arXiv subjects

David Boldrin

Publications and source records attributed to David Boldrin.

9 recordsLinked to original sources

arXiv2026

Insight into the molecular dynamics of barocaloric molecular crystals using quasi-elastic neutron scattering

Barocaloric (BC) molecular crystals exhibit order-disorder phase transitions that o9er a promising basis for solid-state heating and cooling. However, rational design of these next-generation materials requires microscopic understanding of the molecular dynamics that drive the transitional entropy change. Quasi-elastic neutron scattering (QENS) techniques including inelastic fixed-window scans (IFWSs) enable the direct probing of such molecular motions throughout thermodynamic phase transitions. Here we present a QENS-IFWS global fitting methodology for systematically investigating the molecular dynamics underpinning BC performance in molecular crystals. We demonstrate this methodology on neopentyl glycol (NPG) and three derivatives spanning chemically distinct routes to dynamical perturbation. By fixing attempt frequencies across samples, we observe measurable differences in the relative activation energies and mobile fractions of hydroxymethyl and full-molecular rotations. The largest observed e9ect is on the hydroxymethyl rotation activation energies, which is closely related to hydrogen bond network stability, ranging from -3.6% to +11.1% in the derivatives compared to pure NPG. The capability of this global fitting strategy to extract subtle variations in molecular rotations highlights QENS as a versatile design tool for guiding BC molecular crystal design.

cond-mat.mtrl-sci↗

arXiv2026

Investigating nucleation-driven phase transitions in neopentyl molecular crystals using infrared thermography and polarised light microscopy

Sustainable solid-state refrigerants based on barocaloric materials are often limited by thermal hysteresis associated with supercooling effects. Here, we present imaging methods to investigate and compare thermal behaviour and transition kinetics of the barocaloric molecular crystal neopentyl glycol (NPG) with those of a lightly doped derivative, NPG$_{0.99}$PE$_{0.01}$, which incorporates 1 mol % pentaerythritol (PE). We use temperature-dependent polarised light (PL) microscopy and infrared (IR) thermography to correlate phase transition kinetics and local heat-flow with the bulk thermodynamic response obtained from calorimetry. We show that the doped system exhibits reduced supercooling and thermal hysteresis, attributed to increased microstructural disorder and an increase in the number of nucleation events. These findings provide insight into the design of low-hysteresis barocaloric materials for high-efficiency solid-state cooling applications.

cond-mat.mtrl-sci↗

arXiv2025

Confinement in metal-organic frameworks as a route to harnessing liquid barocalorics in the solid-state

Barocaloric (BC) effects at liquid-vapor transitions in hydrofluorocarbons drive most commercial technologies used for heating and cooling in the heating, ventilation and air-conditioning sector. However, these fluids suffer from huge global warming potential and alternative gases are less efficient, toxic or flammable. Solid-solid and solid-liquid BC materials have zero global warming potential and could even improve on current device efficiencies. Whilst solid-liquid BCs typically outperform solid-solid BCs, the latter are advantageous as they avoid leaks and present easier handling and recyclability thus facilitating waste management. Here we confine the solid-liquid BC stearic acid inside the nanopores of a functionalised metal-organic framework (MOF) and demonstrate that the colossal BC properties are retained in a solid-state material. Moreover, the enhanced interactions between the pore surface and the BC material allow a level of active control over the thermal response, as opposed to passive encapsulation. Our results open novel avenues to exploit and tune colossal BC effects in a wide range of combinations of solid-liquid BC materials embedded within functionalized MOFs, without the associated engineering drawbacks.

cond-mat.mtrl-sci↗

arXiv2025

Enhanced reversible barocaloric effect at low pressure in neopentyl plastic crystal solid solutions

The discovery of colossal barocaloric effects in neopentyl glycol (NPG) makes plastic crystals promising candidates for solid-state refrigerants with lower environmental impact than vapour compression fluids. Optimising operational temperatures and low-pressure operability remains challenging without compromising thermodynamic parameters. Here, we implement a strategy to improve the viability of NPG derivatives as barocaloric refrigerants. We blend pentaglycerine (PG) with NPG to lower the phase transition temperature, then dope the blend with 2% pentaerythritol (PE) to improve transition reversibility. In comparison with NPG under the same conditions, this ternary system has a seven-fold increase in reversible isothermal entropy change (|$\Delta S_{(it,rev)}$ | = 13.4 J kg$^{-1}$ K$^{-1}$) and twenty-fold increase in operational temperature span ($\Delta T_{span}$ = 18 K) at pressures of 1 kbar. Synchrotron x-ray diffraction and quasielastic neutron scattering reveal structural and dynamical effects that broaden the temperature range of the first-order phase transition due to intermolecular hydrogen bond network disruption by the molecular dopants. We propose that exploiting the compositional phase space of multi-component molecular blends is effective for designing practicable molecular BCs.

cond-mat.mtrl-sci↗

arXiv2024

Direct observation of thermal hysteresis in the molecular dynamics of barocaloric neopentyl glycol

