic compounds (Transition-Metal)PS3. Low-dimensional magnetic systems such as these provide rich opportunities for studying new physics and the evolution of established behaviours with changing dimensionality. These materials can be exfoliated to monolayer thickness and easily stacked and combined into functional heterostructures. Alternatively, the application of hydrostatic pressure can be used to controllably close the van-der-Waals interplanar gap and tune the crystal structure and electron exchange paths towards a 3D nature. We collect and discuss trends and contrasts in our data from electrical transport, Raman scattering and synchrotron x-ray measurements, as well as insight from theoretical calculations and other results from the literature. We discuss structural transitions with pressure common to all materials measured, and link these to Mott insulator-transitions in these compounds at high pressures. Key new results include magnetotransport and resistivity data in the high-pressure metallic states, which show potentially interesting qualities for a new direction of future work focussed on low temperature transport and quantum critical physics.
RESUMEN
We explore the effects of particle size and solvent environment on the thermodynamic stability of two pairs of polymorphs subjected to ball-mill neat grinding (NG) and liquid assisted grinding (LAG). Two systems were studied: (i) forms I and II of a 1 : 1 theophylline : benzamide cocrystal and (ii) forms A and B of an aromatic disulfide compound. For both systems, the most stable-bulk polymorph converted to the metastable-bulk polymorph upon NG. LAG experiments yielded different outcomes depending on the amount of solvent used. This was further investigated by performing carefully controlled LAG experiments with increasing µL amounts of solvents of different nature. With these experiments, we were able to monitor form A to B and form I to II conversions as a function of solvent concentration and derive polymorph equilibrium curves. The concentration required for a switch in polymorphic outcome was found to be dependent on solvent nature. We propose that these experiments demonstrate a switch in thermodynamic stability of the polymorphs in the milling jar. Form B, the stable-bulk polymorph, has less stable surfaces than form A, thus becoming metastable at the nanoscale when surface effects become important. Ex situ diffraction and electron microscopy data confirm crystal sizes in the order of tens of nanometers after the ball mill grinding experiments reach equilibrium. DFT-d computations of the polymorph particles stabilities support these findings and were used to calculate cross-over sizes of forms A and B as a function of solvent. Attachment energies and surface stabilities of the various crystalline faces exposed were found to be very sensitive to the solvent environment. Our findings suggest that surface effects are significant in polymorphism at the nanoscale and that the outcomes of equilibrium ball-mill NG and LAG experiments are in general controlled by thermodynamics.
RESUMEN
The Gd3Ga5-xAlxO12 (0 ≤ x ≤ 5) solid solution has been prepared using ceramic synthesis routes and the structural and magnetic properties were investigated using x-ray diffraction, magnetic susceptibility, χ, and isothermal magnetisation, M(H), measurements. Our results indicate a contraction of the unit cell and more significant antiferromagnetic interactions as x increases. Despite the decrease in the magnetic polarisation on the application of a field and the corresponding decrease in the change in the magnetic entropy, ΔS, we find that Gd3Al5O12 has a significantly higher observed (17%) and theoretical (14%) ΔS per unit mass than Gd3Ga5O12. The theoretical increase in ΔS per unit volume (7%) is offset by the increased antiferromagnetic interactions in Gd3Al5O12. The differences in ΔS are driven by a decrease in both the mass and the density as Al ions replace Ga ions. These results highlight the importance of changes to the crystal structure when considering materials for solid state magnetic cooling.