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Multicomponent hydrogels offer a tremendous opportunity for preparing useful and exciting materials that cannot be accessed using a single component. Here, we describe an unusual multi‐component low‐molecular weight gelling system that exhibits pH‐responsive behavior involving cooperative hydrogen bonding between the components, allowing it to maintain a gel phase across a wide pH range. Unlike traditional acid‐triggered gels, our system undergoes a change in the underlying molecular packing and maintains the β‐sheet structure both at acidic and basic pH. We further establish that autonomous programming between these two gel states is possible by an enzymatic reaction which allows us to prepare gels with improved mechanical properties., We show that a multicomponent supramolecular gel with two opposite ionizable pendant groups on different components can exhibit unusual phase behavior involving cooperative hydrogen bonding between the components, allowing it to maintain a gel phase across a wide pH range. Autonomous programming between these two gel states is possible by an enzymatic reaction which allows the preparation of gels with improved mechanical properties.

This study focuses on how the mesophase transition behaviour of the phospholipid dioleoyl phosphatidylethanolamine (DOPE) is altered by the presence of 10 nm hydrophobic and 14 nm hydrophilic silica nanoparticles (NPs) at different concentrations. The lamellar to inverted hexagonal phase transition (Lα-HII) of phospholipids is energetically analogous to the membrane fusion process, therefore understanding the Lα-HII transition with nanoparticulate additives is relevant to how membrane fusion may be affected by these additives, in this case the silica NPs. The overriding observation is that the HII/Lα boundaries in the DOPE p-T phase diagram were shifted by the presence of NPs: the hydrophobic NPs enlarged the HII phase region and thus encouraged the inverted hexagonal (HII) phase to occur at lower temperatures, whilst hydrophilic NPs appeared to stabilise the Lα phase region. This effect was also NP-concentration dependent, with a more pronounced effect for higher concentration of the hydrophobic NPs, but the trend was less clear cut for the hydrophilic NPs. There was no evidence that the NPs were intercalated into the mesophases, and as such it was likely that they might have undergone microphase separation and resided at the mesophase domain boundaries. Whilst the loci and exact roles of the NPs invite further investigation, we tentatively discuss these results in terms of both the surface chemistry of the NPs and the effect of their curvature on the elastic bending energy considerations during the mesophase transition.

We use a combination of diamond anvil cell techniques and large volume (multi-anvil press, piston cylinder) devices to study the synthesis, structure and properties of new materials under high pressure conditions. The work often involves the study of structural and phase transformations occurring in the metastable regime, as we explore the phase space determined as a function of the pressure, temperature and chemical composition. The experimental studies are combined with first principles calculations and molecular dynamics simulations, as we determine the structures and properties of new phases and the nature of the transformations between them. Problems currently under investigation include structural studies of transition metal and main group nitrides, oxides and oxynitrides at high pressure, exploration of new solid-state compounds that are formed within the C-N-O system, polyamorphic low- to high-density transitions among amorphous semiconductors such as a-Si, and transformations into metastable forms of the element that occur when its "expanded" clathrate polymorph is compressed.

The structural behavior of the pyrochlore Bi2Hf2O7 with varying temperature has been studied by powder neutron diffraction X-ray diffraction, and Raman scattering. The unit cell undergoes phase changes that are determined by diffraction methods to occur between α-, β-, γ- and δ-polymorphs at around 400, 550 and 900 °C. The δ-Bi2Hf2O7 polymorph corresponds to the ideal cubic pyrochlore phase. Three of the four polymorphs have previously been reported to occur for Bi2Sn2O7. The need to accommodate the active Bi3+ lone pair in the presence of large HfO6 octahedra results in an “icelike” ordering of Bi positions in Bi2Hf2O7 at room temperature. A similar sequence of phase transitions is observed at room temperature with increasing Ti4+ for Hf4+ substitution in the compositional series Bi2Hf2-xTixO7. The structure changes as the space available to the Bi3+ ion increases represent a gradual increase in disorder and symmetry.

