arXiv subjects

Simon A. T. Redfern

Publications and source records attributed to Simon A. T. Redfern.

5 recordsLinked to original sources

arXiv2021

Distinct scaling behaviors of giant electrocaloric cooling performance in low-dimensional organic, relaxor and anti-ferroelectrics

Electrocaloric (EC) materials show promise in eco-friendly solid-state refrigeration and integrable on-chip thermal management. While direct measurement of EC thin-films still remains challenging, a generic theoretical framework for quantifying the cooling properties of rich EC materials including normal-, relaxor-, organic- and anti-ferroelectrics is imperative for exploiting new flexible and room-temperature cooling alternatives. Here, we present a versatile theory that combines Master equation with Maxwell relations and analytically relates the macroscopic cooling responses in EC materials with the intrinsic diffuseness of phase transitions and correlation characteristics. Under increased electric fields, both EC entropy and adiabatic temperature changes increase quadratically initially, followed by further linear growth and eventual gradual saturation. The upper bound of entropy change (dS_max) is limited by distinct correlation volumes (V_cr) and transition diffuseness. The linearity between V_cr and the transition diffuseness is emphasized, while dS_max=300 kJ/(K.m3) is obtained for Pb0.8Ba0.2ZrO3. The dS_max in antiferroelectric Pb0.95Zr0.05TiO3, Pb0.8Ba0.2ZrO3 and polymeric ferroelectrics scales proportionally with V_cr^(-2.2), owing to the one-dimensional structural constraint on lattice-scale depolarization dynamics; whereas dS_max in relaxor and normal ferroelectrics scales as dS_max ~ V_cr^(-0.37), which tallies with a dipolar interaction exponent of 2/3 in EC materials and the well-proven fractional dimensionality of 2.5 for ferroelectric domain walls.

cond-mat.mtrl-sci↗

arXiv2020

Ternary Hypervalent Silicon Hydrides via Lithium at High Pressure

Hydrogen is rarely observed as ligand in hypervalent species, however, we find that high-pressure hydrogenation may stabilise hypervalent hydrogen-rich materials. Focussing on ternary silicon hydrides via lithium doping, we find anions composed of hypervalent silicon with H ligands formed under high pressure. Our results reveal two new hypervalent anions: layered-SiH$_{5}^{-}$ and tricapped trigonal prismatic SiH$_{6}^{2-}$. These differ from octahedral SiH$_{6}^{2-}$ described in earlier studies. In addition, there are further hydrogen-rich structures Li$_{3}$SiH$_{10}$ and Li$_{2}$SiH$_{6+δ}$ which may be stabilised at high pressure. Our work provides pointers to future investigations on hydrogen-rich materials.

cond-mat.mtrl-sci↗

arXiv2020

Hydrogen "penta-graphene-like" structure stabilized by hafnium: a high-temperature conventional superconductor

The recent discovery of H3S and LaH10 superconductors with record high superconducting transition temperatures, Tc, at high pressure, has fueled the search for room-temperature superconductivity in the compressed superhydrides. Here we predict the existence of an unprecedented hexagonal HfH10, with an extraordinarily high Tc of around 213-234 K at 250 GPa. In HfH10, the H atoms are arranged in clusters to form a planar "penta-graphene-like" sublattice, in contrast to the covalent sixfold cubic structure in H3S and clathrate-like structure in LaH10. The Hf atom acts as a "precompressor" and electron donor to the hydrogen sublattice. This "penta-graphene-like" H10 structure is also found in ZrH10, ScH10 and LuH10 at high pressure, each material showing a high Tc ranging from 134 to 220 kelvin. Our study of dense superhydrides with "penta-graphene-like" layered structures opens the door to the exploration and exploitation of a new class of high Tc superconductors.

cond-mat.supr-con↗

arXiv2019

Reverse chemistry of iron in the deep Earth

In this work, we demonstrate a remarkable change of chemical trend of Iron under high pressure that is of great importance for understanding the distribution of elements in the Earth's mantle and core. Using first principles crystal structure search method, we conduct a systematic study of the propensity of p block elements to chemically bind with iron under high pressures ranging from ambient conditions to that of Earth's core. We show that under increasing pressure, iron tends to reverse its chemical nature, changing from an electron donor (reductant) to an electron acceptor, and oxidizes p-block elements in many compounds. Such reverse chemistry has a significant impact on the stoichiometries, bond types and strengths, structures and properties of iron compounds under deep planetary conditions.

physics.chem-ph↗

arXiv2009

The beta to gamma (insulator-metal) transition in BiFeO3

High temperature powder neutron diffraction experiments have been conducted around the reported beta-gamma, insulator-metal phase transition (~ 930 C) in BiFeO3. The results demonstrate that while a small volume contraction is observed at the transition temperature, consistent with an insulator-metal transition, both the beta and gamma phase of BiFeO3 exhibit orthorhombic symmetry i.e. no further increase of symmetry occurs under the present experimental conditions, contrary to previous suggestions. Furthermore we observe the gamma orthorhombic phase to persist up to a temperature of approximately 950 C before complete decomposition into Bi2Fe4O9 (and liquid Bi2O3), which subsequently begins to decompose at approximately 960 C.

cond-mat.mtrl-sci↗