Barocalorics (BCs) are emerging as promising alternatives to vapour-phase refrigerants, which are problematic as they exacerbate climate change when they inevitably leak into the atmosphere. However, the commercialisation of BC refrigerants is significantly hindered by hysteresis in the solid-solid phase transition that would be exploited in a refrigeration cycle. Here, we provide new insight into the hysteresis that is a critical step towards the rational design of viable BCs. By studying the benchmark BC plastic crystal, neopentyl glycol (NPG), we observe directly the liberation of the hydroxyl rotational modes that unlock the hydrogen bond network, distinguishing for the first time the molecular reorientation and hydroxymethyl rotational modes. We showcase the use high-resolution inelastic fixed-window scans in combination with quasielastic neutron scattering (QENS) measurements to build a comprehensive microscopic understanding of the NPG phase transition, directly tracking the molecular dynamics of the phase transition. Hysteresis previously observed in calorimetric studies of NPG is now observed directly as hysteresis in molecular rotational modes, and hence in the formation and disruption of hydrogen bonding. Furthermore, by tracking the thermal activation of three main reorientation modes, we suggest that their fractional excitations may resolve an outstanding discrepancy between measured and calculated entropy change. These results allow for direct study of the molecular dynamics that govern the thermal hysteresis of small molecule energy materials. They will be broadly applicable, as many promising BC material families possess first-order transitions involving molecular reorientations.

cond-mat.mtrl-sci↗

arXiv2023

The Impact of Local Strain Fields in Non-Collinear Antiferromagnetic Films

Antiferromagnets hosting structural or magnetic order that breaks time reversal symmetry are of increasing interest for 'beyond von Neumann computing' applications because the topology of their band structure allows for intrinsic physical properties, exploitable in integrated memory and logic function. One such group are the non-collinear antiferromagnets. Essential for domain manipulation is the existence of small net moments found routinely when the material is synthesised in thin film form and attributed to symmetry-breaking caused by spin canting, either from the Dzyaloshinskii-Moriya interaction or from strain. Although the spin arrangement of these materials makes them highly sensitive to strain, there is little understanding about the influence of local strain fields caused by lattice defects on global properties, such as magnetisation and anomalous Hall effect. This premise is investigated by examining non-collinear films that are either highly lattice mismatched or closely matched to their substrate. In either case, edge dislocation networks are generated and for the former case these extend throughout the entire film thickness, creating large local strain fields. These strain fields allow for finite intrinsic magnetisation in seemly structurally relaxed films and influence the antiferromagnetic domain state and the intrinsic anomalous Hall effect.

cond-mat.mtrl-sci↗

arXiv2019

The anomalous Hall effect in non-collinear antiferromagnetic Mn$_{3}$NiN thin films

We have studied the anomalous Hall effect (AHE) in strained thin films of the frustrated antiferromagnet Mn$_{3}$NiN. The AHE does not follow the conventional relationships with magnetization or longitudinal conductivity and is enhanced relative to that expected from the magnetization in the antiferromagnetic state below $T_{\mathrm{N}} = 260$\,K. This enhancement is consistent with origins from the non-collinear antiferromagnetic structure, as the latter is closely related to that found in Mn$_{3}$Ir and Mn$_{3}$Pt where a large AHE is induced by the Berry curvature. As the Berry phase induced AHE should scale with spin-orbit coupling, yet larger AHE may be found in other members of the chemically flexible Mn$_{3}A$N structure.

cond-mat.str-el↗

arXiv2018

Multi-site exchange enhanced barocaloric response in Mn$_{3}$NiN

We have studied the barocaloric effect (BCE) in the geometrically frustrated antiferromagnet Mn$_{3}$NiN across the N\'{e}el transition temperature. Experimentally we find a larger barocaloric entropy change by a factor of 1.6 than that recently discovered in the isostructural antiperovskite Mn$_{3}$GaN despite greater magnetovolume coupling in the latter. By fitting experimental data to theory we show that the larger BCE of Mn$_{3}$NiN originates from multi-site exchange interactions amongst the local Mn magnetic moments and their coupling with itinerant electron spins. Using this framework, we discuss the route to maximise the BCE in the wider Mn$_{3}$AN family.

cond-mat.str-el↗

arXiv2011

Spin dynamics in the S =1/2 kagome compound vesignieite, Cu3Ba(VO5H)2

We report the study of high quality samples of the frustrated S = 1/2 kagome antiferromagnet vesignieite, Cu3Ba(VO5H)2. Neutron powder diffraction measurements evidence the high quality of the kagome lattice and show no sign of a transition to long-range order. A kink in the susceptibility below T = 9 K is matched to a reduction in paramagnetic-like correlations in the diffraction data and a slowing of the spin dynamics observed by MuSR. Our results point to an exotic quantum state below 9 K with coexistance of both dynamical and small frozen moments \sim 0.1 \muB. We propose that Dzyaloshinsky-Moriya interaction is large enough in this system to stabilize this novel quantum ground state.

cond-mat.str-el↗