We combine high-pressure x-ray diffraction, high-pressure Raman scattering, and optical microscopy to investigate a series of ($1\ensuremath{-}x$)Pb(Mg${}_{1/3}$Nb${}_{2/3}$)O${}_{3}$-$x$PbTiO${}_{3}$ (PMN-$x$PT) solid solutions ($x=0.2$, 0.3, 0.33, 0.35, 0.37, 0.4) in diamond anvil cells up to 20 GPa at 300 K. The Raman spectra show a peak centered at 380 cm${}^{\ensuremath{-}1}$ starting above 6 GPa for all samples, in agreement with previous observations. X-ray diffraction measurements are consistent with this spectral change indicating a structural phase transition; we find that the triplet at the pseudocubic (220) Bragg peak merges into a doublet above 6 GPa. Our results indicate that the morphotropic phase boundary region ($x=0.33\ensuremath{-}0.37$) with the presence of monoclinic symmetry persists up to 7 GPa. The pressure dependence of ferroelectric domains in PMN-0.32PT single crystals was observed using a polarizing optical microscope. The domain wall density decreases with pressure and the domains disappear at a modest pressure of 3 GPa. We propose a pressure-composition phase diagram for PMN-$x$PT solid solutions.

We have combined first-principles calculations and high-pressure experiments to study pressure-induced phase transitions in silicon nitride (Si${}_{3}$N${}_{4}$). Within the quasi-harmonic approximation, we predict that the $\ensuremath{\alpha}$ phase is always metastable relative to the $\ensuremath{\beta}$ phase over a wide pressure-temperature range. Our lattice vibration calculations indicate that there are two significant and competing phonon-softening mechanisms in the $\ensuremath{\beta}$-Si${}_{3}$N${}_{4}$, while phonon softening in the $\ensuremath{\alpha}$-Si${}_{3}$N${}_{4}$ is rather moderate. When the previously observed equilibrium high-pressure and high-temperature $\ensuremath{\beta}$ $\ensuremath{\rightarrow}$ $\ensuremath{\gamma}$ transition is bypassed at room temperature (RT) due to kinetic reasons, the $\ensuremath{\beta}$ phase is predicted to undergo a first-order structural transformation to a denser $P\overline{6}$ phase above 39 GPa. The estimated enthalpy barrier height is less than 70 meV$/$atom, which suggests that the transition is kinetically possible around RT. This predicted new high-pressure metastable phase should be classified as a ``postphenacite'' phase. Our high-pressure x-ray diffraction experiment confirms this predicted RT phase transition around 34 GPa. No similar RT phase transition is predicted for $\ensuremath{\alpha}$-Si${}_{3}$N${}_{4}$. Furthermore, we discuss the differences in the pressure dependencies of phonon modes among the $\ensuremath{\alpha}$, $\ensuremath{\beta}$, and $\ensuremath{\gamma}$ phases and the consequences on their thermal properties. We attribute the phonon modes with negative Gr\"uneisen ratios in the $\ensuremath{\alpha}$ and $\ensuremath{\beta}$ phases as the cause of the predicted negative thermal expansion coefficients (TECs) at low temperatures in these two phases, and predict no negative TECs in the $\ensuremath{\gamma}$ phase.

We combine high-pressure x-ray diffraction, high-pressure Raman scattering, and optical microscopic methods to investigate a series of PMN-xPT solid solutions (x=0.2, 0.3, 0.33, 0.35, 0.37, 0.4) in diamond anvil cells up to 20 GPa at 300 K. The Raman spectra show a new peak centered at 380 cm−1 above 6 GPa for all samples, consistent with previous observations. X-ray diffraction measurements are consistent with this spectral change indicating a structural phase transition. For example, we find that the triplet at the pseudocubic [220] Bragg peak merges into a doublet above 6 GPa. The analyzed results indicate that the morphotropic phase boundary (x=0.33 to 0.37) with monoclinic symmetry persists up to 7 GPa. The pressure dependence of ferroelectric domains in PMN-0.32PT single crystals was observed with a polarizing optical microscope. The domain wall density decreases with pressure and the domains disappears at modest pressure of 3 GPa. This indicates that the high pressure phase of PMN-PT is not a macroscopic polar state. We suggest a phase diagram for the PMN-xPT solid solutions.

The structural behavior of the pyrochlore Bi2Hf2O7 with varying temperature has been studied by powder neutron diffraction X-ray diffraction, and Raman scattering. The unit cell undergoes phase changes that are determined by diffraction methods to occur between α-, β-, γ- and δ-polymorphs at around 400, 550 and 900 °C. The δ-Bi2Hf2O7 polymorph corresponds to the ideal cubic pyrochlore phase. Three of the four polymorphs have previously been reported to occur for Bi2Sn2O7. The need to accommodate the active Bi3+ lone pair in the presence of large HfO6 octahedra results in an “icelike” ordering of Bi positions in Bi2Hf2O7 at room temperature. A similar sequence of phase transitions is observed at room temperature with increasing Ti4+ for Hf4+ substitution in the compositional series Bi2Hf2-xTixO7. The structure changes as the space available to the Bi3+ ion increases represent a gradual increase in disorder and symmetry.