Anthony Edward Phillips

This work discusses two new barocaloric materials, the first 3D printed barocaloric composite in the shape of a gyroid-like heat exchanger, and new potential applications for barocaloric thermal management in hydrogen and Li-ion energy technologies.

There is increasing need for higher performance dielectric polymers for devices in power conversion systems for renewable energy generation and electric vehicles. In particular, materials with higher dielectric permittivity, lower loss and the ability to operate at higher temperatures. We have developed a counter intuitive method to achieve this, the melt blending of multiple immiscible polymers, in an approach that mimics high entropy materials design. We demonstrate that using this approach we can significantly exceed the rule-of-mixtures for the dielectric constant (>250%), whilst surprisingly retaining a low loss tangent. The materials show increased thermal stability up to 150 °C, which opens up the possibility of the wider application of dielectric polymers. We provide a consistent model to describe the behaviour based on the use of polymers with different glass transition temperatures to frustrate the de-blending of the immiscible polymers during melt processing. This produces highly amorphous and disordered polymer blends with increased inter-chain spacing (free volume) and increased rotational freedom of the polar groups in polar nano regions. This approach has wide applicability to other polar polymer blends and is scalable.

There is increasing need for higher performance dielectric polymers for devices in power conversion systems for renewable energy generation and electric vehicles. In particular, materials with higher relative dielectric permittivity, lower loss and the ability to operate at higher temperatures. We have developed a novel and counter intuitive method to achieve this, the melt blending of multiple immiscible polymers, in an approach that mimics high entropy materials design. We demonstrate that using this approach we can significantly exceed the rule-of-mixtures for the relative dielectric permittivity (> 250%), whilst surprisingly, retaining low dielectric loss. The materials show increased thermal stability up to 150°C, which opens up the possibility of the wider application of dielectric polymers. We provide a consistent model to describe the behaviour based on the use of polymers with different glass transitions temperatures to frustrate the de-blending of the immiscible polymers during melt processing. This produces highly amorphous and disordered polymer blends with increased inter-chain spacing (free volume) and increased mobility of the polar groups in polar nano regions. This new approach has wide applicability to other polar polymer blends and is scalable.

We report detailed structural studies of the low-temperature ferroelectric phases of [NH4]M(HCO2)3, where M = Mn2+ and Zn2+, finding that a third of the NH4+ cations remain subtly rotationally disordered to low temperature in both compounds. All NH4+ cations within the channels are well separated from each other, with significant hydrogen bonds only with the anionic M(HCO2)3 framework. Complementary studies of the dynamics using 2H solid state NMR and quasielastic neutron scattering indicate significant motion in both paraelectric and ferroelectric phases, which evolves gradually with increasing temperature with no abrupt change at the phase transition. Nudged elastic band calculations suggest that the activation barrier for flipping between “up” and “down” orientations of the NH4+ cations is low in the ferroelectric phase, with the NH4+ cations primarily interacting with the framework rather than the neighbouring molecular cations. It is likely this motion that is responsible for scrambling the NH4+ cation orientation locally in the ferroelectric phase. We propose that this disorder, with the same basic motion active above and below the phase transition, induces the significant dielectric relaxation associated with these materials’ relaxor-like dielectric properties.

This work introduces a completely rewritten version of the program RMCProfile (version 7), big-box, reverse Monte Carlo modelling software for analysis of total scattering data. The major new feature of RMCProfile7 is the ability to refine multiple phases simultaneously, which is relevant for many current research areas such as energy materials, catalysis and engineering. Other new features include improved support for molecular potentials and rigid-body refinements, as well as multiple different data sets. An empirical resolution correction and calculation of the pair distribution function as a back-Fourier transform are now also available. RMCProfile7 is freely available for download at https://rmcprofile.ornl.gov/.

Organometallic-sandwich salts are well-known materials that undergo order-disorder phase transitions, leading to a high-temperature phase characterized by the total or partial disorder of ionic species. Their potential for barocaloric applications has not previously been explored. Here, we focus on two salts derived from metallocenes with the formula [Cp₂M][PF₆] where Cp: cyclopentadienyl anion (C₅H₅)^- and M: Fe³⁺ or Co³⁺. These molecular salts exhibit two solid-solid phase transitions, shifting from a well-ordered crystalline phase at low temperatures to an orientationally disordered phase in the case of the Fe-compound, and a partially disordered phase in the case of the Co-compound above room temperature. We find a significant entropy change (∼40 J K^-1 kg^-1) and a moderate volume change (∼2%) associated with these phase transitions. Additionally, we observe that these transitions are highly sensitive to external applied pressure, leading to substantial barocaloric effects (exceeding 20 K kbar^-1). Very interestingly, we obtain substantial values for reversible adiabatic temperature change (ΔT_rev > 10 K) under an applied pressure as low as 1 kbar, comparable to those observed in the most promising barocaloric materials. We prove that the unique structure and chemical bonding of these sandwich organometallic cations are responsible for their interesting and unusual barocaloric response. These findings position the metallocenium family as promising candidates for eco-friendly solid-state refrigeration technologies.

Using X-ray diffraction and measurements of the pair distribution function we show that negative thermal expansion in SmB ₆ arises from charge transfer rather than the traditional tension effect.

The phase transitions and orientational order of the methylammonium molecular ion in a deuterated sample of [CH₃NH₃]PbI₃ have been studied using neutron total scattering and Reverse Monte Carlo methods. From analysis of the C–N bond orientational distribution functions, obtained both by forming histograms and by analysis using orthonormal spherical functions, we showed that the molecular ions were found to have an almost completely uniform distribution of orientations in the high-temperature cubic phase. The C–N bonds then show progressively increasing order on cooling within the intermediate phase, and complete order in the low-temperature phase. Analysis of the network of connected PbI₆ octahedra has shown that the rigidity of the PbI₆ octahedra is similar to that in the related cubic perovskite CsPbI₃, and is intermediate between the oxide perovskite SrTiO₃ and the more flexible ScF₃. Both the fluctuations of the I–Pb–I right angle and the orientations of the PbI₆ show normal variation with temperature across the range of temperatures of all three phases. The Pb–I bond has been found to be highly anharmonic, comparable to the same bond in CsPbI₃. We find no evidence for static ferroelectric distortions at any temperature, but dynamic fluctuations in the local dipole moment are found to be larger than in some other materials.

Correction for ‘Local structure and lithium-ion diffusion pathway of cubic Li₇La₃Zr₂O₁₂ studied by total scattering and the Reverse Monte Carlo method’ by Haolai Tian et al., J. Mater. Chem. A, 2023, 11, 25516–25533, https://doi.org/10.1039/D3TA04495E.